Platelet Activation in Response to Phlebotomy

ABSTRACT. Plasma concentrations of β-thromboglobulin (BTG) and platelet factor 4 (PF4) were measured in three consecutive blood samples from 29 healthy male blood donors. The first sample was collected after 15 min of rest, the second immediately after a phlebotomy of 450 ml blood and the third after a further 15 min of rest. The mean baseline plasma BTG and PF4 values were 68 × 5 and 13.8 ± 0.7 ng/ml, respectively. The second sample's mean plasma values of these two platelet-specific proteins were significantly higher (88 ± 9 and 24.1 ± 4.1 ng/ml, respectively). The plasma concentrations of BTG and PF4 in the last sample, however, had returned to baseline levels. It is concluded that significant but short-lasting platelet activation and secretion take place in healthy subjects in response to an acute but comparatively mild blood loss even though the platelets do not participate in the process of hemostasia.     

Platelet Aggregation in Primary Raynaud's Phenomenon and the Effect of Enalapril Administration

Platelet aggregation using a single platelet counting technique in whole blood, was determined on 18 patients with primary Raynaud's phenomenon and 17 age-matched controls. Platelet aggregation in the Raynaud's patients was also assessed during a double-blind, crossover trial to investigate the efficacy of the angiotensin converting enzyme (ACE) inhibitor, enalapril. There were no differences in platelet aggregation to collagen, arachidonic acid, ADP or PAF, or in plasma levels of beta-thromboglobulin (BTG), platelet factor 4 (PF4) or thromboxane B(2) (TxB(2)) between the Raynaud's group and the normal controls. Similarly, there were no differences in these parameters in the Raynaud's group during treatment with enalapril when compared to placebo. It is concluded that patients with primary Raynaud's phenomenon have no evidence of abnormal platelet aggregation or increased platelet activation, and that platelet aggregation is not affected by enalapril.     

Beta thromboglobulin and platelet factor 4 in bronchoalveolar lavage fluid of patients with systemic sclerosis

OBJECTIVE: To evaluate concentrations of the platelet activation markers beta thromboglobulin (BTG) and platelet factor 4 (PF4) in bronchoalveolar lavage fluid (BALF) from patients with systemic sclerosis with and without scleroderma interstitial lung disease (SLD). METHODS: BTG and PF-4 were measured by enzyme immunoassay in BALF from 37 patients with systemic sclerosis. Controls were 10 healthy subjects. BALF was collected during routine bronchoscopy from the right middle lobe. SLD was diagnosed by high resolution computed tomography of the lungs. RESULTS: BTG was detected in 11 of the patients with systemic sclerosis (29.7%) and PF4 was found in eight (21.6%). Mean (SD) concentrations of BTG and PF4 in BALF from patients with detectable levels of these platelet activation markers were 106.9 (69.8) and 35.2 (17.4) IU/ml, respectively. The BTG:PF4 ratio was more than 2:1, indicating in vivo release. Both markers were found exclusively in patients with SLD. SLD patients with detectable platelet activation markers had a significantly shorter disease duration than those with undetectable BTG/PF4. CONCLUSIONS: The study provides evidence that activation of blood platelets takes place within the lungs of patients with SLD and may contribute to the development of lung fibrosis.     

Plasma beta-thromboglobulin as a measure of platelet activity: Effect of risk factors and findings in ischemic heart disease and after acute myocardial infarction

The plasma concentration of beta-thromboglobulin (BTG), a platelet-specific protein released during platelet aggregation, is considered a sensitive marker of in vivo platelet activity. The mean plasma level in 133 asymptomatic individuals was 32.3 ± 1.1 ng/ml, and there was no difference between those with no risk factors (32.2 ± 1.2 ng/ml, n = 56), those who smoked (31.8 ± 1.8 ng/ml, n = 45), those with hyperlipidemia (32.8 ± 1.7 ng/ml, n = 15), and those exposed to both of these risk factors (34.1 ± 2.7 ng/ml, n = 17). The mean plasma BTG level in 104 patients with symptomatic ischemic heart disease was significantly elevated (40.9 ± 1.4 ng/ml, p < 0.01), but there was considerable overlap with normal levels. Although no difference was found between patients with no risk factors (38.1 ± 4.0 ng/ml, n = 13) and those with only 1 risk factor (37.0 ± 1.8 ng/ml, n = 44), patients with 2 or more risk factors had a significantly elevated plasma BTG level (45.2 ± 2.2 ng/ml, n = 47, p < 0.01). It is concluded that risk factors themselves do not increase platelet activity, but that patients with vascular disease have activated platelets that may contribute to the progression of the disease. Plasma BTG was also measured serially for 10 days in 29 patients after hospitalization with acute ischemic cardiac pain. Although the median plasma level was elevated above normal there were no acute changes in plasma BTG after either acute infarction (n = 22) or acute ischemia (n = 7), except in 2 patients in whom pericardial friction rubs developed. Thus, measurement of systemic plasma BTG did not detect platelet involvement in acute coronary occlusion or acute ischemia.     

