Oral contraceptives and platelet aggregation NIRMALA IRA FRED
SHEVDE,
I. SUSSMAN,
M.D.**
ROSNER,
VALENTINA Jamaica
and
M.D.*
M.D. PACHOLUK,
Stony
Brook,
New
M.S. York
There are conflicting data in the literature concerning the effects of oral contraceptives on platelet aggregation. We studied 26 healthy women who used oral contraceptives for at least three months. The control group consisted of 25 healthy women of childbearing age, who were receiving no medications. No differences in platelet aggregation in response to adenosine diphosphate, epinephrine, and collagen were found between the two groups. (AM. J. OBSTET. GYNECOL. 132: 303, 1976.)
THE ASSOCIATION OF thromboembolic disease with contraceptive use is well extablished.‘-3 However, the mechanism of this apparent hypercoagulability is still unclear. Several investigators have shown changes in the plasma levels of various clotting factors in women using oral contraceptives. 4-6 There are conflicting data on the effects of such medications on platelet aggregation. Some investigators have demonstrated increased platelet aggregation,?, 8 whereas in other studies *no change in platelet function was found.g, lo The present study was undertaken to determine whether or not oral contraceptive pills affect platelet aggregation.
Material and methods Twenty-six healthy women with a mean age of 25 years (range, 16 to 40) were studied. These women had used various preparations of oral contraceptives for at least three months. The control group consisted of 25 healthy women of childbearing age (age range, 25 to 35 years, with a mean of 28), who were receiving no mediFrom the Divisia of Hematolom, Queens Hospital Center, Affiliation if the Long%& Jewish-Hillside Medical Center. and the Debartment of Medicine, Health Sciences Center; State Univbrsity of N&u York at ‘Stony Brook. Received for publication
November
Revised and accepted February
22, 1977.
28, 1978.
Reprint requests: Dr. Fred Rosner, Division of Hematology, 82-68 164th St. Jamaica, New York 11432. *Former Afjliation Center.
Fellow
in Hematology, Queueem Hospital Center, Long Island Jewish-Hillside Medical
of the
**Research Associate in Hen&at&g?, Center, Bronx, New York. 0002-9378/‘78/190303+04$00.40/0
0
Montejore
1978 The C. V. Mosby
Medical
Co
cations. No subject had ingested aspirin or similar compounds known to affect platelet function during the 10 day period prior to study. Smoking histories were not obtained. All blood samples were obtained in the morning after an overnight fast. A 19 gauge needle was used to draw 27 ml. of venous blood into a plastic syringe from the antecubital vein. The blood was mixed with 0.1 volume of 3.8 per cent trisodium citrate and plateletrich plasma was obtained by centrifugation at 1,500 x g for thFee minutes at 20” C. Platelet-poor plasma was prepared by further centrifugation at 2,800 X g for 20 minutes at 4” C. Platelet-aggregation studies were performed on a dual-channel aggregometer* connected to a recorder,t according to a modification of the method of Weiss and Rogers. ” Then 20 and 25 ~1 of aggregating agent were added to cuvettes containing platelet-rich plasma that had been allowed to equilibrate for one minute at 37” C. The aggregating agents used were adenosine diphosphate$ (ADP) in final concentrations of 4 and 2 PM, epinephrines in a final concentration of 5 PM, and soluble collagen11 in concentrations of 0.079, 0.039, and 0.0197 mg. per milliliter. Those samples showing aggregation at a collagen concentration of 0.0197 mg. per milliliter were further tested at 0.00985 mg. per milliliter. The platelet-aggregation studies were expressed and *Payton Associates, Inc., Buffalo, New York. Scribe, Omni Spectra, Inc., Farmington, Michigan. *Sigma Chemical Co., St. Louis, Missouri. PParke, Davis & Co., Detroit, Michigan. IlWorthington Biochemical Corp., Freehold, New Jersey.
303
304
et al.
Shevde
Table
I. Platelet
aggregation
in women
using oral contraceptives
-
study group Aggregating agent Epinephrine ,4DP: 4d4
(4 PM)
2 CLM
Control gwup
Maximum aggregatian (man k S.D.) (%)
No.
Mtaximum
aggwgahm
(mean 2 S.D.) (%)
NO.
I’ zlalur
23
90.37
5
0.43
N.S.*
75.92 + 3.18 66.2 2 3.98
25 24
81.11 68.93
t 2.6 -t 2.03
N.S. N.S.
