- Email: [email protected]
Maternal Serum Interleukin-6 During Pregnancy and During Term and Preterm Labor
Maternal Serum Interleukin-6 During Pregnancy and During Term and Preterm Labor PHILLIP C. GREIG, MD, AMY P. MURTHA, MD, CATHY J. JIMMERSON, WILLIAM N. P. HERBERT, MD, BEATRICE ROITMAN-JOHNSON, AND JEAN ALLEN, PhD Objective: To determine the normal concentrations of maternal serum interleukin-6 during the second and third trimesters of pregnancy and the different stages of term and preterm labor, and to examine the clinical usefulness of measuring this cytokine in the serum of women in preterm labor to diagnose asymptomatic intrauterine infections. Methods: Maternal serum interleukin-6 concentrations were measured cross-sectionally in 315 gravidas in their second and third trimesters and during term and preterm labor. Placentas from women who delivered preterm were examined for histologic chorioamnionitis. Results: At term, women in labor had significantly elevated median maternal serum interleukin-6 concentrations compared with those at term not in labor (4.7 pg/mL versus 2.2 pg/mL, P < .001). Women admitted in preterm labor who delivered had significantly higher median interleukin-6 concentrations than did those in preterm labor who responded to tocolysis (9.3 pg/mL versus 1.9 pg/mL, P < .001). Women in preterm labor who delivered preterm with evidence of chorioamnionitis had significantly higher serum concentrations of interleukin-6 than did those in preterm labor who delivered in the absence of chorioamnionitis (15.9 pg/mL versus 4.6 pg/mL, P 5 .006). Conclusion: Compared with antepartum gravidas, those in term or preterm labor had significantly higher concentrations of maternal serum interleukin-6 concentrations; extremely elevated levels were found in patients whose preterm labor was associated with intrauterine infection. (Obstet Gynecol 1997;90:465–9. © 1997 by The American College of Obstetricians and Gynecologists.)
Recent data regarding preterm labor indicate a strong association with intrauterine infection.1–3 Such infection, identified by the presence of histologic chorioamnionitis, has been found in 58 – 88% of patients in preterm labor who fail tocolysis and deliver prematureFrom the Department of Obstetrics and Gynecology, Duke University Medical Center, Durham, North Carolina, and R & D Systems, Minneapolis, Minnesota.
VOL. 90, NO. 3, SEPTEMBER 1997
ly.4 – 6 In vitro studies7,8 suggest that bacterial invasion of the uterus and the subsequent inflammatory response stimulate the production of prostaglandins and other substances that may promote uterine contractions and cervical ripening. Because the majority of these intrauterine infections are asymptomatic, their diagnosis is hampered by the absence of an accurate test for use in patients in preterm labor. Recently, considerable attention has been focused on a specific inflammatory mediator, interleukin-6.3,4,6,9 –12 Produced by both immune and nonimmune cells, including those found in the decidua, amnion, chorion, and placenta,13,14 this cytokine has been shown to propagate the local inflammatory cascade and to stimulate increased prostaglandin production.12 Elevated amniotic fluid interleukin-6 concentrations have been found in gravidas whose preterm labor was associated with an intrauterine infection.3,4,6,10 An elevated amniotic fluid interleukin-6 concentration appears to be the most sensitive marker for both early intrauterine infections limited to the fetal membranes and symptomatic intra-amniotic infection.15–17 Clinically, however, a reluctance to perform amniocentesis in patients in preterm labor limits the usefulness of this test for the identification of intrauterine infection. A simpler and less invasive method of measuring interleukin-6 would be useful for the identification of asymptomatic intrauterine infections in patients in preterm labor. Laham et al18 found elevations in maternal serum interleukin-6 concentration in patients in preterm labor who had symptomatic intrauterine infection, but none in preterm labor whose infection was asymptomatic were included. Interleukin-6 is a good choice for this measurement in the systemic circulation because of its known endocrine function as a mediator of the acute-phase response in the liver. Our study was designed to answer the following questions: 1) What are
0029-7844/97/$17.00 PII S0029-7844(97)00294-9
465
the normal concentrations of maternal serum interleukin-6 during the second and third trimesters of pregnancy and during the different stages of term labor? 2) Are maternal serum interleukin-6 concentrations elevated in patients in preterm labor who have asymptomatic infection? 3) What is the clinical usefulness of maternal serum interleukin-6 concentrations in diagnosing an asymptomatic intrauterine infection in patients in preterm labor?