Platelet BTG release in vitro induced by mechanical and chemical stimulus: correlation with the aggregation curve parameters

The in vitro release of B-thromboglobulin (BTG) from platelets after stirring the platelet rich plasma (PRP) 5 min (37 degrees C/1000 rpm) was studied in a group of 13 healthy subjects before and after 'in vivo' aspirin ingestion, 1 g/d for 3 d. A significant reduction in the platelet BTG release (31%) was observed after aspirin ingestion in comparison to the pre-aspirin values, suggesting that the BTG release by stirring is related to a platelet activation process. Small doses of collagen (0.625 microgram/ml) and thrombin (0.1 U/ml) always released platelet BTG, whether or not there was a visible platelet aggregation, suggesting that platelet activation is not necessarily followed by visible platelet aggregation. A significant correlation was found between the BTG released by stimulation with collagen (2.5 microgram/ml) and thrombin (0.2 U/ml) and the maximum velocity and intensity of the aggregation curve parameters.     

Imbalance of Angiopoietin-1 and Angiopoetin-2 in Severe Dengue and Relationship with Thrombocytopenia,Endothelial Activation,and Vascular Stability

The pathogenesis of plasma leakage during dengue hemorrhagic fever/dengue shock syndrome (DHF/DSS) is largely unknown. Angiopoietins are key regulators of vascular integrity: Angiopoietin-1 is stored in platelets and maintains vascular integrity, and endothelium-derived angiopoietin-2 promotes vascular leakage. We determined angiopoietin-1 and angiopoietin-2 levels in a cohort of children in Indonesia with DHF/DSS and related them to plasma leakage markers. Patients with DHF/DSS had reduced angiopoietin-1 and increased angiopoietin-2 plasma levels on the day of admission when compared with levels at discharge and in healthy controls. There was an inverse correlation between angiopoietin-1 and markers of plasma leakage and a positive correlation between angiopoietin-2 and markers of plasma leakage. Angiopoietin-1 levels followed the same trend as the soluble platelet activation marker P-selectin and correlated with platelet counts. Dengue-associated thrombocytopenia and endothelial activation are associated with an imbalance in angiopoietin-2: angiopoietin-1 plasma levels. This imbalance may contribute to the transient plasma leakage in DHF/DSS.     

Effects of interleukin-2 administration on platelet function in cancer patients

Platelet function in 16 patients with metastatlc renal cell carcinoma and melanoma was studied sequentially over the first 96 hr of treatment with moderate and highdose inter-leukin-2 (IL-2). During the first 96 hr of therapy, an increased ex vivo platelet maximal aggregation (MA) response to ADP, epinephrine, and arachidonlc acld was paralleled by a decrease In the peripheral platelet count. Plasma speclmens from patients receiving the moderate dose schedule showed a significant IL-2 induced secretory response of the platelet α-granule components β-thromboglobulin (BTG) and platelet factor 4 (PF4) and the eicosanoid thromboxane B₂ (TBX₂) as measured by RIA. The increase in TXB₂ was highly correlated with MA when analyzed by bivariate regression analysls, whereas the addition of PF4 to TXB₂ in a multiple regression analysls further Increased their correlation to MA. The observed decrease In peripheral platelet count correlated significantly with MA and PF4 secretion. High-dose IL-2-treated patlents showed a statistically significant increase in the percentage of large platelets exceeding 12 fl in diameter and platelet responsiveness to hypotonic shock. These observations suggest that IL-2 therapy results in a reduced peripheral platelet pool, with an increased proportion of the remaining pool of platelets larger, more viable, and actlvated. © 1994 Wiley-Liss, Inc.     

