The effect of low-dose heparin therapy on serum platelet and transaminase levels

JOURNAL

OF SURGICAL

RESEARCH

28, 297-305 (1980)

The Effect of Low-Dose Heparin Therapy on Serum and Transaminase Level&

Platelet

JEFFREY R. SAFFLE, M.D.,2 JOHN Russo, JR.,PHARM. D., GEORGE E. DUKES,JR.,PHARM. D., AND GLENN D. WARDEN, M.D. The Departments

of Surgery and Pharmacy Practice, University of Utah Colleges of Medicine 50 North Medical Drive, Salt Lake City, Utah 84132

and Pharmacy,

Presented at the Annual Meeting of the Association for Academic Surgery, Great Gorge, New Jersey, November 11- 14, 1979 Review of recent literature permits characterization of a thrombocytopenic syndrome associated with full- or low-dose heparin therapy, the incidence, severity, and etiology of which remain controversial. This study was undertaken to determine the occurrence of thrombocytopenia accompanying subcutaneous heparin administration to healthy volunteers. Thirty-nine subjects received a regimen of 5000 USP units of heparin injected every 12 hr for 10 days. Serum liver enzymes were also monitored in a subgroup of the population. Two available heparin preparations, hog mucosa and beef lung extracts, were compared for the occurrence of these associated abnormalities. There was no thrombocytopeniaobserved in any person. However, as a group there was a significant (P = 0.01) downward linear trend in the mean platelet count during heparin administration, with a marked increase after cessation of the drug. Significant elevations of transaminase (SGOT and SGPT) levels occurred in 62% of the subjects, also returning to normal after heparin was discontinued. The enzyme pattern was characteristic of hepatocellular damage. The incidence of this finding was higher in subjects receiving hog mucosal heparin (93%) than in those receiving bovine heparin (37%, P < 0.05). Though not exhibiting thrombocytopenia, the subjects did have a decrease in mean platelet count. The etiology of the hypertransaminasemia is unknown, though the temporal correlation with the decrease in platelets suggests that the two are related, possibly through an immunologic mechanism. The enzyme elevations could result in inaccurate diagnosis based on these enzymes or pose potential danger to patients with marginal liver function. Patients receiving heparin should have liver enzymes as well as platelet counts followed regularly.

INTRODUCTION

of thrombocytopenia following rechallenge with the drug [18, 241. The potential Thrombocytopenia accompanying the adclinical significance of this complication is ministration of therapeutic doses of hepsuggested by reports of hemorrhage, reat-in has been frequently reported [2, 5, 9, current thromboembolism, and a high mor18, 19,22-24,321. More recently, thrombotality rate [4, 15, 2.51. cytopenia associated with low-dose heparin Hypertransaminasemia is a less recogtherapy has also been reported [lo, 14, 15, nized side effect of heparin whose potential 18, 20, 25, 261. This side effect is characclinical significance and mechanism remain terized by a predictable time of onset and to be determined [29]. Hypertransamiduration and, in some patients, a recurrence nasemia associated with heparin has not been reported to produce clinical illness. ’ This investigation was supported by Public Health Services Research Grant No. RR-64 from the Division However, the determination of transamiof Research Resources, and by a grant from the nase levels is an established diagnostic aide Upjohn Company, Kalamazoo, Michigan 49001. for pulmonary infarction, hepatic damage, 2 Address reprint requests to Jeffrey R. Saffle, M.D., Department of Surgery, University of Utah College of and myocardial infarction which could be misdiagnosed in the presence of heparin Medicine, 50 North Medical Drive, Salt Lake City, Utah 84132. administration. 297

0022-4804/80/040297-09$01.00/O Copyright 0 1980 by Academic press, Inc. All rights of reproduction in any form reserved.