87.85 ?I 3.3 -i 3.8
10
22
89.01 r+ 1% 86.10 t 1.65
N.S.
22
88.78
26 25 14 23
81.59
2
0.98
Collagen:
0.079 mg./ml. 0.039 mg./ml.
N.S.
*Not significant. Table
II. Selected coagulation
factors in women
using oral contraceptives
stuuy group Ck&gfhW
Control goup
No.
Mean + S.D.
No.
Fibrinogen (mg./lOO ml.) Factor VIII Factor VII/X
26 25 19
281.3 ‘- 17.35 88.76 2 3.9 96.3 k 3.43
Factor
19
114.05
25 22 12 12
X
evaluated as the percentage of maximum aggregation,” the rate of maximum aggregation, and the lag time before onset of aggregation with collagen. The rate of maximum aggregation was defined as the slope (millimeters per minute) of the tangent to the steepest portion of the aggregation curve. Ristocetin-induced aggregation of platelet-rich plasma was not tested in this study since, in our experience, such aggregation does not correlate with factor VIII procoagulant activity when the latter is normal or increased (as found in this study) because maximal aggregation always occurs. Fibrinogen was assayed by the biuret method. Factor VIII was assayed by a one-stage kaolin-activated partial thromboplastin time with the use of factor VIIIdeficient plasma substrate. Factors X and VII/X were assayed by a one-stage assay with commercially available VII/X-deficient plasma.* One unit of any coaguiation factor was defined as the amount of that factor present in 1 ml. of pooled normal plasma (20 donors). Student’s t test was used in the statistical analyses. Factor VII antigen and Von Willebrand factor were not measured in this study.
Results The percentage of maximal aggregation of platelets in response IO various concentrations of epinephrine, ADP, and colIagen is shown in Table I. There was no *General Diagnostic Div., Warner-Lambert Morris Plains, New Jersey.
Pharm. Co.,
+
5.16
Mean 2 S.D. 271 79.77
IL 14.25 t
90.25 ” 94.91
4.55 3.81
t 10.6
P
value N.S. N.S. N.S. N.S.
significant difference between the aggregation induced by the three agents in the two groups of women. Furthermore, both groups exhibited a similar pattern in response to decreasing concentrations of coilagen. When collagen was diluted to a final concentration of 0.0195 mg. per milliliter, only two patients in the control group and two in the study group showed aggregation while the others had no aggregation at all at that dilution. These four individuals did not demonstrate any aggregation at the next dilution. Platelet-aggregation lag times and rates were also compared and, again, no significant differences were observed. There was no variation in platelet aggregation at different times in the menstrual cycle. Coagulation factors I. VIII. and VII/X were slightly increased in the study group compared to the control group (Table II and Fig. 1). However, these lindings were not statistically significant.
Comment The use of oral contraceptive pills has been associated with the development of deep vein thrombosis and pulmonary embolism.‘-” Although the tnechanism of this complication has not been clearly elucidated, an increase in blood clotting factors has been postulated.‘-g* Is Our study shows no statistically significant change in clotting factors but our results are in the direction of those of the other studies. Recently, attention has been focused on rhe higher incidence of cerebral and coronary arterial thrombosis in women using
Volume Number
132 3
Oral contraceptives and platelet aggregation
CONTROL Sl’JW
305
m GROUP
100
270 -
90
240 210 -
80 bk 70
180 -
4 60
I50
-
$ 50
120 -
El 40 8
90
-
'a 30
60 -
20
30 -
IO
O-
0 FACTOR XIX
Fig. 1. Selected
coagulation
factors
oral contraceptive pills.’ Hyperaggregation of platelets has been thought to play a role in this increased incidence of arterial thrombosis. Poller and associates6 found platelet aggregation to be significantly accelerated with the Chandler’s tube technique used to induce coagulation in women using combined estrogen-progestin oral contraceptives, though this was less apparent in the third trimester of pregnancy. Women using a pure progestogen, chlormadinone acetate, did not show this change up to the sixth month of study. In addition, the accelerated platelet aggregation resulting from conventional oral contraceptive use became normal one month after a change to the progestogen. There was no change in the platelet-aggregation response to ADP during oral contraception. However, the Chandler’s tube method is also affected by an increase in clotting factors. When the same investigators7 tested platelet aggregation with Born’s method, they were unable to demonstrate hyperaggregation. On the other hand, other worker9. I3 have demonstrated increased platelet aggregation with the Born method. McGrath and Castaldi,8 also with the Born method, found an increase in platelet aggregation with
REFERENCES
1. Inman, W. H., and Vessey, M. P.: Investigation of deaths from pulmonary, coronary and cerebral thrombosis and embolism in women of childbearing age, Br. Med. J. 2: 193, 1968.
in women
FACTOR ml/x
using
FACTOR x
oral contraceptives.