Effect of Storage on Platelet Release and Aggregation Responses

Abstract. The effect of storage conditions on the aggregation and release responses of platelets have been investigated, using ADP as activating agent and β-thromboglobulin (BTG) as a measure of the release reaction. Spontaneous release of BTG during storage was also investigated. Platelets held at room temperature maintained their ability to aggregate and release BTG in response to ADP after 3 days storage. After storage for 1 day at 4°C platelets showed normal aggregation in response to ADP but did not release BTG. After 4–6 days, platelets stored at room temperature showed a smaller spontaneous loss of BTG than those stored at 4°C. Spontaneous release of BTG from platelets in whole blood stored at 4°C was significantly lower than from platelets stored as concentrates, although platelets prepared from whole blood stored for 1 day showed neither release nor aggregation in response to ADP stimulation. We suggest that ADP-stimulated BTG release may be a sensitive test of platelet function during storage.     

Enhanced platelet release reaction in insulin-dependent and insulin-independent diabetic patients

Plasma beta thromboglobulin (BTG) and platelet factor 4 (PF4) levels were measured and correlated with plasma lipid and lipoprotein patterns in 20 insulin-independent diabetics, in 10 insulin-dependent diabetics and in 30 healthy controls matched to patients as to age, sex and weight. Increased plasma BTG and PF4 levels both in insulin-dependent and in insulin-independent diabetics indicate enhanced in vivo platelet activation and release reaction in these patients. No difference in BTG and PF4 values between these two groups of diabetic patients was observed in our study. A marked correlation between plasma PF4 values and plasma glucose levels was shown in insulin-dependent diabetics only whereas a significant positive correlation with plasma triglycerides and plasma triglyceride VLDL and a negative correlation with cholesterol-HDL was shown in insulin-independent diabetics.     

Patients with paroxysmal atrial fibrillation but not paroxysmal supraventricular tachycardia display evidence of platelet activation during arrhythmia

It is well known that chronic atrial fibrillation (CAF) and paroxysmal atrial fibrillation (PAF) are associated with hypercoagulable state. However, pathological hemostatic changes during the paroxysmal supraventricular tachycardia (PSVT) have not yet been elucidated. To determine platelet activity in patients with PSVT, PAF and CAF, we examined the levels of beta-thromboglobulin (BTG) and platelet factor 4 (PF4) during tachyarrhythmia attacks. We measured the levels of BTG and PF4, as an index of platelet activation in 15 patients with PAF (9 men, mean age 45+/-11), and 14 patients with PSVT (8 men, mean age 40+/-10). Levels were compared to 22 age and sex matched healthy controls in sinus rhythm and with 25 patients with CAF (16 men, mean age 51+/-12). Blood samples were taken during arrhythmia and 24 hours after conversion to sinus rhythm. Patients taking medications or have clinical conditions that may affect the BTG and PF4 levels were excluded. In patients with PAF, BTG and PF4 levels were significantly higher than in controls (p<0.009, and p=0.002, respectively), and in patients with PSVT (p<0.004, and p=0.009, respectively), however, BTG and PF4 levels were significantly lower than CAF patients (p=0.002, and p=0.02, respectively). Moreover, BTG and PF4 levels were significantly decreased 24 hours after conversion to sinus rhythm (p<0.0001), and p=0.004, respectively). Although BTG and PF4 levels in patients with PSVT were significantly lower than in patients with PAF (p=0.04, and p=0.009, respectively) and CAF (p=0.0001, and p=0.0001, respectively), BTG and PF4 levels were similar to controls and did not change significantly after recovery to sinus rhythm (p=NS for all). These results indicate that there was no platelet activation in patients with PSVT during tachyarrhythmia but significantly increased platelet activity in PAF and CAF patients. There was a significant decrement of the platelet activity to a level of control subjects twenty-four hours after cardioversion of PAF.     

Effects of the platelet inhibitor ticlopidine on exercise tolerance in stable angina pectoris

Coronary blood flow might be reduced by platelet aggregatesor by vasospasm induced by platelet-produced thromboxane A2.Therefore the effects of the platelet inhibitor ticlopidine(500 mg daily) on platelet function and on exercise tolerancewere investigated in a double-blind placebo-controlled studyin 38 middle-aged men with stable incapacitating angina pectoris.Before and after 4 and 8 weeks of treatment, exercise testswere performed in warm and cold environments. The in vitro plateletreactivity to ADP was determined at rest and the plasma levelsof beta-thromboglobulin (BTG) and platelet factor 4 (PF4) weremeasured before and immediately after exercise. There were nosigns of increased platelet activity at rest or after exerciseas judged by the levels of BTG and PF4. Despite a potent inhibitionof platelet reactivity to ADP in vitro during ticlopidine treatment,the exercise tolerance was reduced in exercise tests in bothwarm and cold environments and in daily life. Therefore plateletactivity does not seem to play any significant role in exercisetolerance in the stable phase of angina pectoris.     