298

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RESEARCH:

The purpose of this study was to determine the frequency and significance of thrombocytopenia accompanying a standard course of low-dose heparin therapy and to monitor changes in serum transaminase levels during heparin adminstration. Healthy volunteers were used to eliminate any contribution of altered platelet or hepatic enzyme homeostasis or immune dysfunction arising from illness. Each subject received one of the commercially available heparin preparations- beef lung or hog intestinal mucosa extract-and data were evaluated for significant changes attributed to the source of the heparin.

VOL. 28, NO. 4, APRIL

1980

171. Five thousand USP units of heparin was injected subcutaneously into the skin of the abdominal wall or the anterior thigh. Doses were given every 12 hr for 10 days. Subjects were assigned to receive either beef lung heparin (Lot No. 959 FW and 610 EU) or porcine intestinal mucosa heparin (Lot No. 541DS). All heparin was provided by the Upjohn Co., Kalamazoo, Michigan 49001, Data Collection

Following baseline data collection, platelet counts were obtained daily throughout the period of heparin administration and 12 hr following the final dose. Bleeding time, fibrinogen levels, and serum PT and PTT MATERIALS AND METHODS were obtained before heparin dose 10 and Subject Selection 12 hr following the final dose. When some Forty normal volunteers were recruited of the initial subjects were found on routine for the study and informed consent was monitoring to have developed hypertransobtained. Eligibility to enter the study was aminasemia, the study design was modified based on satisfactory medical and drug for the remaining 22 subjects. Alkaline phosphatase, SGOT, SGPT, LDH, and total histories, physical exam, and the following baseline laboratory studies: CBC with dif- and direct bilirubin levels were obtained on ferential and platelet count (Technicon Days 5, 8, and 11 during heparin adminisHemalog, Technicon Instrument Corp., tration, and at subsequent 3-day intervals Tarrytown, N. Y. 10591), urinalysis, serum until elevated values returned to normal. Subjects were questioned daily for the prothrombin time (PT), serum partial thromboplastin time (PTT), fibrinogen (6), appearance of petechiae, ecchymoses, or bleeding time (21), and automated 12- frank bleeding. Thrombocytopenia was dechannel screening exam (Technicon SMAC, fined as a drop in platelet count to less than Technicon Instrument Corp.) which in- 150,000/mm3 on 2 consecutive days of cluded serum bilirubin, lactic dehydro- therapy [5]. The study design called for any patient developing thrombocytopenia, genase (LDH), serum glutamic-oxaloacetic or any clinical evidence of abnormal bleedtransaminase (SGOT), serum glutamic-pyruvic transaminase (SGPT), and alkaline ing, to have heparin discontinued, and platephosphatase (alk phos). Subjects on any let levels were monitored daily until remedication or those with histories sug- covery. In addition, the study plan called gesting abnormal bleeding or heparin al- for the performance of platelet aggregation lergy were disqualified. Subjects took no studies on any subject who remained thrommedication, and no alcoholic beverages bocytopenic for more than 3 days. were consumed for 10 days prior to, or Data Analysis during, the study.

Heparin Administration

Each volunteer received a standard course of subcutaneous heparin administration [ 1,

The data were analyzed using standard analysis of variance procedures. The Biomedical Computer Programs (P2V) were used to analyze the platelet count trend

SAFFLE

ET AL.: COMPLICATIONS

TABLE

HEPARIN

299

THERAPY

Thrombocytopenia

1

STUDY POPULATIONCHARACTERISTICS Number of subjects Mean age Men Women Receiving beef heparin Receiving hog heparin

OF LOW-DOSE

39 26.6 2 3.94” 17 22 25 14

(1Figures represent mean and one standard error of the mean.

analysis, platelet count by sex, and platelet count by heparin source two-way ANOVAs and the transaminase ANOVAs. Analyses of the coagulation tests and liver function enzymes were done according to the oneway ANOVA procedure by Winer with modification for missing data [33]. Data for the incidence of hypertransaminasemia were analyzed with the x2 formula. All analyses were done at the University of Utah Computer Center. RESULTS