ADP and collagen. Zahavi and associates,13 employing a fragilograph, demonstrated increased platelet aggregation with ADP. Recent work by Carvalho and associates9 has failed to show any increased platelet aggregation among women using oral contraceptives. Our study confirms these findings. Both rate and maximal aggregation were the same in the two groups of women. Furthermore, both groups responded to the same minimum dose of coliagen. No difference4 in platelet aggregation could be documented, regardless ?f the type of analysis employed. Differences between this and previous studies cannot be explained on the basis of the type of estrogen involved. The dosage of estrogen in all studies was similar, namely, 50 pg of ethinyl estradiol. Zahavi and associates13 were able to demonstrate increased platelet aggregation throughout the mCnstrua1 cycle. Poller and associates’ and McGrath and Castaldi8 tested women only at midcycle. Our patients were tested at various times during the menstrual cycle as were the patients of Carvalho and associates.” When our data were analyzed according to the time of menstrual cycle, no differences were observed.
2. Sartwell, P. E., Masi, A. T., Arthes, F. G., Greene, G. R., and Smith, H. E.: Thromboembolism and oral contraceptives: An epidemiologic case control study, Am. J. Epidemiol. 90: 365, 1969. 3. Drill, V. A.:.Oral contraceptives and thromboembolic dis-
306
Shevde
octobcr 1, lY7X XIII. J. Ohstet. Gynerol.
et al.
ease. I. Prospective and retrospective studies. II. Estrogen content oforal contraceptives, J. A. M. A. 219: 583, 1972. 4. Hougie, C., Rutherford, R. N., Banks. A. L., and Coburn, W. A.: Effect of a progestin-estrogen oral contraceptive on blood clotting factors, Metabolism 14: 411, 1965. 5. Poller, L., and Thomson, J. M.: Clotting factors during oral contraception: Further report, Br. Med. J. 2: 23, 1966. 6. Poller. L., Tabiowo, A., and Thomson, J. M.: Effects of low-dose oral contraceptives on blood coagulation, Br. Med. J. 3: 218. 1968. Poller, L., Priest. C. M., and Thomson, J. M.: Platelet aggregation during oral contraception. Br. Med. J. 4: 273, 1969. McCrath, K. M., and Castaldi, P. A.: Changes in coagulation factors and platelet function in response to progestational agents, Hemostasis 4: 65, 1975. Carvalho, A. C. A., Vaillancourt, R. A., Cabral, R. B., Lees, R. S., and Colman. R. W.: Coagulation abnor-
Informdon
10.
11.
12.
13.
malities in women taking oral contraccptivcx. ,I. ?,. hf :\. 237: 875, 1977. Strolin-Benedetti, M., Guttv. D., and Strrtiin. I’.: ..I cornparative study of the effect of oral corrtraceptives and cigarette smoking on platelet adhesiveness, Hemostasis 5: 14. 1976. Weiss, H. J., and Rogers, J.: ‘l’hrombocytopathia due to abnormalities in platelet release reactiotr-studies on six unrelated patients, Blood 39: 187, 1972. Meade, T. W., Brozovic, M.. Chakrabarti, K.. Hovbarth, D. .J., North, W. R. S., and Stirling, Y.: An epidemiological study of the haemostatic and other effects of oral contraceptives, Br. J. Haematol. 34: 1976. Zahavi. J., Dreyfuss, F.. Kalef; M., and Solzrman. N.: Adenosine diphosphate-induced platelet aggregation in healthy women with and without a combined and a sequential contraceptive pill. Ahr. J. Orxtt r. Gvsrx:ot.. 117: 107. 1973.
for au#ors
Most of the provisions of the Copyright Act of 1976 became effective on January 1. 1978. Therefore, all manuscripts must be accompanied by the following statement, signed by each author: “The undersigned author(s) transfers all copyright ownership of the
manuscript entitled (title of article) to The C. V. Mosby Company in the event the work is published. The author(s) warrants that the article is original, is not under consideration by another journal, and has not been previously published.” Authors will be consulted, when possible, regarding republication of their material.