Differential transmyocardial platelet behavior in response to pacing and ergonovine-induced myocardial ischemia

In 17 anginal patients with critical narrowing of the left anterior descending artery, we studied the effects of acute ischemia, either induced by atrial pacing or by ergonovine, on transmyocardial platelet behavior. Six other patients with atypical chest pain and normal coronary arteries served as controls. Simultaneous arterial and great cardiac vein samples were drawn during control and ischemia to measure the levels of platelet factor four (PF4) and beta-thromboglobulin (BTG). During pacing-induced ischemia the great cardiac vein-arterial differences of PF4 and BTG decreased significantly, indicating a reduced platelet aggregability; no significant changes were observed in the control patients. By contrast, when ischemia resulted from ergonovine-induced spasm of the left anterior descending artery (five patients), the great cardiac vein-arterial differences increased, indicating enhanced platelet aggregability. Again no differences were observed in the patients with a negative ergonovine test. The results of our study suggest that the transcardiac platelet behavior may vary during different ischemic conditions. When ischemia is due to increased myocardial demands and flow is normal or increased, myocardial metabolites released from the ischemic area may oppose platelet aggregation. By contrast, spasm and the stagnant flow resulting from it may enhance platelet aggregation.     

Desialylation of plasma proteins in severe dengue infection: possible role of oxidative stress

Oxidative stress in dengue infection has been suggested. This study was carried out to explore the plasma protein oxidation and its sialic acid content in dengue infection. Thirty-two dengue hemorrhagic fever (DHF), 25 dengue shock syndrome (DSS), 29 dengue fever (DF), and 63 healthy controls were included in this study. The extent of carbonylation, sulphydryl content, and desialylation of plasma protein was estimated in acute phase sample. Significantly higher levels of protein carbonyls and lower levels of sialic acid and sulphydryl groups were found in DHF and DSS compared with DF using one-way analysis of variance. Regression analysis showed that desialylation is dependent on protein carbonyls in DHF/DSS. This study indicates that, in dengue infection, plasma proteins undergo increased levels of desialylation, which can be attributed to the oxidative stress. Future studies on sialylation status of endothelium and platelets can show light into the pathogenesis of the dengue infection.     

Influence of the dengue serotype, previous dengue infection, and plasma viral load on clinical presentation and outcome during a dengue-2 and dengue-4 co-epidemic

Martinique experienced a dengue outbreak with co-circulation of DENV-2 and DENV-4. In an emergency department-based study, we analyzed whether the clinical presentation and outcome of adult patients were related to serotype, immune status, or plasma viral load. Of the 146 adult patients who had confirmed dengue infection, 91 (62.3%) were classified as having classic dengue fever, 11 (7.5%) fulfilled World Health Organization criteria for dengue hemorrhagic fever (DHF) or dengue shock syndrome (DSS), 21 other patients (14.4%) presented with at least one typical feature of DHF/DSS [i.e., internal hemorrhage, plasma leakage, marked thrombocytopenia (platelet count < or = 50,000 platelets/mm(3)) and/or shock], and 23 further patients (15.8%) had unusual manifestations. Four patients died. Severe illness was more frequent in patients with secondary dengue infection (odds ratio, 7.18; 95% confidence interval, 3.1-16.7; P < 0.001). Multivariate regression analysis showed that gastrointestinal symptoms and other unusual manifestations were independently associated with DENV-2 infection, whereas cough and DHF/DSS features were independently associated with secondary immune response. A high plasma viral load was associated with DENV-2 infection, increased serum liver enzymes, and with DHF/DSS features in patients presenting after the third day of illness. The most severe cases of dengue resulted from the combined effects of DENV-2 and secondary infection.     