No significant lowering of the platelet count as defined by Bell et al. [5] occurred in any of the subjects. However, as a group, a significant (P = 0.001) decrease in the mean platelet count occurred, beginning on the third day of heparin administration (Table 2, Fig. 1). A significant linear downward trend (P = 0.001) occurred in the mean platelet count over the 10 study days. A subgroup of 12 patients also had platelet counts drawn on Day 15, 4 days after discontinuing the drug. This mean count showed a marked increase (P < 0.009) above both the Day 11 nadir and the preheparin screening value, suggesting a rebound effect. No statistically significant difference was detected -in the mean platelet counts of subjects receiving the two different sources of heparin. There was a significant (P < 0.05) difference in the mean platelet counts between men and women. This difference was present in the screening lab values, and its magnitude did not change throughout the study.

Subjects Thirty-nine subjects completed the study protocol. Their characteristics are summarized in Table 1. One of the original 40 qualifying subjects developed viral hepatitis coincident with the study and was not included in the final data analysis. Three subjects were noted to have mild unconjugated hyperbilirnbinemia on their screening laboratory examinations. Since only this parameter, thought to represent Gilbert’s syndrome, was abnormal, their data were utilized in the final analysis. It was originally planned to assign subjects to receive either porcine or beef heparin according to a randomized design. However, difficulty in obtaining the porcine intestinal mucosa preparation necessitated giving beef lung heparin to 25 of the 39 subjects. No other difference was discernible between the two groups, and abandoning the randomized design did not introduce any detectable bias.

TABLE

2

PLATELET~OIJNTSDURING HEPARIN ADMINISTRATION

Day of study Baseline 2 3 4 5 6 7 8 9 10 11 15

Platelet count (X 1000, 275 271 251 258 264 247 256 254 242 235 232 286

” 2 2 k ? + k k 2 + 2 k

55 45.9 52.9 50.1 47.4 43.2 54.0 62.8 38.7 41.8 36.6 33.0

P value* NS 0.001 0.04 NS 0.01 0.01 0.03 0.01 0.01 0.01 0.009

o Figures represent mean platelet counts for the group on the day indicated, and standard errors of the means. b P values given reflect the comparis6n of the value on the day shown to the baseline mean platelet count, by analysis of variance.

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JOURNAL OF SURGICAL RESEARCH: VOL. 28, NO. 4, APRIL 1980 *Pt0.05

BY ANALYSIS VARIANCE

OF

*

P

FIG. 1. Values of mean platelet counts in 39 volunteers in association with a course of subcutaneous heparin therapy. Values shown are for the group mean on the day indicated. The period of heparin administration is indicated by the bar. Asterisks indicate values which are statistically significantly different (P < 0.05) from the baseline (“screen”) value.

No statistically significant changes were observed in clinical or laboratory tests of coagulation (fibrinogen, PT, PTT, bleeding time). Because no patient became thrombocytopenic, platelet aggregation studies were not performed. Transaminases

Of the initial 18 subjects entered into the study, 6 had no transaminase determination. Seven of 12 subjects who had transaminase levels obtained on Day 11 showed abnormal elevation of their serum SGPT. Therefore, the study protocol was altered to include measurements of hepatic enzymes at regular intervals during and following the course of heparin administration. A total of 33 patients had at least one transaminase value measured. The incidence of elevated values is displayed in Table 3. Sixty-one percent of the subjects examined had at least one abnormally elevated level. A significant difference in the incidence of this abnormality was found between the patients who received porcine intestinal mucosa derived heparin (93%) and those who received beef lung extract (37%, P < 0.05). Differences could not be attributed to the sex of the subjects.