Materials and Methods Maternal serum interleukin-6 concentrations were measured cross-sectionally in women during pregnancy, term labor, and preterm labor at the Duke University Medical Center from December 15, 1994 to December 15, 1995. The study was approved by the Duke University Medical Center Institutional Review Board, and subjects gave informed, written consent for blood sampling and inclusion in the study. Pregnant women were enrolled from the prenatal clinics at the time of routine blood testing (first obstetric visit, maternal serum alphafetoprotein measurements, maternal glucose screening, and third-trimester syphilis serology). Patients in term and preterm labor and patients admitted at 40 – 42 weeks’ gestation for induction of labor were also identified and enrolled in the study. Subjects were divided into five groups for analysis, with gestational age expressed as completed weeks: women at 22–34 weeks who were not in labor (n 5 57); women at term (after 36 weeks) who were not in labor (n 5 54); women at term (after 36 weeks) who were in labor and progressed to delivery, with rupture of the fetal membranes being preceded by active labor (n 5 148); women at 22–34 weeks who were evaluated for preterm labor and did not deliver during the index hospitalization and who also had regular uterine contractions and a cervix dilated at least 1 cm (n 5 31); and women at 22–34 weeks in preterm labor with intact membranes who failed tocolysis and delivered within 5 days of admission (n 5 25). We attempted to enroll into the study an unbiased cross-section of our obstetric population that is seen in our clinics and in labor and delivery. Patients not enrolled into the study either were not approached by the staff or refused to participate. Before the administration of any medications (ie, antibiotics or steroids), maternal venous blood was collected into a 10-mL sterile, silicone-coated test tube, refrigerated at 5C, and later centrifuged at 600 3 g for 10 minutes. The serum sample was then divided into aliquots that were stored at 270C for subsequent batch analysis. Interleukin-6 was measured in duplicate using an enzyme-linked immunosorbent assay kit (R & D Systems, Minneapolis,
466 Greig et al
Maternal Serum IL-6 in Preterm Labor
MN) by one of the authors (BRJ), who was blinded to each patient’s clinical history. This was the most sensitive commercially produced assay for interleukin-6 available at the time our study was initiated. It does not cross-react with any other known human cytokines and has an interassay and intra-assay coefficient of variation both less than 5%. The lowest standard was 2 pg/mL, with lesser values extrapolated from the standard curve. Preterm labor was defined as ten or more uterine contractions per hour with documented cervical effacement or dilation. Patients in preterm labor underwent uterine activity monitoring, a sterile speculum examination to identify rupture of the fetal membranes, and, if that examination was negative, digital evaluation of the cervix. Cultures were obtained for Neisseria gonorrhoeae, Chlamydia trachomatis, and group B streptococcus. Patients thought to be in preterm labor were treated initially with intravenous hydration and with 0.25 mg terbutaline subcutaneously. If contractions persisted and cervical change was noted, patients were treated with magnesium sulfate administered as a 4 – 6 g bolus intravenously, followed by a maintenance rate of 2–3 g/hour. This dosage was adjusted to inhibit contractions and to maintain maternal serum concentration between 4 and 8 mg/dL. Oral indomethacin or ibuprofen was used occasionally in conjunction with magnesium sulfate at the discretion of the attending staff perinatologist. Ampicillin was given intravenously to all preterm labor patients at a rate of 1 g every 6 hours until the results of the group B streptococcus culture were available. Placentas from 20 of the 25 (80%) patients who delivered preterm were examined for the presence of histologic chorioamnionitis by Duke University Medical Center staff pathologists using the criteria of Blanc.19 Subjects were excluded from analysis if they had evidence of the following: premature rupture of the fetal membranes, human immunodeficiency virus infection, or clinical signs of intrauterine infection (including uterine tenderness, fetal tachycardia, or maternal temperature grater than 37.8C). Patients with clinical signs of intrauterine infection were excluded because the purpose of the study was to evaluate maternal serum interleukin-6 concentrations in gravidas with asymptomatic intrauterine infection. Because the interleukin-6 concentrations were not distributed normally within each group, the Mann-Whitney U test was used for analysis. P # .006 was considered statistically significant to correct for the nine analyses performed. Receiver operating characteristic (ROC) curve analysis was used to determine the optimal maternal serum interleukin-6 concentration for the prediction of preterm delivery and histologic chorioamnionitis.
