- Email: [email protected]
The tap test: Confirmation of a simple, rapid, inexpensive, and reliable indicator of fetal pulmonary maturity
Wen et al.
ing, physical stature, and educational attainment on the incidence of low birth weight. Am J Epidemiol 1985; 121 :843-55. 16. Edwards LA, Alton IR, Barrada MI, et aI. Pregnancy in the underweight woman. Am J OBSTET GYNECOL 1979; 135:297-302. 17. Simpson JW, Lawless RW, Mitchell AC. Responsibility of the obstetrician to the fetus . II. Influence of prepregnancy weight and pregnancy weight gain on birth weight. Obstet GynecoI1975;45 :481-7 .
January 1990 Am J Obstet Gynecol
18. Miller HC, Hassanein K, Hensleigh P. Maternal factors in the incidences of low birth weight infants among black and white mothers. Pediatr Res 1978;12:1016-9. 19. Miller HC. A model for studying the pathogenesis and incidence of low-birth-weight infants. Am J Dis Child 1983;137:323-7. 20. Miller HC. Intrauterine growth retardation. Am J Dis Child 1981;135:944-8.
The tap test: Confirmation of a simple, rapid, inexpensive, and reliable indicator of fetal pulmonary maturity Michael L. Socol, MD Chicago, Illinois The tap test was previously described, and preliminary experience suggested it might be a rapid, inexpensive, and reliable indicator of fetal lung maturity. In this expanded series of 332 patients delivered of infants within 72 hours of amniotic fluid analysis, the predictive values for mature test results at 2, 5, and 10 minutes were 98.9% (182 of 184), 97.4% (221 of 227), and 97.1% (233 of 240), respectively. Predictive values for immature test results were 41.2% (61 of 148), 54.3% (57 of 105), and 60.9% (56 of 92). For the phospholipid profile the predictive value was 96.9% (186 of 192) for a mature test result and 40.7% (57 of 140) for an immature result. These observations, coupled with its methodologic simplicity, make the tap test a good first step in a cascade scheme of tests for fetal lung maturity and a valuable test in a facility where the phospholipid profile is not available 24 hours a day. (AM J OBSTET GVNECOL 1990;162:218-22.)
Key words: Tap test, fetal pulmonary maturity
The phospholipid profile is the most accepted test of fetal lung maturity.' It has a high sensitivity and predictive value for assessing maturity; however, it has only modest specificity and a low predictive value for immaturity. Further, the phospholipid profile is a laborintensive and costly test. These limitations prompted the recommendation by Garite et al! of a cascade scheme, whereby the fastest and simplest test would be performed first. Only if this test suggested immaturity would additional tests be performed; multiple results
From the Section 011 Matemal-Fetal Medlcme, Department of Obstetnes and Gynecology, Northwestern University Medical School and Prentice Women's Hospttal and Materntty Center of Northwestern Memonal Hospital. R ecetved February 14,1989; revised June 19.1989; accepted July 11, 1989. R epnnt requests: Michael L. Socol, MD . 333 East Superior St., #410, Chzcago, 1L 60611. 611 /15222
218
Table I. Indications for amniotic fluid analysis in 332 patients Premature rupture of membranes Preterm labor Placenta previa Hypertension Diabetes Class A Class B-F Repeat cesarean section Isoimmunization Suspected intrauterine growth retardation Congenital anomaly Other
106 74 33 26
TOTAL
332
13 25 17 14 8 6 10
suggesting immaturity would increase the likelihood that the fetus was actually immature. Examples of such tests include the shake test,3 fluorescence polarization: and most recently amniotic fluid urea nitrogen. 5
Volume 162 Number I
Tap test
'--1 J.. .). ____
•
1 )-.
--
219
.
.
, \
A
8
Fig. L In mature amniotic fluid five bubbles or less remain in the ether (top) layer (Al. Bubbles in the amniotic fluid (boltom) laye r are ignored (Bl.
The tap test was previously described a~ a rapid and inexpensive measurement of fetal lung maturity." In that preliminary communica tion , it was found to be comparable to the phospholipid profile in predicting fetal pulmonary status. The purpose of this puplication is to relate an expanded experience with the tap test in 332 patients delivered of infants within 72 hours of amniotic fluid analysis.