SHEVDE,
I. SUSSMAN,
M.D.**
ROSNER,
VALENTINA Jamaica
and
M.D.*
M.D. PACHOLUK,
Stony
Brook,
New
M.S. York
There are conflicting data in the literature concerning the effects of oral contraceptives on platelet aggregation. We studied 26 healthy women who used oral contraceptives for at least three months. The control group consisted of 25 healthy women of childbearing age, who were receiving no medications. No differences in platelet aggregation in response to adenosine diphosphate, epinephrine, and collagen were found between the two groups. (AM. J. OBSTET. GYNECOL. 132: 303, 1976.)
THE ASSOCIATION OF thromboembolic disease with contraceptive use is well extablished.‘-3 However, the mechanism of this apparent hypercoagulability is still unclear. Several investigators have shown changes in the plasma levels of various clotting factors in women using oral contraceptives. 4-6 There are conflicting data on the effects of such medications on platelet aggregation. Some investigators have demonstrated increased platelet aggregation,?, 8 whereas in other studies *no change in platelet function was found.g, lo The present study was undertaken to determine whether or not oral contraceptive pills affect platelet aggregation.
Material and methods Twenty-six healthy women with a mean age of 25 years (range, 16 to 40) were studied. These women had used various preparations of oral contraceptives for at least three months. The control group consisted of 25 healthy women of childbearing age (age range, 25 to 35 years, with a mean of 28), who were receiving no mediFrom the Divisia of Hematolom, Queens Hospital Center, Affiliation if the Long%& Jewish-Hillside Medical Center. and the Debartment of Medicine, Health Sciences Center; State Univbrsity of N&u York at ‘Stony Brook. Received for publication
November
Revised and accepted February
22, 1977.
28, 1978.
Reprint requests: Dr. Fred Rosner, Division of Hematology, 82-68 164th St. Jamaica, New York 11432. *Former Afjliation Center.
Fellow
in Hematology, Queueem Hospital Center, Long Island Jewish-Hillside Medical
of the
**Research Associate in Hen&at&g?, Center, Bronx, New York. 0002-9378/‘78/190303+04$00.40/0
0
Montejore
1978 The C. V. Mosby
Medical
Co
cations. No subject had ingested aspirin or similar compounds known to affect platelet function during the 10 day period prior to study. Smoking histories were not obtained. All blood samples were obtained in the morning after an overnight fast. A 19 gauge needle was used to draw 27 ml. of venous blood into a plastic syringe from the antecubital vein. The blood was mixed with 0.1 volume of 3.8 per cent trisodium citrate and plateletrich plasma was obtained by centrifugation at 1,500 x g for thFee minutes at 20” C. Platelet-poor plasma was prepared by further centrifugation at 2,800 X g for 20 minutes at 4” C. Platelet-aggregation studies were performed on a dual-channel aggregometer* connected to a recorder,t according to a modification of the method of Weiss and Rogers. ” Then 20 and 25 ~1 of aggregating agent were added to cuvettes containing platelet-rich plasma that had been allowed to equilibrate for one minute at 37” C. The aggregating agents used were adenosine diphosphate$ (ADP) in final concentrations of 4 and 2 PM, epinephrines in a final concentration of 5 PM, and soluble collagen11 in concentrations of 0.079, 0.039, and 0.0197 mg. per milliliter. Those samples showing aggregation at a collagen concentration of 0.0197 mg. per milliliter were further tested at 0.00985 mg. per milliliter. The platelet-aggregation studies were expressed and *Payton Associates, Inc., Buffalo, New York. Scribe, Omni Spectra, Inc., Farmington, Michigan. *Sigma Chemical Co., St. Louis, Missouri. PParke, Davis & Co., Detroit, Michigan. IlWorthington Biochemical Corp., Freehold, New Jersey.
303
304
et al.
Shevde
Table
I. Platelet
aggregation
in women
using oral contraceptives
-
study group Aggregating agent Epinephrine ,4DP: 4d4
(4 PM)
2 CLM
Control gwup
Maximum aggregatian (man k S.D.) (%)
No.
Mtaximum
aggwgahm
(mean 2 S.D.) (%)
NO.
I’ zlalur
23
90.37
5
0.43
N.S.*
75.92 + 3.18 66.2 2 3.98
25 24
81.11 68.93
t 2.6 -t 2.03
N.S. N.S.