Obstetrics & Gynecology
Figure 1. Distribution of maternal serum interleukin-6 concentrations among patients in the five study groups. The median values for each group are represented by the short horizontal lines. PTL 5 preterm labor.
Results The distribution of maternal serum interleukin-6 concentrations from the 315 women who met the entry criteria and had data available for analysis can be seen in Figure 1. The racial distribution of the study population was 62% black, 37% white, and 1% Hispanic. Patients in labor at term had significantly elevated median (range) interleukin-6 concentrations compared with those in the nonlaboring patients at term (4.7 [1.0 –79.0] pg/mL versus 2.2 [0.1–37.8] pg/mL, P , .001). When the patients at term in the active phase of labor (n 5 60, cervical dilation equal to or greater than 4 cm) were compared with those in the latent phase of labor (n 5 88, cervical dilation less than 4 cm), there was no significant difference between the median (range) maternal serum interleukin-6 concentrations (4.8 [1.2– 79.0] pg/mL versus 4.7 [1.0 –19.7] pg/mL, P 5 0.7). Median (range) interleukin-6 concentrations in patients in preterm labor without delivery were not significantly different from interleukin-6 concentrations found in preterm patients who were without labor (1.9 [0 –7.7] pg/mL versus 1.6 [0.1– 6.9] pg/mL, P 5 0.8). Median (range) maternal serum interleukin-6 concentrations from patients in preterm labor who delivered during the index admission were significantly higher than those found in each of the other four groups of patients (9.3 [2.1–102] pg/mL, P , .001). When patients in the preterm delivery group were analyzed by the presence of histologic chorioamnionitis at delivery, those with histologic signs of infection had significantly higher median concentrations of interleukin-6 than did
VOL. 90, NO. 3, SEPTEMBER 1997
those who delivered prematurely without signs of infection (15.9 pg/mL versus 4.6 pg/mL, P 5 .006) (Figure 2). This group of patients who delivered pre-
Figure 2. Distribution of maternal serum interleukin-6 values among patients in preterm labor who delivered preterm with and without the presence of histologic chorioamnionitis. The short horizontal lines represent median values for each group, and the continuous line at 6 pg/mL represents the interleukin-6 cutoff value used in the analysis. 1Histochorio 5 histologic chorioamnionitis present; 2Histochorio 5 histologic chorioamnionitis absent.
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Table 1. Maternal Serum Interleukin-6 Concentration Exceeding 6 pg/mL to Identify Tocolytic Failure and Preterm Delivery
Sensitivity Specificity Positive predictive value Negative predictive value
Failed tocolysis (all patients)
Failed tocolysis with the presence of histologic chorioamnionitis
80% (20/25) 94% (29/31) 95% (20/21) 85% (29/34)
100% (14/14) 67% (4/6) 88% (14/16) 100% (4/4)
maturely without signs of histologic chorioamnionitis had median serum interleukin-6 concentrations very similar to those in labor at term (4.6 pg/mL versus 4.7 pg/mL, P 5 0.9). Using ROC curve analysis, we found that maternal serum concentrations exceeding 6 pg/mL gave the optimal sensitivity and specificity for the identification of both preterm delivery and histologic chorioamnionitis (Table 1). The positive predictive value of an elevated interleukin-6 concentration greater than 6 pg/mL for the identification of histologic chorioamnionitis was 88%; there were two cases of elevated maternal serum interleukin-6 concentration in the absence of histologic chorioamnionitis. One of these cases (interleukin-6 concentration 20.7 pg/mL) was a patient who was later found to have had a urinary tract infection at the time of admission; the other (interleukin-6 concentration 6.2 pg/mL) had a large placental abruption noted at the time of delivery and on pathologic examination of the placenta.