Material and methods The tap test is performed by mixing approximately 1 ml of amniotic fluid with 1 drop of 6N hydrochloric acid (concentrated hydrochloric acid diluted 1: 1) and then adding approximately 1.5 ml of diethyl ether. The 16 x 150 mm test tube is briskly tapped three or four times, which creates an estimated 200 to 300 bubbles in the ether layer. In amniotic fluid from a mature fetus the bubbles quickly rise to the surface and break down; in amniotic fluid from an immature fetus the bubbles are stable or break down slowly. In this study the tap test was read at 2, 5, and 10 minutes. with the cutoff for maturity set at five bubbles . If no more than five bubbles persisted in the ether laye r, the results were considered mature (Figs. 1 and 2). The occasional bub-
Table II. Gestational age distribution of patients in the study Weeks of gestatIOn
Re;plratury dlltreH s,vndmme
26-27 28-30 3 1-33 34-36
6 30 21 5
No respiratory
distress I.vndrome
2
~37
I
5 65 106 91
TOTAL
63
269
Total 8 35 86 III
92 332
bles that were confined to the a mniotic fluid layer and not dispersed in the ether layer were ignored. Borderline tests were performed in duplicate, and if one reading was mature, it was considered indicative of fetal lung ma turity. All tap tests were performed by a single investigator (M. L. S.) who was unawa re of the patient's clinical course. The amniotic fluid was obtained by the resident house staff, either by amniocentesis or from a freely flowing vaginal pool. If the tap test or phospholipid
220
Socol
January 1990 Am J Obstet Gynecol
-. G
-..iJIIII , J
"
.'
.. 'J t' ...
i ~
•
•
..~
¢1';
•
....
j .<
''''
,
,
Fig. 2. In immature amniotic fluid the bubbles remain stable (A) or slowly break down (B). More than five bubbles persist in the ether layer.
Table III. Comparison of tap test results versus phospholipid profile Ammotlc fluid sample
True mature
False mature
True unmature
False Immature
Clear Tap test 2 min 5 min 10 min Phospholipid profile
135 159 162 139
2 4 4 3
42 40 40 41
47 23 20 43
Blood-tinged Tap test 2 min 5 min 10 min Phospholipid profile
20 24 26 19
0 I 2
10 9 8 9
8 4 2 9
Meconium-stained Tap test 2 min 5 min 10 min Phospholipid profile
12 13 14 12
0 0 0 0
Vaginal pool Tap test 2 min 5 min 10 min Phospholipid profile
15 25 31 16
0 I I 2
8 7 7 6
27 17
Total population Tap test 2 min 5 min 10 min Phospholipid profile
182 221 233 186
2 6 7 6
61 57 56 57
87 48 36 83
I
5 4 3 5
II
26
Tap test
Volume 162 Number I
221
Table IV. Sensitivity, specificity, and predictive values for the tap test and phospholipid profile Tap test Validlly mdlcatoTs Clear amniotic fluid samples Sensitivity Specificity Predictive value Mature Immature All samples Sensitivity Specificity Predictive value Mature Immature
2 min
I
5 mm
I
10 min
P hospholzpid profile
95.5 (42 / 44) 74.2 (1351182)
90.9 (40 / 44) 87.4 (159 / 182)
90.9 (40/44) 89.0 (1621182)
93 .2 (41144) 76.4 (1391182)
98.5 (135/137) 47.2 (42 / 89)
97.5 (1591163) 63.5 (40/63)
97.6 (1621166) 66.7 (40 / 60)
97.9 (139/142) 48.8 (41184)
96.8 (61163) 67.7 (182/269)
90.5 (57/63) 82.2 (221 /269)
88.9 (56 / 63) 86.6 (233/269)
90.5 (57/63) 69.1 (186/269)
98.9 (1821184) 41.2 (61/148)
97 .4 (221/227) 54.3 (5711 05)
97.1 (233/240) 60.9 (56/92)
96.9 (186/192) 40.7 (571140)
profile could not be performed immediately, the amniotic fluid was refrigerated, but all amniotic fluid was analyzed within 24 hours. Before administration of the tap test, amniotic fluid that had been contaminated by blood or meconium was spun for 5 minutes in a tabletop centrifuge at 400 g. Vaginal pool amniotic fluid was similarly treated to remove any contaminating mucus. Clear amniotic fluid obtained by amniocentesis was not centrifuged unless there were large amounts of vernix. The phospholipid profile was performed in the hospital chemistry laboratory by one-dimensional planimetric thin-layer chromatography after centrifugation for 5 minutes at 800 g and acetone precipitation.' Phosphatidylglycerol was qualitatively determined as negative, trace, or positive by comparison to a reference standard. A lecithin/sphingomyelin ratio of ;:::2: 1 or the presence of phosphatidylglycerol (trace or positive) was considered indicative of maturity. Clinical decisions were made by the resident house staff on the basis of the phospholipid profile. The sensitivity, specificity, and predictive values for the tap test and the phospholipid profile were determined by comparing the results of these tests with the actual incidence of neonatal respiratory distress syndrome. The diagnosis of respiratory distress syndrome was made by the attending neonatologist and was based on standard clinical, blood gas, and radiologic findings. 8 These included chest retraction, grunting, cyanosis, a need for oxygen supplementation for a mimimum of 24 hours, and a ground-glass appearance with air bronchograms on chest x-ray film. Gestational age was determined by the best fit of menstrual dates, ultrasonography (if available), and the Dubowitz examination. Results
The indications prompting amniotic fluid analysis are listed in Table I. The gestational age distribution of the study population is presented in Table II. Seventy-two percent (240 of 332) of the fetuses were delivered before term.