Individual values of one transaminase, SGPT, are displayed as a scatter diagram in Fig. 2. Progressive increases in both the magnitude and the frequency of abnormal SGPT values occurred during the course of heparin administration. Cessation of heparin was accompanied by a slow decline of values to within the normal range, usually within 6 days. In every individual, values of SGOT were lower than simultaneous determinations of SGPT. Mean values of liver function enzymes for the group are presented in Table 4. Serum LDH, while not reaching abnormal levels in any subject, did show a significant (P < 0.001) increase in mean values during the study which paralleled the rise in transaminase determinations (Fig. 3). Serum total and direct bilirubin and serum alkaline phosphatase did not change during or after the course of heparin administration. To rule out the possibility that heparin may have some direct effect on transaminase determinations as performed by automated machinery, serum from 10volunteers was assayed in duplicate on the day of their baseline laboratory draw, prior to the administration of heparin. To one tube, 100 USP units of either beef lung or hog mucosal heparin was added, and a duplicate tube contained no heparin. There was no TABLE 3 INCIDENCE OF ELEVATED TRANSAMINASES ASSOCIATED WITH LOW-DOSE HEPARIN ADMINISTRATION

Sex

Type of heparin given Beef Hog Total

Men 4/8 (50%)” 7/7 (100%) lb15

(73%)

Women

Total (%)

3/11 (27%) 7/19 (36.8%)* 6/7 (86%) 13/14 (93%)* 9/18 (50%) 20/33 (61%)

a Results are expressed as the number of subjects who demonstrate increased transaminase values as a fraction of the number of subjects receiving the drug. * Difference between two groups P < 0.05 by x2 analysis.

SAFFLE ET AL.: COMPLICATIONS

301

OF LOW-DOSE HEPARIN THERAPY 207 0 270 0

2100 1450 :

I30 0 MALE HOG HEPARIN o MALE BEEF HEPARIN . FEMALE HOG HEPARIN n FEMALE BEEF HEPARIN

l2o II0

go” 0

0 .

i t

100 890

1

c z .

80

2

70

HEPARIN ADMINISTRATION 0

,[

q 0

0

0

; GO 3 UPPER LIMIT OF NORMAL SGPT

: 40 (I v) 30 20 2

=fm 8

D ’ E

IO 50 i

.

OJ a,,,,,,,,,,,,, SCREEN

, 4

2

6

8 IO DAY OF STUDY

I2

I4

,

,

,

16

, 18

FIG. 2. Individual values of SGPT in 39 volunteers in association with a course of subcutaneous heparin administration. Values for each individual determination are shown for the day indicated. The period of heparin administration is indicated by the bar. The upper limit of normal of SGPT is indicated by the solid line. The legend indicates the sex of the subject and the source of the heparin administered.

DISCUSSION

difference in the values of transaminases assayed in the duplicate tubes, with all values in the normal range. No subject developed symptoms of abnormal bleeding or any subjective findings of liver disease at any time prior to, during, or after the study.

More than a dozen reports of thrombocytopenia associated with heparin therapy have been published in the past 10 years [2-5, 9-11,

14-16, 18, 19, 22-26, 30-321.

The incidence ranges from 0 to 30% of the patients at risk, according to five studies

TABLE 4 VALUES

OF LIVER

ENZYMES

DURING

AND AFTER HEPARIN

ADMINISTRATION

Day of study Test LDH b S:OT

Screen

5

8

11

15

19

149 2 20”

181 + 37 co.01 32 + 14
185 2 35 co.01 46 in 29 co.01 75 r 48
195 * 35 co.01 46 t 26 co.01 84 2 57 co.01 62.4 f 16 NS 0.71 -e 0.32 NS

175 -e 40 co.01 21 f 9 co.01 50 -c_31 co.01 58.8 2 13 NS 0.69 f 0.42 NS

160 ” 29 co.01 17 rfr 12 NS 22 + 15 NS 54.9 2 15 co.01 0.62 f 0.43 NS

15.8 5 3.7

P

SGPT

15 & 6.6

P

Alk phos

65.2 -t 26

P

Bilirubin P

0.65 k 0.53

a Values given are means c standard errors of the means. *P values determined by analysis of variance are shown.