Discussion We found maternal serum interleukin-6 concentrations to be elevated in patients in labor at term, suggesting a possible role for maternal circulating interleukin-6 in both the initiation and the maintenance of normal term labor. However, surprisingly, there was not a difference between these concentrations during the latent compared to the active phase of labor. Patients in preterm labor who failed tocolysis but had no sign of histologic chorioamnionitis also had elevated maternal serum interleukin-6 concentrations, similar to those found in patients in term labor. These findings suggest a possible common, immunologically mediated mechanism involved in both term and preterm labor, a mechanism not associated with intrauterine infection. Elevations of the amniotic fluid interleukin-6 concentration also have been found during normal term labor.4 Interleukin-6 and the immune system appear to be involved not only under conditions of intrauterine infection but also dur-
468 Greig et al
Maternal Serum IL-6 in Preterm Labor
ing normal labor at term. A longitudinal study design would have been better to identify changes in interleukin-6 concentrations throughout pregnancy and with labor. But, this cross-sectional study was still very useful in identifying some interesting differences in interleukin-6 concentrations between the groups studied. Future studies using a longitudinal study design would be useful to clarify further the role of maternal serum interleukin-6 in parturition. Maternal serum interleukin-6 concentrations were significantly elevated in patients in preterm labor who failed tocolysis and were even higher in the patients with asymptomatic histologic chorioamnionitis. Very elevated maternal serum interleukin-6 concentrations associated with asymptomatic histologic chorioamnionitis also correlate with previously documented elevations found in amniotic fluid interleukin-6 concentrations.3,4,6 Immune system interleukin-6 activation in preterm labor that is associated with intrauterine infection appears to be different from that in preterm labor without infection. These data suggest a highly activated interleukin-6 immune response during subclinical intrauterine infection as well as a significant but lesser activation in non–infection-related preterm parturition. The ability to identify those patients in preterm labor with an asymptomatic intrauterine infection and those who are at the highest risk for preterm delivery could be an extremely important research and clinical tool. Excluding noninfected patients and those at low risk of preterm delivery will improve the study design of clinical trials of preterm labor therapies. Based on MEDLINE search of studies published in the last 10 years, as well as a thorough review of the references cited in these identified published studies, our study is the first to evaluate the association between elevated maternal serum interleukin-6 concentrations in patients in preterm labor and an asymptomatic intrauterine infection that was defined by the presence of histologic chorioamnionitis. Our findings indicate that elevated maternal serum IL-6 concentrations may be an excellent predictor of preterm labor and delivery in patients with and without subclinical intrauterine infection.
References 1. Romero R, Sirtori M, Oyarzun E. Infection and labor: V. Prevalence, microbiology, and clinical significance of intraamniotic infection in women with preterm labor and intact membranes. Am J Obstet Gynecol 1989;161:817–24. 2. Romero R, Salafia CM, Athanassiadis AP, Hanaoka S. The relationship between acute inflammatory lesions of the preterm placenta and amniotic fluid microbiology. Am J Obstet Gynecol 1992;166:1382– 8. 3. Hillier SL, Witkin SS, Krohn MA, Watts DH, Kiviat NB, Eschenbach DA. The relationship of amniotic fluid cytokines and preterm
Obstetrics & Gynecology
4.
5.
6.
7.
8.
9. 10.
11.
12.
13. 14.
15.