The results of the tap test and the phospholipid profile for the subsets (clear, blood-tinged, meconiumstained, and vaginal pool amniotic fluid) and the total population are listed in Table III. From these data the sensitivity, specificity, and predictive values can be calculated (Table IV). The predictive values for a mature tap test from clear amniotic fluid obtained by amniocentesis were 98.5% (135 of 137) at 2 minutes, 97.5% (159 of 163) at 5 minutes, and 97.6% (162 of 166) at 10 minutes; the predictive values for an immature test were 47 .2% (42 of 89),63 .5% (40 of 63), and 66.7% (40 of 60), respectively. For the phospholipid profile the predictive values were 97 .9% (139 of 142) for a mature test result and 48.8% (41 of84) for an immature test result. When the entire population was analyzed, the predictive values for a mature tap test result were 98.9% (182 of 184) at 2 minutes, 97.4% (221 of 227) at 5 minutes, and 97.1 % (233 of 240) at 10 minutes; the predictive values for an immature test result were 41.2 % (61 of 148) at 2 minutes, 54.3 % (57 of 105) at 5 minutes, and 60.9% (56 of 92) at 10 minutes. For the phospholipid profile the predictive values were 96.9 % (186 of 192) for the mature test result and 40.7% (57 of 140) for an immature test result. Comment
These data support the previously published preliminary observation that the tap test can be used as a reliable indicator of fetal pulmonary maturity.6 Predictive values for mature tap test results at 2, 5, and 10 minutes were comparable to those of the phospholipid profile. The predictive value for an immature tap test result at 2 minutes was also comparable to that of the phospholipid profile, whereas an immature tap test result at 5 or 10 minutes appeared to be more accurate than the phospholipid profile in predicting fetal lung immaturity. A potential disadvantage of the tap test is that it is semiquantitative, with an arbitrary cutoff for maturity. Further, the time periods at which the tap test were
222
Socol
read were chosen somewhat capriciously. Nonetheless, in this prospective study the tap test has been shown to be comparable in reliability to the phospholipid profile in a diverse patient population. It is rapid, inexpensive, and easy to perform and thus a good choice for a cascade scheme of fetal lung assessment or for use in a facility where the phospholipid profile is not available 24 hours a day. REFERENCES I . Kulovich MV, Hallman MB, Gluck L. The lung profile. 1. Normal pregnancy. AM J OBSTLT GYl'ECOL 1979: 135:5763. 2. Garite TJ, Freeman RK, Nageotte MP. Fetal maturity cascade: a rapid and cost-effective method for fetal lung maturity testing. Obstet Gynecol 1986;67:619-22. 3. Clements .fA. Platzker ACG, Tierney DF. et al. Assessment
January 1990 Am J Obslet Gynecol
4.
5. 6. 7.