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IOO9 k J

SGOT SGPT m----aLDH

60-

-200

2 t-i 12 603 s $! 40*z I 1 203 E 0

K -160: 7 t-l -160 ! 3 I -140 9 ,

SCREEN

,I "EPlRlNlDYl"lSrllT,O" I 1 5 IO

0

1 15

, 20

120

DAY OF STUDY

FIG. 3. Mean values of SGOT, SGPT, and LDH for the group of 39 volunteers in association with a course of subcutaneous heparin administration. Values shown represent the mean for the group of each determination on the day indicated. The period of heparin administration is indicated by the bar.

that have attempted to quantitate its occurrence [13, 16, 18, 20, 231. These studies show no significant difference in the incidence of thrombocyiopenia between .males and females. The age distribution, primarily between the fifth and the eighth decades, is consistent with the greater incidence of venous thrombosis and pulmonary embolism with advancing age [8, 271. There appears to be no increased risk associated with the source of heparin used [5, 231. Summarization of reports published to’ date permits characterization of this thrombocytopenic syndrome. Documentation of thrombocytopenia is usually made within 10 days of starting therapy. Among the findings requiring evaluation of the platelet count are deteriorating respiratory’ status with hemoptysis and pleuritic chest pain; purpura, petechiae, and spontaneous bleeding from venipuncture sites and the intestines; and confusion and other positive neurologic signs. When the heparin is discontinued, the platelet count returns to normal within 5 to 6 days. The morbidity associated with heparin-related thrombocytopenia is high and includes hemorrhagic and thromboembolic episodes. Mortality statistics reveal that of the 61 cases of heparin-associated thrombocytopenia reported in the past 10 years, 13 (21%) have died [3, 14, 18, 19,301. Investigations into the etiology of heparm-associated thrombocytopenia have pro-

duced evidence supporting both consumptive and autoimmune etiologic mechanisms. Natelson et al. [22] reported a patient in whom heparin therapy was followed by thrombocytopenia, hypofibrinogenemia, and diffuse hemorrhage. Since then reports have described patients with apparent consumptive coagulopathy and varying hemorrhagic and thrombotic complications ]2,9, 14, 19, 23-251. In a prospective study, Bell et al. [S] sought evidence of disseminated intravascular coagulation (DIC) in 16 thrombocytopenic patients. Ten of these were found to have elevated titers of fibrin-degradation products, and five had, in addition, moderately decreased fibrinogen levels. More evidence favors an immunologic etiology of heparin-associated thrombocytopenia. In the same patient with DIC, Natelson et al. [22] demonstrated enhanced aggregation of platelet-rich plasma on the addition of heparin, while plasma from a control patient did not show enhanced aggregation. This observation has been confirmed repeatedly. Rhodes ef al. [25] demonstrated that preincubation of serum from thrombocytopenic patients with normal platelets removed the agglutinin that caused aggregation in the presence of heparin, suggesting that binding of this fraction to platelets was an integral step. Wahl et al. [32] showed that free heparin could inhibit the passive agglutination of

SAFFLE

ET AL.: COMPLICATIONS

heparin-labeled sheep erythrocytes by thrombocytopenic serum. Heparin concentration showed the biphasic response typical of antigen-antibody reactions. These authors also demonstrated complement-fixing ability in patient serum which was fractionated to y-globulin. Green et al. [ll] demonstrated binding of radiolabeled heparin to platelets in the presence of thrombocytopenic serum. Analysis of lysates of such mixtures suggested the presence of an immunoglobulin, though specific anti-IgG or -1gM antisera failed to inhibit the aggregation reaction. In contrast, Babcock et al. 121 could retard platelet factor- three release in this setting by preincubation with anti-IgG, while Wahl et al. 1321 absorbed out the active factor from his passive hemagglutination reaction with an anti-IgM antiserum. The present study has prospectively examined 39 healthy volunteers for the development of thrombocytopenia during a standard course of low-dose heparin. In every patient, platelet counts remained well within normal limits throughout the treatment period. In addition, there were no changes in laboratory parameters of consumption. Platelet aggregation was not examined, since no patient became thrombocytopenic. Taken as a group, however, our volunteers showed a gradual decrease in mean platelet counts during the period of heparin administration over a time course which is identical to that reported in clinical thrombocytopenia accompanying heparin, suggesting a phenomenon which parallels previous reports, but which is less severe. It would appear that the potential for thrombocytopenia associated with heparin administration varies greatly in different clinical settings, a concept which agrees with the varied incidence and severity of the syndrome in previous reports. This has several potential explanations. The ages of the groups compared, while considerably different, have no known effect on the platelet count. It is possible that heparinrelated thrombocytopenia is in fact dose-