delivery, amniotic fluid infection, histologic chorioamnionitis, and chorioamnion infection. Am J Obstet Gynecol 1993;81:941– 8. Greig PC, Ernest JM, Teot L, Erikson M, Talley R. Amniotic fluid interleukin-6 concentrations correlate with histologic chorioamnionitis and amniotic fluid cultures in patients in premature labor with intact membranes. Am J Obstet Gynecol 1993;169:1035– 44. Cherouny PH, Pankuch GA, Botti JJ, Appelbaum PC. The presence of amniotic fluid leukoattractants accurately identifies histologic chorioamnionitis and predicts tocolytic efficacy in patients with idiopathic preterm labor. Am J Obstet Gynecol 1992;167:683– 8. Yoon BH, Romero R, Kim CJ, Jun JK, Gomez R, Choi JH, et al. Amniotic fluid interleukin-6: A sensitive test for antenatal diagnosis of acute inflammatory lesions for preterm placenta and prediction of perinatal morbidity. Am J Obstet Gynecol 1995;172:960 –70. van der Elst C, Lopez Bernal A, Sinclair-Smith C. The role of chorioamnionitis and prostaglandins in preterm labor. Obstet Gynecol 1991;77:672– 6. Romero R, Emamian M, Wan M. Prostaglandin concentrations in amniotic fluid of women with intra-amniotic infection and preterm labor. Am J Obstet Gynecol 1987;157:1461–7. Guzick DS, Winn K. The association of chorioamnionitis with preterm delivery. Obstet Gynecol 1985;65:11– 6. Romero R, Yoon BH, Kenney JS, Gomez R, Allison AC, Sehgal PB. Amniotic fluid interleukin-6 determinations are of diagnostic and prognostic value in preterm labor. Am J Reprod Immunol 1993;30: 167– 83. Dudley DJ, Hunter C, Mitchell MD, Varner MW. Clinical value of amniotic fluid interleukin-6 determinations in the management of preterm labour. Br J Obstet Gynaecol 1994;101:592–7. Mitchell MD, Dudley DJ, Edwin SS, Schiller SL. Interleukin-6 stimulates prostaglandin production by human amnion and decidual cells. Eur J Pharmacol 1991;192:189 –91. Mitchell MD, Trautman MS, Dudley DJ. Cytokine networking in the placenta. Placenta 1993;14:249 –75. Dudley DJ, Trautman MS, Araneo BA, Edwin SS, Mitchell MD. Decidual cell biosynthesis of interleukin-6: Regulation by inflammatory cytokines. J Clin Endocrinol Metab 1992;74:884 –9. Andrews WW, Hauth JC, Goldenberg RL, Gomez R, Romero R, Cassell GH. Amniotic fluid interlukin-6: Correlation with upper
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16.
17.
18.
19.
genital tract microbial colonization and gestational age in women delivered after spontaneous labor versus indicated delivery. Am J Obstet Gynecol 1995;173:606 –12. Coultrip LL, Lien JM, Gomez R, Kapernick P, Khoury A, Grossman JH. The value of amniotic fluid interleukin-6 determination in patients with preterm labor and intact membranes in the detection of microbial invasion of the amniotic cavity. Am J Obstet Gynecol 1994;171:901–11. Romero R, Yoon BH, Mazor M, Gomez R, Diamond MP. The diagnostic and prognostic value of amniotic fluid white blood cell count, glucose, interleukin-6, and Gram stain in patients with preterm labor and intact membranes. Am J Obstet Gynecol 1993; 169:805–16. Laham N, Rice GE, Bishop GJ, Hansen MB, Bendtzen K, Brennecke SP. Elevated plasma interleukin 6: A biochemical marker of human preterm labour. Gynecol Obstet Invest 1993;36:145–7. Blanc WA. Pathology of the placenta, membranes, and umbilical cord in bacterial, fungal, and viral infections in man. In: Naeye RL, Kissane JM, Kaufman N, eds. Perinatal diseases. International Academy of Pathology Monograph. Baltimore: Williams & Wilkins, 1981.
Address reprint requests to:
Phillip C. Greig, MD 890 West Faris Road Suite 450, Box 3 Greenville, SC 29605-4253 E-mail: [email protected]
Received December 12, 1996. Received in revised form May 1, 1997. Accepted May 9, 1997. Copyright © 1997 by The American College of Obstetricians and Gynecologists. Published by Elsevier Science Inc.
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469
Recent data regarding preterm labor indicate a strong association with intrauterine infection.1–3 Such infection, identified by the presence of histologic chorioamnionitis, has been found in 58 – 88% of patients in preterm labor who fail tocolysis and deliver prematureFrom the Department of Obstetrics and Gynecology, Duke University Medical Center, Durham, North Carolina, and R & D Systems, Minneapolis, Minnesota.