8.
of the risk of respiratory-distress syndrome by a rapid test for surfactant in the amniotic fluid. N Engl J Med 1972;286: 1077-81. Shinitzky M, Goldfisher A, Bruck A, et al. A new method for assessment of fetal lung maturity. Br J Obstet Gynaecol 1976;83:838-44. Almeida OD, Kitay DZ. Amniotic fluid urea nitrogen in the prediction of respiratory distress syndrome. AM J OBSTET GVNECOL 1988;159:465-8. Socol ML, Sing E, Depp OR. The tap test: a rapid indicator of fetal pulmonary maturity. AM J OBSTET GVI'
ing, physical stature, and educational attainment on the incidence of low birth weight. Am J Epidemiol 1985; 121 :843-55. 16. Edwards LA, Alton IR, Barrada MI, et aI. Pregnancy in the underweight woman. Am J OBSTET GYNECOL 1979; 135:297-302. 17. Simpson JW, Lawless RW, Mitchell AC. Responsibility of the obstetrician to the fetus . II. Influence of prepregnancy weight and pregnancy weight gain on birth weight. Obstet GynecoI1975;45 :481-7 .
January 1990 Am J Obstet Gynecol
18. Miller HC, Hassanein K, Hensleigh P. Maternal factors in the incidences of low birth weight infants among black and white mothers. Pediatr Res 1978;12:1016-9. 19. Miller HC. A model for studying the pathogenesis and incidence of low-birth-weight infants. Am J Dis Child 1983;137:323-7. 20. Miller HC. Intrauterine growth retardation. Am J Dis Child 1981;135:944-8.
The tap test: Confirmation of a simple, rapid, inexpensive, and reliable indicator of fetal pulmonary maturity Michael L. Socol, MD Chicago, Illinois The tap test was previously described, and preliminary experience suggested it might be a rapid, inexpensive, and reliable indicator of fetal lung maturity. In this expanded series of 332 patients delivered of infants within 72 hours of amniotic fluid analysis, the predictive values for mature test results at 2, 5, and 10 minutes were 98.9% (182 of 184), 97.4% (221 of 227), and 97.1% (233 of 240), respectively. Predictive values for immature test results were 41.2% (61 of 148), 54.3% (57 of 105), and 60.9% (56 of 92). For the phospholipid profile the predictive value was 96.9% (186 of 192) for a mature test result and 40.7% (57 of 140) for an immature result. These observations, coupled with its methodologic simplicity, make the tap test a good first step in a cascade scheme of tests for fetal lung maturity and a valuable test in a facility where the phospholipid profile is not available 24 hours a day. (AM J OBSTET GVNECOL 1990;162:218-22.)
Key words: Tap test, fetal pulmonary maturity
The phospholipid profile is the most accepted test of fetal lung maturity.' It has a high sensitivity and predictive value for assessing maturity; however, it has only modest specificity and a low predictive value for immaturity. Further, the phospholipid profile is a laborintensive and costly test. These limitations prompted the recommendation by Garite et al! of a cascade scheme, whereby the fastest and simplest test would be performed first. Only if this test suggested immaturity would additional tests be performed; multiple results
From the Section 011 Matemal-Fetal Medlcme, Department of Obstetnes and Gynecology, Northwestern University Medical School and Prentice Women's Hospttal and Materntty Center of Northwestern Memonal Hospital. R ecetved February 14,1989; revised June 19.1989; accepted July 11, 1989. R epnnt requests: Michael L. Socol, MD . 333 East Superior St., #410, Chzcago, 1L 60611. 611 /15222
218
Table I. Indications for amniotic fluid analysis in 332 patients Premature rupture of membranes Preterm labor Placenta previa Hypertension Diabetes Class A Class B-F Repeat cesarean section Isoimmunization Suspected intrauterine growth retardation Congenital anomaly Other
106 74 33 26
TOTAL
332
13 25 17 14 8 6 10
suggesting immaturity would increase the likelihood that the fetus was actually immature. Examples of such tests include the shake test,3 fluorescence polarization: and most recently amniotic fluid urea nitrogen. 5
Volume 162 Number I
Tap test
'--1 J.. .). ____
•
1 )-.
--
219
.
.
, \
A
8
Fig. L In mature amniotic fluid five bubbles or less remain in the ether (top) layer (Al. Bubbles in the amniotic fluid (boltom) laye r are ignored (Bl.
The tap test was previously described a~ a rapid and inexpensive measurement of fetal lung maturity." In that preliminary communica tion , it was found to be comparable to the phospholipid profile in predicting fetal pulmonary status. The purpose of this puplication is to relate an expanded experience with the tap test in 332 patients delivered of infants within 72 hours of amniotic fluid analysis.