87.85 ?I 3.3 -i 3.8
10
22
89.01 r+ 1% 86.10 t 1.65
N.S.
22
88.78
26 25 14 23
81.59
2
0.98
Collagen:
0.079 mg./ml. 0.039 mg./ml.
N.S.
*Not significant. Table
II. Selected coagulation
factors in women
using oral contraceptives
stuuy group Ck&gfhW
Control goup
No.
Mean + S.D.
No.
Fibrinogen (mg./lOO ml.) Factor VIII Factor VII/X
26 25 19
281.3 ‘- 17.35 88.76 2 3.9 96.3 k 3.43
Factor
19
114.05
25 22 12 12
X
evaluated as the percentage of maximum aggregation,” the rate of maximum aggregation, and the lag time before onset of aggregation with collagen. The rate of maximum aggregation was defined as the slope (millimeters per minute) of the tangent to the steepest portion of the aggregation curve. Ristocetin-induced aggregation of platelet-rich plasma was not tested in this study since, in our experience, such aggregation does not correlate with factor VIII procoagulant activity when the latter is normal or increased (as found in this study) because maximal aggregation always occurs. Fibrinogen was assayed by the biuret method. Factor VIII was assayed by a one-stage kaolin-activated partial thromboplastin time with the use of factor VIIIdeficient plasma substrate. Factors X and VII/X were assayed by a one-stage assay with commercially available VII/X-deficient plasma.* One unit of any coaguiation factor was defined as the amount of that factor present in 1 ml. of pooled normal plasma (20 donors). Student’s t test was used in the statistical analyses. Factor VII antigen and Von Willebrand factor were not measured in this study.
Results The percentage of maximal aggregation of platelets in response IO various concentrations of epinephrine, ADP, and colIagen is shown in Table I. There was no *General Diagnostic Div., Warner-Lambert Morris Plains, New Jersey.
Pharm. Co.,
+
5.16
Mean 2 S.D. 271 79.77
IL 14.25 t
90.25 ” 94.91
4.55 3.81
t 10.6
P
value N.S. N.S. N.S. N.S.
significant difference between the aggregation induced by the three agents in the two groups of women. Furthermore, both groups exhibited a similar pattern in response to decreasing concentrations of coilagen. When collagen was diluted to a final concentration of 0.0195 mg. per milliliter, only two patients in the control group and two in the study group showed aggregation while the others had no aggregation at all at that dilution. These four individuals did not demonstrate any aggregation at the next dilution. Platelet-aggregation lag times and rates were also compared and, again, no significant differences were observed. There was no variation in platelet aggregation at different times in the menstrual cycle. Coagulation factors I. VIII. and VII/X were slightly increased in the study group compared to the control group (Table II and Fig. 1). However, these lindings were not statistically significant.
Comment The use of oral contraceptive pills has been associated with the development of deep vein thrombosis and pulmonary embolism.‘-” Although the tnechanism of this complication has not been clearly elucidated, an increase in blood clotting factors has been postulated.‘-g* Is Our study shows no statistically significant change in clotting factors but our results are in the direction of those of the other studies. Recently, attention has been focused on rhe higher incidence of cerebral and coronary arterial thrombosis in women using
Volume Number
132 3
Oral contraceptives and platelet aggregation
CONTROL Sl’JW
305
m GROUP
100
270 -
90
240 210 -
80 bk 70
180 -
4 60
I50
-
$ 50
120 -
El 40 8
90
-
'a 30
60 -
20
30 -
IO
O-
0 FACTOR XIX
Fig. 1. Selected
coagulation
factors
oral contraceptive pills.’ Hyperaggregation of platelets has been thought to play a role in this increased incidence of arterial thrombosis. Poller and associates6 found platelet aggregation to be significantly accelerated with the Chandler’s tube technique used to induce coagulation in women using combined estrogen-progestin oral contraceptives, though this was less apparent in the third trimester of pregnancy. Women using a pure progestogen, chlormadinone acetate, did not show this change up to the sixth month of study. In addition, the accelerated platelet aggregation resulting from conventional oral contraceptive use became normal one month after a change to the progestogen. There was no change in the platelet-aggregation response to ADP during oral contraception. However, the Chandler’s tube method is also affected by an increase in clotting factors. When the same investigators7 tested platelet aggregation with Born’s method, they were unable to demonstrate hyperaggregation. On the other hand, other worker9. I3 have demonstrated increased platelet aggregation with the Born method. McGrath and Castaldi,8 also with the Born method, found an increase in platelet aggregation with
REFERENCES
1. Inman, W. H., and Vessey, M. P.: Investigation of deaths from pulmonary, coronary and cerebral thrombosis and embolism in women of childbearing age, Br. Med. J. 2: 193, 1968.
in women
FACTOR ml/x
using
FACTOR x
oral contraceptives.