VOL. 90, NO. 3, SEPTEMBER 1997
ly.4 – 6 In vitro studies7,8 suggest that bacterial invasion of the uterus and the subsequent inflammatory response stimulate the production of prostaglandins and other substances that may promote uterine contractions and cervical ripening. Because the majority of these intrauterine infections are asymptomatic, their diagnosis is hampered by the absence of an accurate test for use in patients in preterm labor. Recently, considerable attention has been focused on a specific inflammatory mediator, interleukin-6.3,4,6,9 –12 Produced by both immune and nonimmune cells, including those found in the decidua, amnion, chorion, and placenta,13,14 this cytokine has been shown to propagate the local inflammatory cascade and to stimulate increased prostaglandin production.12 Elevated amniotic fluid interleukin-6 concentrations have been found in gravidas whose preterm labor was associated with an intrauterine infection.3,4,6,10 An elevated amniotic fluid interleukin-6 concentration appears to be the most sensitive marker for both early intrauterine infections limited to the fetal membranes and symptomatic intra-amniotic infection.15–17 Clinically, however, a reluctance to perform amniocentesis in patients in preterm labor limits the usefulness of this test for the identification of intrauterine infection. A simpler and less invasive method of measuring interleukin-6 would be useful for the identification of asymptomatic intrauterine infections in patients in preterm labor. Laham et al18 found elevations in maternal serum interleukin-6 concentration in patients in preterm labor who had symptomatic intrauterine infection, but none in preterm labor whose infection was asymptomatic were included. Interleukin-6 is a good choice for this measurement in the systemic circulation because of its known endocrine function as a mediator of the acute-phase response in the liver. Our study was designed to answer the following questions: 1) What are
0029-7844/97/$17.00 PII S0029-7844(97)00294-9
465
the normal concentrations of maternal serum interleukin-6 during the second and third trimesters of pregnancy and during the different stages of term labor? 2) Are maternal serum interleukin-6 concentrations elevated in patients in preterm labor who have asymptomatic infection? 3) What is the clinical usefulness of maternal serum interleukin-6 concentrations in diagnosing an asymptomatic intrauterine infection in patients in preterm labor?
Materials and Methods Maternal serum interleukin-6 concentrations were measured cross-sectionally in women during pregnancy, term labor, and preterm labor at the Duke University Medical Center from December 15, 1994 to December 15, 1995. The study was approved by the Duke University Medical Center Institutional Review Board, and subjects gave informed, written consent for blood sampling and inclusion in the study. Pregnant women were enrolled from the prenatal clinics at the time of routine blood testing (first obstetric visit, maternal serum alphafetoprotein measurements, maternal glucose screening, and third-trimester syphilis serology). Patients in term and preterm labor and patients admitted at 40 – 42 weeks’ gestation for induction of labor were also identified and enrolled in the study. Subjects were divided into five groups for analysis, with gestational age expressed as completed weeks: women at 22–34 weeks who were not in labor (n 5 57); women at term (after 36 weeks) who were not in labor (n 5 54); women at term (after 36 weeks) who were in labor and progressed to delivery, with rupture of the fetal membranes being preceded by active labor (n 5 148); women at 22–34 weeks who were evaluated for preterm labor and did not deliver during the index hospitalization and who also had regular uterine contractions and a cervix dilated at least 1 cm (n 5 31); and women at 22–34 weeks in preterm labor with intact membranes who failed tocolysis and delivered within 5 days of admission (n 5 25). We attempted to enroll into the study an unbiased cross-section of our obstetric population that is seen in our clinics and in labor and delivery. Patients not enrolled into the study either were not approached by the staff or refused to participate. Before the administration of any medications (ie, antibiotics or steroids), maternal venous blood was collected into a 10-mL sterile, silicone-coated test tube, refrigerated at 5C, and later centrifuged at 600 3 g for 10 minutes. The serum sample was then divided into aliquots that were stored at 270C for subsequent batch analysis. Interleukin-6 was measured in duplicate using an enzyme-linked immunosorbent assay kit (R & D Systems, Minneapolis,
466 Greig et al
Maternal Serum IL-6 in Preterm Labor