Material and methods The tap test is performed by mixing approximately 1 ml of amniotic fluid with 1 drop of 6N hydrochloric acid (concentrated hydrochloric acid diluted 1: 1) and then adding approximately 1.5 ml of diethyl ether. The 16 x 150 mm test tube is briskly tapped three or four times, which creates an estimated 200 to 300 bubbles in the ether layer. In amniotic fluid from a mature fetus the bubbles quickly rise to the surface and break down; in amniotic fluid from an immature fetus the bubbles are stable or break down slowly. In this study the tap test was read at 2, 5, and 10 minutes. with the cutoff for maturity set at five bubbles . If no more than five bubbles persisted in the ether laye r, the results were considered mature (Figs. 1 and 2). The occasional bub-
Table II. Gestational age distribution of patients in the study Weeks of gestatIOn
Re;plratury dlltreH s,vndmme
26-27 28-30 3 1-33 34-36
6 30 21 5
No respiratory
distress I.vndrome
2
~37
I
5 65 106 91
TOTAL
63
269
Total 8 35 86 III
92 332
bles that were confined to the a mniotic fluid layer and not dispersed in the ether layer were ignored. Borderline tests were performed in duplicate, and if one reading was mature, it was considered indicative of fetal lung ma turity. All tap tests were performed by a single investigator (M. L. S.) who was unawa re of the patient's clinical course. The amniotic fluid was obtained by the resident house staff, either by amniocentesis or from a freely flowing vaginal pool. If the tap test or phospholipid
220
Socol
January 1990 Am J Obstet Gynecol
-. G
-..iJIIII , J
"
.'
.. 'J t' ...
i ~
•
•
..~
¢1';
•
....
j .<
''''
,
,
Fig. 2. In immature amniotic fluid the bubbles remain stable (A) or slowly break down (B). More than five bubbles persist in the ether layer.
Table III. Comparison of tap test results versus phospholipid profile Ammotlc fluid sample
True mature
False mature
True unmature
False Immature
Clear Tap test 2 min 5 min 10 min Phospholipid profile
135 159 162 139
2 4 4 3
42 40 40 41
47 23 20 43
Blood-tinged Tap test 2 min 5 min 10 min Phospholipid profile
20 24 26 19
0 I 2
10 9 8 9
8 4 2 9
Meconium-stained Tap test 2 min 5 min 10 min Phospholipid profile
12 13 14 12
0 0 0 0
Vaginal pool Tap test 2 min 5 min 10 min Phospholipid profile
15 25 31 16
0 I I 2
8 7 7 6
27 17
Total population Tap test 2 min 5 min 10 min Phospholipid profile
182 221 233 186
2 6 7 6
61 57 56 57
87 48 36 83
I
5 4 3 5
II
26
Tap test
Volume 162 Number I
221
Table IV. Sensitivity, specificity, and predictive values for the tap test and phospholipid profile Tap test Validlly mdlcatoTs Clear amniotic fluid samples Sensitivity Specificity Predictive value Mature Immature All samples Sensitivity Specificity Predictive value Mature Immature
2 min
I
5 mm
I
10 min
P hospholzpid profile
95.5 (42 / 44) 74.2 (1351182)
90.9 (40 / 44) 87.4 (159 / 182)
90.9 (40/44) 89.0 (1621182)
93 .2 (41144) 76.4 (1391182)
98.5 (135/137) 47.2 (42 / 89)
97.5 (1591163) 63.5 (40/63)
97.6 (1621166) 66.7 (40 / 60)
97.9 (139/142) 48.8 (41184)
96.8 (61163) 67.7 (182/269)
90.5 (57/63) 82.2 (221 /269)
88.9 (56 / 63) 86.6 (233/269)
90.5 (57/63) 69.1 (186/269)
98.9 (1821184) 41.2 (61/148)
97 .4 (221/227) 54.3 (5711 05)
97.1 (233/240) 60.9 (56/92)
96.9 (186/192) 40.7 (571140)
profile could not be performed immediately, the amniotic fluid was refrigerated, but all amniotic fluid was analyzed within 24 hours. Before administration of the tap test, amniotic fluid that had been contaminated by blood or meconium was spun for 5 minutes in a tabletop centrifuge at 400 g. Vaginal pool amniotic fluid was similarly treated to remove any contaminating mucus. Clear amniotic fluid obtained by amniocentesis was not centrifuged unless there were large amounts of vernix. The phospholipid profile was performed in the hospital chemistry laboratory by one-dimensional planimetric thin-layer chromatography after centrifugation for 5 minutes at 800 g and acetone precipitation.' Phosphatidylglycerol was qualitatively determined as negative, trace, or positive by comparison to a reference standard. A lecithin/sphingomyelin ratio of ;:::2: 1 or the presence of phosphatidylglycerol (trace or positive) was considered indicative of maturity. Clinical decisions were made by the resident house staff on the basis of the phospholipid profile. The sensitivity, specificity, and predictive values for the tap test and the phospholipid profile were determined by comparing the results of these tests with the actual incidence of neonatal respiratory distress syndrome. The diagnosis of respiratory distress syndrome was made by the attending neonatologist and was based on standard clinical, blood gas, and radiologic findings. 8 These included chest retraction, grunting, cyanosis, a need for oxygen supplementation for a mimimum of 24 hours, and a ground-glass appearance with air bronchograms on chest x-ray film. Gestational age was determined by the best fit of menstrual dates, ultrasonography (if available), and the Dubowitz examination. Results
The indications prompting amniotic fluid analysis are listed in Table I. The gestational age distribution of the study population is presented in Table II. Seventy-two percent (240 of 332) of the fetuses were delivered before term.