OF LOW-DOSE

HEPARIN

THERAPY

303

related. However, Bell et al. [5] found no correlation between thrombocytopenia and heparin dose. Also, several reports of thrombocytopenia associated with doses of heparin identical to those used in this study argue against this. The most unique feature of our sample population is their underlying coagulation status. The patients reported previously were suffering from deep vein thrombosis, myocardial infarction, or pulmonary embolism or were postoperative from surgery. In addition, some had carcinomatosis 1251. All of these disorders are known to be accompanied by enhanced platelet and fibrinogen consumption and increased platelet adhesiveness. Harker et al. 1121 and Slichter et al. [28] have demonstrated a twofold increase in platelet and fibrinogen turnover in patients with venous thrombosis, and a threefold increase after major elective surgery. In their studies, patients with purely immune thrombocytopenic syndromes (ITP, TTP, etc.) had normal fibrinogen kinetics, while many patients with heparin-associated thrombocytopenia have enhanced fibrinogen consumption. This suggests that a purely autoimmune etiology is inconsistent with the clinical syndrome of heparin-related thrombocytopenia. It is probable that both consumptive and autoimmune mechanisms contribute to the thrombocytopenic syndrome. In our study the finding of a significant drop in mean platelet count with heparin in the absence of demonstrable thrombocytopenia may be a baseline reaction to heparin, possibly autoimmune in nature, with which the stresses of major illness or surgery combine to produce significant thrombocytopenia in the acutely ill patient. Less information on heparin-associated hypertransaminasemia is available. Sonnenblick et al. 1291noted a transient rise in patients’ serum transaminase values during a course of full-dose heparin therapy. In a prospective study, 10 of 14 in-patients showed elevated SGOT and SGPT levels during a regimen of therapeutic hepariniza-

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tion, and a return to normal values with cessation of therapy. A 61% incidence of hypertransaminasemia occurred in our study population. The phenomenon raises questions as to the source of the elevated enzymes and the causative mechanisms of this abnormality. Both transaminases have several sources, including skeletal muscle, which could conceivably have been damaged by improper subcutaneous injections. Injections given in this study were administered by one of the authors (JS) and were well confined to the subcutaneous fat of the anterior abdominal wall or thigh. Also, the enzyme picture demonstrated by our subjects was typical of hepatocellular injury. The SGPT value was in every case higher than SGOT, and was paralleled by a smaller but significant rise in LDH. LDH isoenzymes were performed in one patient, and were normal. The enzyme elevations observed in our subjects, as well as the decline in mean platelet counts, match temporally the observations of Sonnenblick et al. [29] and the thrombocytopenic syndrome reviewed previously. The inference that the three phenomena are causally related is obvious. Antiplatelet antibodies stimulated by heparin are at least partially responsible for thrombocytopenia, and they could certainly induce hepatocellular damage as well. The very rapid changes in enzymes observed could also be the result of direct hepatotoxicity. Such toxicity is apparently not dose related, however, since the hypertransaminasemia is no different in magnitude from that observed by Sonnenblick to accompany full-dose heparinization. It is of interest to note that a significantly increased incidence of hypertransaminasemia accompanies the administration of porcine as opposed to bovine heparin. Prior to this finding no difference had been found in the potential for side effects of the two compounds. Nelson et al. [23] evaluated 38 consecutive patients receiving porcine mucosal heparin and found that 9 (24%) devel-