MN) by one of the authors (BRJ), who was blinded to each patient’s clinical history. This was the most sensitive commercially produced assay for interleukin-6 available at the time our study was initiated. It does not cross-react with any other known human cytokines and has an interassay and intra-assay coefficient of variation both less than 5%. The lowest standard was 2 pg/mL, with lesser values extrapolated from the standard curve. Preterm labor was defined as ten or more uterine contractions per hour with documented cervical effacement or dilation. Patients in preterm labor underwent uterine activity monitoring, a sterile speculum examination to identify rupture of the fetal membranes, and, if that examination was negative, digital evaluation of the cervix. Cultures were obtained for Neisseria gonorrhoeae, Chlamydia trachomatis, and group B streptococcus. Patients thought to be in preterm labor were treated initially with intravenous hydration and with 0.25 mg terbutaline subcutaneously. If contractions persisted and cervical change was noted, patients were treated with magnesium sulfate administered as a 4 – 6 g bolus intravenously, followed by a maintenance rate of 2–3 g/hour. This dosage was adjusted to inhibit contractions and to maintain maternal serum concentration between 4 and 8 mg/dL. Oral indomethacin or ibuprofen was used occasionally in conjunction with magnesium sulfate at the discretion of the attending staff perinatologist. Ampicillin was given intravenously to all preterm labor patients at a rate of 1 g every 6 hours until the results of the group B streptococcus culture were available. Placentas from 20 of the 25 (80%) patients who delivered preterm were examined for the presence of histologic chorioamnionitis by Duke University Medical Center staff pathologists using the criteria of Blanc.19 Subjects were excluded from analysis if they had evidence of the following: premature rupture of the fetal membranes, human immunodeficiency virus infection, or clinical signs of intrauterine infection (including uterine tenderness, fetal tachycardia, or maternal temperature grater than 37.8C). Patients with clinical signs of intrauterine infection were excluded because the purpose of the study was to evaluate maternal serum interleukin-6 concentrations in gravidas with asymptomatic intrauterine infection. Because the interleukin-6 concentrations were not distributed normally within each group, the Mann-Whitney U test was used for analysis. P # .006 was considered statistically significant to correct for the nine analyses performed. Receiver operating characteristic (ROC) curve analysis was used to determine the optimal maternal serum interleukin-6 concentration for the prediction of preterm delivery and histologic chorioamnionitis.
Obstetrics & Gynecology
Figure 1. Distribution of maternal serum interleukin-6 concentrations among patients in the five study groups. The median values for each group are represented by the short horizontal lines. PTL 5 preterm labor.
Results The distribution of maternal serum interleukin-6 concentrations from the 315 women who met the entry criteria and had data available for analysis can be seen in Figure 1. The racial distribution of the study population was 62% black, 37% white, and 1% Hispanic. Patients in labor at term had significantly elevated median (range) interleukin-6 concentrations compared with those in the nonlaboring patients at term (4.7 [1.0 –79.0] pg/mL versus 2.2 [0.1–37.8] pg/mL, P , .001). When the patients at term in the active phase of labor (n 5 60, cervical dilation equal to or greater than 4 cm) were compared with those in the latent phase of labor (n 5 88, cervical dilation less than 4 cm), there was no significant difference between the median (range) maternal serum interleukin-6 concentrations (4.8 [1.2– 79.0] pg/mL versus 4.7 [1.0 –19.7] pg/mL, P 5 0.7). Median (range) interleukin-6 concentrations in patients in preterm labor without delivery were not significantly different from interleukin-6 concentrations found in preterm patients who were without labor (1.9 [0 –7.7] pg/mL versus 1.6 [0.1– 6.9] pg/mL, P 5 0.8). Median (range) maternal serum interleukin-6 concentrations from patients in preterm labor who delivered during the index admission were significantly higher than those found in each of the other four groups of patients (9.3 [2.1–102] pg/mL, P , .001). When patients in the preterm delivery group were analyzed by the presence of histologic chorioamnionitis at delivery, those with histologic signs of infection had significantly higher median concentrations of interleukin-6 than did
VOL. 90, NO. 3, SEPTEMBER 1997
those who delivered prematurely without signs of infection (15.9 pg/mL versus 4.6 pg/mL, P 5 .006) (Figure 2). This group of patients who delivered pre-
Figure 2. Distribution of maternal serum interleukin-6 values among patients in preterm labor who delivered preterm with and without the presence of histologic chorioamnionitis. The short horizontal lines represent median values for each group, and the continuous line at 6 pg/mL represents the interleukin-6 cutoff value used in the analysis. 1Histochorio 5 histologic chorioamnionitis present; 2Histochorio 5 histologic chorioamnionitis absent.