The results of the tap test and the phospholipid profile for the subsets (clear, blood-tinged, meconiumstained, and vaginal pool amniotic fluid) and the total population are listed in Table III. From these data the sensitivity, specificity, and predictive values can be calculated (Table IV). The predictive values for a mature tap test from clear amniotic fluid obtained by amniocentesis were 98.5% (135 of 137) at 2 minutes, 97.5% (159 of 163) at 5 minutes, and 97.6% (162 of 166) at 10 minutes; the predictive values for an immature test were 47 .2% (42 of 89),63 .5% (40 of 63), and 66.7% (40 of 60), respectively. For the phospholipid profile the predictive values were 97 .9% (139 of 142) for a mature test result and 48.8% (41 of84) for an immature test result. When the entire population was analyzed, the predictive values for a mature tap test result were 98.9% (182 of 184) at 2 minutes, 97.4% (221 of 227) at 5 minutes, and 97.1 % (233 of 240) at 10 minutes; the predictive values for an immature test result were 41.2 % (61 of 148) at 2 minutes, 54.3 % (57 of 105) at 5 minutes, and 60.9% (56 of 92) at 10 minutes. For the phospholipid profile the predictive values were 96.9 % (186 of 192) for the mature test result and 40.7% (57 of 140) for an immature test result. Comment
These data support the previously published preliminary observation that the tap test can be used as a reliable indicator of fetal pulmonary maturity.6 Predictive values for mature tap test results at 2, 5, and 10 minutes were comparable to those of the phospholipid profile. The predictive value for an immature tap test result at 2 minutes was also comparable to that of the phospholipid profile, whereas an immature tap test result at 5 or 10 minutes appeared to be more accurate than the phospholipid profile in predicting fetal lung immaturity. A potential disadvantage of the tap test is that it is semiquantitative, with an arbitrary cutoff for maturity. Further, the time periods at which the tap test were
222
Socol
read were chosen somewhat capriciously. Nonetheless, in this prospective study the tap test has been shown to be comparable in reliability to the phospholipid profile in a diverse patient population. It is rapid, inexpensive, and easy to perform and thus a good choice for a cascade scheme of fetal lung assessment or for use in a facility where the phospholipid profile is not available 24 hours a day. REFERENCES I . Kulovich MV, Hallman MB, Gluck L. The lung profile. 1. Normal pregnancy. AM J OBSTLT GYl'ECOL 1979: 135:5763. 2. Garite TJ, Freeman RK, Nageotte MP. Fetal maturity cascade: a rapid and cost-effective method for fetal lung maturity testing. Obstet Gynecol 1986;67:619-22. 3. Clements .fA. Platzker ACG, Tierney DF. et al. Assessment
January 1990 Am J Obslet Gynecol
4.
5. 6. 7.
8.
of the risk of respiratory-distress syndrome by a rapid test for surfactant in the amniotic fluid. N Engl J Med 1972;286: 1077-81. Shinitzky M, Goldfisher A, Bruck A, et al. A new method for assessment of fetal lung maturity. Br J Obstet Gynaecol 1976;83:838-44. Almeida OD, Kitay DZ. Amniotic fluid urea nitrogen in the prediction of respiratory distress syndrome. AM J OBSTET GVNECOL 1988;159:465-8. Socol ML, Sing E, Depp OR. The tap test: a rapid indicator of fetal pulmonary maturity. AM J OBSTET GVI'