oped thrombocytopenia, while Bell et al. [5] followed 52 patients receiving beef lung heparin. Thrombocytopenia occurred in 16 (30%) of their population. Although both heparin derivatives are complex and impure mucopolysaccharide compounds, they have demonstrated cross-reactivity in eliciting thrombocytopenia [ 11, 231. This may represent in both preparations a common antigenic locus. The incidence of hypertransaminasemia suggests at least a small difference in reaction to the two compounds. The potential for hepatocellular damage from heparin administration has both diagnostic and therapeutic implications. Enzyme changes are used to confirm the diagnosis of myocardial infarction and pulmonary embolus, as well as other disorders. Though reliance on alternative diagnostic tests should prevent erroneous diagnosis, it is important for the clinician to remember the potential for heparin to alter these enzymes. The effect of such hepatocellular damage on patients with marginal liver function, or accompanying long-term heparin exposure, remains to be elucidated. The potential reactions accompanying full- or low-dose heparin therapy appear to be multiple, and their etiologies complex. We found a much smaller effect on platelet count in our healthy volunteers than have previous studies in hospitalized patients. Our finding of significant hypertransaminasemia in over half of our population was equal in frequency and magnitude to that described in a previous report. Until more information on the etiology and mechanism of these phenomena is available, the potential for thrombocytopenia and hypertransaminasemia should be considered when beginning heparin therapy. Liver enzymes, platelet counts, and other coagulation tests should be monitored in patients receiving heparin in any form. REFERENCES 1. American Heart Association Council on Thrombosis. Prevention of venous thromboembolism in

SAFFLE

2. 3. 4.

5.

6.

7. 8.

ET AL.: COMPLICATIONS

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surgical patients by low-dose heparin. Circula?ion 55: 423A, 1917. Babcock, R. B., Dumper, C. W., and Scharfman, W. B. Heparin-induced immune thrombocytopenia. N. Engl. .I. Med. 295: 231, 1976. Babcock, R. B., Dumper, C. W., and Scharfman, W. B. Thrombocytopenia during heparin therapy. N. Engl. J. Med. 295: 1201, 1976. Baird, R. A., and Convery, F. R. Arterial thromboembolism in patients receiving systemic heparin therapy. J. Bone Joint Surg. 59A: 1061, 1977. Bell, W. R., Tomasula, P. A., Alving, B. M., and Duffy, T. P. Thrombocytopenia occurring during the administration of heparin. Ann. Intern. Med. 85: 155, 1976. Biggs, R. (Ed.) Human Blood Coagulation Haemostasis and Thrombosis, 2nd ed. Oxford: Blackwell Scientific, 1976. Bygdeman, S., and Eliasch, H. Platelet adhesiveness in myocardial infarction in relation to clinical course. Acta Med. &and. 199: 475, 1976. Coon, W. W., and Coller, F. A. Some epidemiologic considerations of thromboembolism. Surg.

embolism by low doses of heparin. Lancer 1: 45, 1975. 18. Kapsch, D. N., Adelstein, E. H., Rhodes, G. R., and Silver, D. Heparin-induced thrombocytopenia, thrombosis, and hemorrhage. Surgery 86: 148, 1979. 19. Klein, H. G., and Bell, W. R. Disseminated intravascular coagulation during heparin therapy. Ann. Intern. Med. 80: 477, 1974. 20. Malcolm, E. D., and Wigmore, T. A. Thrombocytopenia induced by low-dose subcutaneous heparin. Lance? 1: 444, 1978. 21. Mielkle, C. H., Jr., Kaneshiro, M. M., Maher, I. A., Weiner, J. M., and Rapaport, S. 1. The standardized normal Ivy bleeding time and its prolongation by aspirin. Blood 34~ 204, 1969. 22. Natelson, E. A., Lynch, E. C., Alfrey, C. P., and Gross, J. B. Heparin-induced thrombocytopenia: An unexpected response to treatment of consumptive coagulopathy. Ann. Intern. Med. 71: 1121, 1969. 23. Nelson, J. C., Lenner, R. G., Goldstein, R., and Cagin, N. A. Heparin-induced thrombocytopenia.