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Table 1. Maternal Serum Interleukin-6 Concentration Exceeding 6 pg/mL to Identify Tocolytic Failure and Preterm Delivery
Sensitivity Specificity Positive predictive value Negative predictive value
Failed tocolysis (all patients)
Failed tocolysis with the presence of histologic chorioamnionitis
80% (20/25) 94% (29/31) 95% (20/21) 85% (29/34)
100% (14/14) 67% (4/6) 88% (14/16) 100% (4/4)
maturely without signs of histologic chorioamnionitis had median serum interleukin-6 concentrations very similar to those in labor at term (4.6 pg/mL versus 4.7 pg/mL, P 5 0.9). Using ROC curve analysis, we found that maternal serum concentrations exceeding 6 pg/mL gave the optimal sensitivity and specificity for the identification of both preterm delivery and histologic chorioamnionitis (Table 1). The positive predictive value of an elevated interleukin-6 concentration greater than 6 pg/mL for the identification of histologic chorioamnionitis was 88%; there were two cases of elevated maternal serum interleukin-6 concentration in the absence of histologic chorioamnionitis. One of these cases (interleukin-6 concentration 20.7 pg/mL) was a patient who was later found to have had a urinary tract infection at the time of admission; the other (interleukin-6 concentration 6.2 pg/mL) had a large placental abruption noted at the time of delivery and on pathologic examination of the placenta.
Discussion We found maternal serum interleukin-6 concentrations to be elevated in patients in labor at term, suggesting a possible role for maternal circulating interleukin-6 in both the initiation and the maintenance of normal term labor. However, surprisingly, there was not a difference between these concentrations during the latent compared to the active phase of labor. Patients in preterm labor who failed tocolysis but had no sign of histologic chorioamnionitis also had elevated maternal serum interleukin-6 concentrations, similar to those found in patients in term labor. These findings suggest a possible common, immunologically mediated mechanism involved in both term and preterm labor, a mechanism not associated with intrauterine infection. Elevations of the amniotic fluid interleukin-6 concentration also have been found during normal term labor.4 Interleukin-6 and the immune system appear to be involved not only under conditions of intrauterine infection but also dur-
468 Greig et al
Maternal Serum IL-6 in Preterm Labor
ing normal labor at term. A longitudinal study design would have been better to identify changes in interleukin-6 concentrations throughout pregnancy and with labor. But, this cross-sectional study was still very useful in identifying some interesting differences in interleukin-6 concentrations between the groups studied. Future studies using a longitudinal study design would be useful to clarify further the role of maternal serum interleukin-6 in parturition. Maternal serum interleukin-6 concentrations were significantly elevated in patients in preterm labor who failed tocolysis and were even higher in the patients with asymptomatic histologic chorioamnionitis. Very elevated maternal serum interleukin-6 concentrations associated with asymptomatic histologic chorioamnionitis also correlate with previously documented elevations found in amniotic fluid interleukin-6 concentrations.3,4,6 Immune system interleukin-6 activation in preterm labor that is associated with intrauterine infection appears to be different from that in preterm labor without infection. These data suggest a highly activated interleukin-6 immune response during subclinical intrauterine infection as well as a significant but lesser activation in non–infection-related preterm parturition. The ability to identify those patients in preterm labor with an asymptomatic intrauterine infection and those who are at the highest risk for preterm delivery could be an extremely important research and clinical tool. Excluding noninfected patients and those at low risk of preterm delivery will improve the study design of clinical trials of preterm labor therapies. Based on MEDLINE search of studies published in the last 10 years, as well as a thorough review of the references cited in these identified published studies, our study is the first to evaluate the association between elevated maternal serum interleukin-6 concentrations in patients in preterm labor and an asymptomatic intrauterine infection that was defined by the presence of histologic chorioamnionitis. Our findings indicate that elevated maternal serum IL-6 concentrations may be an excellent predictor of preterm labor and delivery in patients with and without subclinical intrauterine infection.
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Address reprint requests to:
Phillip C. Greig, MD 890 West Faris Road Suite 450, Box 3 Greenville, SC 29605-4253 E-mail: [email protected]
Received December 12, 1996. Received in revised form May 1, 1997. Accepted May 9, 1997. Copyright © 1997 by The American College of Obstetricians and Gynecologists. Published by Elsevier Science Inc.
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