Hemostatic effects of hormonal stimulation in patients with metastatic prostate cancer

Hemostatic abnormalities are common in patients with metastatic malignancy and are attributed, in part, to materials secreted by tumor cells. Tumor stimulation might therefore cause further perturbation of hemostasis. This article reports observations on the effects of androgen stimulation on multiple hemostatic parameters in patients with metastatic prostate cancer. Testosterone was given before chemotherapy in an experimental protocol designed to increase tumor sensitivity to cytotoxic agents. The following parameters were measured on day 0 (before) and days 2 and 4 of fluoxymesterone administration: PT, APTT, platelet count, plasma betathromboglobulin (BTG), platelet factor 4 (PF4), fibrinogen, fibrin(ogen) split products (FSP), factor VIII coagulant activity (VIII C), von Willebrand factor antigen (vWF Ag), fibrinopeptide A (FPA), antithrombin III (AT III), and protein C antigen (PC). Ten patients were studied during 17 cycles of hormonal stimulation. Baseline levels of BTG, PF4, fibrinogen, FSP, factor VIII C, vWF Ag, and FPA were significantly elevated compared with normal control. Although androgen stimulation resulted in elevation of BTG, FPA, and FSP levels by day 4 in many patients, the changes for the entire group were not statistically significant. Other parameters remained unchanged or were only slightly elevated. Two patients developed laboratory evidence of disseminated intravascular coagulation (DIC) but were clinically unaffected. Our data suggest that most patients with metastatic prostate cancer show evidence of ongoing activation of platelets, coagulation, and fibrinolysis. In a few individual patients, androgen stimulation of this hormonally dependent tumor may cause further activation of platelets, coagulation, and fibrinolysis.     

Heparin-induced thrombocytopenia: new evidence for the dynamic binding of purified anti-PF4-heparin antibodies to platelets and the resultant platelet activation

Immune heparin-induced thrombocytopenia (HIT) is associated with antibodies directed against a complex of platelet factor 4 (PF4) and heparin. We were able to affinity purify anti-PF4-heparin IgG (HIT IgG) from the plasma of 2 patients with HIT. Under conditions that were more physiological and sensitive than those in previous studies, we observed that this HIT IgG caused platelet aggregation on the addition of heparin. Platelets activated with HIT IgG increased their release and surface expression of PF4. We quantitated, for the first time, the binding of affinity-purified HIT iodine 125-IgG to platelets as they activated in a plasma milieu. Binding of the HIT IgG was dependent on heparin and required some degree of platelet activation. Blocking the platelet FcgammaRII with the monoclonal antibody IV.3 did not prevent HIT IgG binding to activated platelets. We concluded that anti-PF4-heparin IgG is the component in these HIT plasmas that induces platelet aggregation. The Fab region of HIT IgG binds to PF4-heparin on the surface of activated platelets. We propose that only then does the Fc portion of the bound IgG further activate the same or adjacent platelets through the Fc receptor. Our data support a dynamic model of platelet activation in which released PF4 enhances further antibody binding and more release.     

Plasma concentrations of platelet-specific proteins and serum thromboxane B2 production in response to treatment with dipyridamole. A pilot study of 27 post-myocardial infarction patients

In the present study 27 post-myocardial infarction patients were treated with Persantin capsules (Depot-Kapseln, sustained release form), containing 200 mg dipyridamole, b.i.d. over a period of 3 weeks. Baseline levels for plasma beta-thromboglobulin (BTG), platelet factor 4 (PF4), and serum thromboxane B2 (TXB2) were obtained on day 0 and subsequently on days 1, 3, 5, 7, 14 and 21. The baseline levels for plasma BTG and PF4 as well as for serum TXB2 significantly exceeded those for a control group consisting of healthy subjects. The plasma values for BTG and PF4 remained unchanged during the whole study period. During the first week of study the levels for serum X TXB2 were unchanged; however, on days 14 and 21 the means for TXB2 dropped significantly. There is experimental work to suggest that dipyridamole may exert an inhibitory effect on platelet thromboxane biosynthesis. The present results support this concept.     

A prospective study of haemostatic parameters in relation to the clinical course of myeloproliferative disorders

Abstract: Platelet function and the clinical course of the disease were prospectively investigated in 29 patients with myeloproliferative disorders. Serial determinations (median: 5 investigations per patient within 17 months) of platelet aggregation, plasma and intraplatelet concentrations of β-thromboglobulin (βTG) and platelet factor 4 (PF4), and of fibrino-peptide A (FPA) plasma levels were carried out. In the chronic phase of polycythaemia vera, patients with thrombohaemorrhagic complications during the study period had higher platelet count, more severe platelet aggregation defects, and increased plasma levels of βTG and FPA compared to patients without complications. However, thrombohaemorrhagic complications were not predicted by changes in these parameters in the individual patient during the chronic disease phase. When patients with chronic myelogenous leukaemia entered blast crisis, bleeding complications were related to thrombocytopenia, impaired platelet function and low intraplatelet concentrations of βTG and PF4. Cytoreduction by chemotherapy in the chronic phase of CML did not alter βTG and PF4 plasma levels, whereas treatment of polycythaemia rubra vera by venesection favourably influenced platelet α-granule secretion and increased intraplatelet concentrations of βTG and PF4.     