Gynecol.

24. Rhodes, G. R., Dixon, R. H., and Silver, D. Heparin-induced thrombocytopenia with thrombotic-hemorrhagic manifestations. Surg. Gynecol.

Obstet. 109: 487, 1959.

9. Fratantoni, J. C., Pollet, R., and Gralnick, H. R. Heparin-induced thrombocytopenia: confirmation of diagnosis with in vitro methods. Blood 45: 395, 1975. 10. Galle, P. C., Muss, H. B., McGrath, K. M., Stuart, J. J., and Homesley, H. D. Thrombocytopenia in two patients treated with low-dose heparin. Obstet. Gynecol. 52: 9s, 1978. 11. Green, D., Harris, K., Reynolds, N., Roberts, M., and Patterson, R. Heparin immune thrombocytopenia: evidence for a heparin-platelet complex as the antigenic determinant. .I. Lab. Clin. Med. 91: 167, 1978.

12. Harker, L. A., and Slichter, S. J. Platelet and fibrinogen consumption in man. N. Engl. J. Med. 287: 999, 1972. 13. Heiden, D., Mielke, C. H., Jr., and Rodvien, R. Impairment by heparin of primary hemostasis and platelet 14C S-hydroxytryptamine release. Brit. .I. Haematol.

36: 427, 1977.

14. Hrushesky, W. J. Subcutaneous heparin-induced thrombocytopenia. Arch. Intern. Med. 138: 1489, 1978.

151 Hrushesky, W. J. Thrombocytopenia induced by low-dose subcutaneous heparin. Lancet 2: 2186, 1977. 16. Justus, P. G., Junija, H. S., and Noyes, W. D. Heparin-associated thrombocytopenia. Clin. Res. 24: 17A, 1976. 17. Kakkar, V. V., Corrigan, T. P., and Fossard, D. P. Prevention of fatal postoperative pulmonary

Arch. Intern.

Med. 138: 548, 1978.

Obstet. 136: 409, 1973.

25. Rhodes, G. R., Dixon, R. H., and Silver, D. Heparin-induced thrombocytopenia: Eight cases with thrombotic-hemorrhagic complications. Ann. Surg. 186: 752, 1977. 26. Searcy, R. A.,and Marple, R. N. Heparin thrombocytopenia. Ann. Intern. Med. 86: 364, 1977. 27. Sevitt, S., and Gallagher, N. Venous thrombosis and pulmonary embolism: A clinico-pathological study in injured and burned patients. Brit. J. Surg. 48: 475, l%l. 28. Slichter, S. J., Funk, D. D., Leandoer, L. E., and Harker, L. A. Kinetic evaluation of haemostasis during surgery and wound healing. Brit. J. Haematol. 27: 115, 1974. 29. Sonnenblick, M., Oren, A., and Jacobsohn, W. Hypertransaminasemia with heparin therapy. Brit. Med. J. 3: 77, 1975. 30. Stevensen, M. M. Thrombocytopenia during heparin therapy. N. Engl. J. Med. 295: 1200, 1976. 31. Trowbridge, A. A., Caraveo, J., Green, J. B., Amaral, B., and Stone, M. J. Heparin-related immune thrombocytopenia. Amer. J. Med. 65: 277, 1978. 32. Wahl, T. O., Lipschitz, D. A., and Stechschulte, D. J. Thrombocytopenia associated with antiheparin antibody. JAMA 240: 2560, 1978. 33. Winer, B. J. Statistical Principles in -Experimental Design. New York: McGraw-Hill, 1971.