ADP and collagen. Zahavi and associates,13 employing a fragilograph, demonstrated increased platelet aggregation with ADP. Recent work by Carvalho and associates9 has failed to show any increased platelet aggregation among women using oral contraceptives. Our study confirms these findings. Both rate and maximal aggregation were the same in the two groups of women. Furthermore, both groups responded to the same minimum dose of coliagen. No difference4 in platelet aggregation could be documented, regardless ?f the type of analysis employed. Differences between this and previous studies cannot be explained on the basis of the type of estrogen involved. The dosage of estrogen in all studies was similar, namely, 50 pg of ethinyl estradiol. Zahavi and associates13 were able to demonstrate increased platelet aggregation throughout the mCnstrua1 cycle. Poller and associates’ and McGrath and Castaldi8 tested women only at midcycle. Our patients were tested at various times during the menstrual cycle as were the patients of Carvalho and associates.” When our data were analyzed according to the time of menstrual cycle, no differences were observed.
2. Sartwell, P. E., Masi, A. T., Arthes, F. G., Greene, G. R., and Smith, H. E.: Thromboembolism and oral contraceptives: An epidemiologic case control study, Am. J. Epidemiol. 90: 365, 1969. 3. Drill, V. A.:.Oral contraceptives and thromboembolic dis-
306
Shevde
octobcr 1, lY7X XIII. J. Ohstet. Gynerol.
et al.
ease. I. Prospective and retrospective studies. II. Estrogen content oforal contraceptives, J. A. M. A. 219: 583, 1972. 4. Hougie, C., Rutherford, R. N., Banks. A. L., and Coburn, W. A.: Effect of a progestin-estrogen oral contraceptive on blood clotting factors, Metabolism 14: 411, 1965. 5. Poller, L., and Thomson, J. M.: Clotting factors during oral contraception: Further report, Br. Med. J. 2: 23, 1966. 6. Poller. L., Tabiowo, A., and Thomson, J. M.: Effects of low-dose oral contraceptives on blood coagulation, Br. Med. J. 3: 218. 1968. Poller, L., Priest. C. M., and Thomson, J. M.: Platelet aggregation during oral contraception. Br. Med. J. 4: 273, 1969. McCrath, K. M., and Castaldi, P. A.: Changes in coagulation factors and platelet function in response to progestational agents, Hemostasis 4: 65, 1975. Carvalho, A. C. A., Vaillancourt, R. A., Cabral, R. B., Lees, R. S., and Colman. R. W.: Coagulation abnor-
Informdon
10.
11.
12.
13.
malities in women taking oral contraccptivcx. ,I. ?,. hf :\. 237: 875, 1977. Strolin-Benedetti, M., Guttv. D., and Strrtiin. I’.: ..I cornparative study of the effect of oral corrtraceptives and cigarette smoking on platelet adhesiveness, Hemostasis 5: 14. 1976. Weiss, H. J., and Rogers, J.: ‘l’hrombocytopathia due to abnormalities in platelet release reactiotr-studies on six unrelated patients, Blood 39: 187, 1972. Meade, T. W., Brozovic, M.. Chakrabarti, K.. Hovbarth, D. .J., North, W. R. S., and Stirling, Y.: An epidemiological study of the haemostatic and other effects of oral contraceptives, Br. J. Haematol. 34: 1976. Zahavi. J., Dreyfuss, F.. Kalef; M., and Solzrman. N.: Adenosine diphosphate-induced platelet aggregation in healthy women with and without a combined and a sequential contraceptive pill. Ahr. J. Orxtt r. Gvsrx:ot.. 117: 107. 1973.
for au#ors
Most of the provisions of the Copyright Act of 1976 became effective on January 1. 1978. Therefore, all manuscripts must be accompanied by the following statement, signed by each author: “The undersigned author(s) transfers all copyright ownership of the
manuscript entitled (title of article) to The C. V. Mosby Company in the event the work is published. The author(s) warrants that the article is original, is not under consideration by another journal, and has not been previously published.” Authors will be consulted, when possible, regarding republication of their material.