Changes in platelet ATP secretion and aggregation during pregnancy and in preeclampsia.

BACKGROUND: Platelet secretion plays an important role in the aggregation of platelets. However, the quantitative relationship between platelet aggregation and secretion of ATP during pregnancy and in pre-eclampsia has yet to be clarified. This study is designed to determine whether platelet count, volume, aggregation, and the amount of secreted ATP change in healthy, nonpregnant women, nonpreeclamptic pregnant women, and preeclamptic pregnant women and whether beta-thromboglobulin (BTG) and platelet factor 4 (PF-4) concentrations alter in nonpreeclamptic and pre-eclamptic women. METHODS: Peripheral blood was collected from 114 women. Nonpreeclamptic pregnant women were divided into four groups (gestational weeks 10, 20, 30, and 35). Platelet aggregation and ATP secretion were investigated with the use of a lumi-aggregometer. BTG and PF-4 concentrations in peripheral blood were determined in 12 pregnant and 11 preeclamptic women. RESULTS: The amount of secreted ATP upon induction by 5 microM ADP increased significantly (P < 0.05-0.01) with gestational age. On the other hand, the amount of secreted ATP induced by 5 microg/mL collagen reached the maximal value from gestational weeks 20 to 35 in nonpreeclamptic women. Significantly more platelet aggregation was induced by the ADP and collagen in nonpreeclamptic women in gestational weeks 20 and 30 than in the gestational weeks 10 or 35 (P < 0.05-0.005). The amount of secreted ATP and platelet count were significantly lower (P < 0.05) in preeclampsia than in normal pregnancy. The BTG and PF-4 concentrations were significantly higher (P < 0.05) in preeclampsia than in normal pregnancy. CONCLUSIONS: The sensitivity of platelets for ATP secretion may intensify with progression of pregnancy. In normal pregnancy, around gestational week 35, the platelets may exhibit weaker ability to aggregate but maintain the capacity to secrete ATP. In preeclampsia, secreted ATP decreased because platelets may be stimulated to undergo a partial secretion.     

Platelet-released proteins as molecular markers for the activation process

The desire to have a specific, sensitive marker for platelet activation was originally thought to lie in the development of RIAs for BTG and PF4. Although this wish has not been denied, the interpretation of the information obtained from such an analysis has proved far less rewarding. The principal challenge of these procedures is based upon the lack of a cause and effect relationship between a given disease and platelet activation, coupled with the differential clearance rate and mechanism for each of the discussed proteins. Thus, we have seen that the renal clearance rate and mechanism for each of the discussed proteins. Thus, we have seen that the renal clearance of a patient should be noted prior to interpreting the elevation of BTG. Similarly, since PF4 is removed from the circulation so rapidly, its plasma values tend to be lower than BTG by a factor of 5, although the significance of the BTG to PF4 ratio is questioned. Administration of heparin results in a heparin-induced increase in plasma PF4 levels but not for BTG, and this PF4 increase can be as great as 20-fold. PF4 and BTG values are also directly increased by pressure increases. Taken individually, these mediators each compromise the ability to correlate the significance of platelet protein increases with any single pathologic condition. When viewed collectively, an analysis of platelet-released proteins is best interpreted as an indication that the functional integrity of the platelet has been perturbed; the direct relationship of disease processes to platelet release is far from certain and is simply documented by the use of such described procedures. The final note of caution for these assays is to be found in the recent summary analysis of the standardization of both BTG and PF4. In this study, considerably greater variation among laboratories was noted for PF4 than was seen for BTG, and the study directors concluded that comparisons of results between laboratories should be regarded as unreliable due mainly to the use of different standards for each protein in a given laboratory. The final note of caution for these assays is to be found in the recent summary analysis of the standardization of both BTG and PF4. In this study, considerably greater variation among laboratories was noted for PF4 than was seen for BTG, and the study directors concluded that comparisons of results between laboratories should be regarded as unreliable due mainly to the use of different standards for each protein in a given laboratory.