A Critical Investigation into the Existence of Circulating Platelet Aggregates

Influence of mechanical trauma of blood and hemolysis on PFA-100 testing

Although the appropriate quality of samples is essential for platelet function testing, information is lacking on interference from mechanical trauma of blood and hemolysis on PFA-100 analyzer. Citrated blood collected from nine healthy volunteers was divided into three aliquots. The first aliquot ('A') was processed without further manipulation, whereas the second and third were subjected to mechanical trauma by two ('aliquot B') or four passages ('aliquot C') through a very fine needle (30 gauge) to produce hemolysis and cell trauma mimicking poor sample collection. Samples were tested on PFA-100 and Advia 2120, and plasma then separated and tested for lactate dehydrogenase (LDH) and hemolysis index. Negligible hemolysis was present in aliquot A (hemolysis index 0.2 ± 0.1, cell-free hemoglobin 0-0.5 g/l), whereas an increasing amount was present in aliquots B (hemolysis index of 13.1 ± 1.8, cell-free hemoglobin 6.0-6.5 g/l) and C (hemolysis index 24.0 ± 1.1, cell-free hemoglobin 11.5-12.0 g/l). Increases in LDH, and concomitant reductions in platelet and red blood cell counts were observed in aliquots B and C. In hemolyzed aliquots B, four out of nine samples yielded 'flow obstruction' with both PFA-100 agonist cartridges, whereas the closure times were dramatically prolonged in the remaining five samples. In hemolyzed aliquots C, flow obstruction was recorded in six of nine samples for collagen and ADP and all samples for collagen and epinephrine, whereas closure times of collagen and ADP in the remaining three samples were dramatically prolonged. Mechanical trauma of blood causing hemolysis makes PFA-100 testing unreliable. When flow obstructions are observed, the potential presence of hemolysis should be investigated.

The effect of different antithrombotic regimens on platelet aggregation after myocardial infarction

Platelet aggregate ratio (PAR) was measured according to the method of Wu & Hoak in 143 patients after acute myocardial infarction (AMI) and in 54 controls. A PAR < 1 expresses the presence of platelet aggregates. The patients were randomized to aspirin 160 mg/d, or warfarin, or aspirin 75 mg/d + warfarin. In patients on aspirin, PAR was measured 24 h after aspirin intake, and in 76 patients also 2 h after aspirin. The median PAR in patients on warfarin was 0.85, on warfarin + aspirin 0.91 and on aspirin alone 0.94, all significantly lower than the median PAR of 0.97 in the controls. In 14 patients on aspirin the PARs were below a cut-off point of 0.82 (secondary aspirin non-responders). PAR increase significantly 2 h after aspirin intake. In two patients, however, PAR remained low (primary aspirin non-responders). It is concluded that some patients do not seem to respond to aspirin, the clinical implication of which has yet to be determined.

Circulating platelet aggregates indicative of in vivo platelet activation in pulmonary hypertension

The authors investigated the existence of circulating cellular aggregates in 12 patients with moderate to severe pulmonary hypertension, using scanning electron microscopy. Peripheral venous blood was collected in the presence of 11.5 mM buffered ethylenediaminetetraacetic acid, in order to disperse freshly formed disaggregable aggregates. Irreversible aggregates represented by platelet clusters and/or platelet attachment to either leukocytes or red cells were identified in 7 patients with pulmonary hypertension. Endogenous platelet activation was further confirmed by a significant increase in plasma levels of beta-thromboglobulin in comparison with controls (33.8 +/- 14.1 vs 22.7 +/- 11.5 ng/mL respectively, p < 0.025). The presence of irreversible aggregates in the blood stream strongly suggests that cell-cell interactions actually occur in vivo in these patients. If so, therapeutic measures aimed at preventing in situ thrombosis and its consequences may be beneficial in this disorder.

Platelet antibody binding and spontaneous aggregation in 21 lupus anticoagulant patients

In order to ascertain the role of the antiphospholipid antibody in the pathogenesis of thrombotic disorders, a study of 21 lupus anticoagulant-positive patients was done by a number of serologic and functional platelet tests. In immunofluorescent studies, we found that 80% showed a mitochondrial pattern on HEp cells and all patients gave an ubiquitously intense staining of donor platelets. By a microscopic spontaneous aggregation test, all lupus anticoagulant-positive patients showed accelerated platelet aggregation which was calcium-independent. The results show that the plasma of lupus anticoagulant-positive patients contains a platelet-binding antibody and causes spontaneous agglutination of platelets. These properties may play a role in the thrombogenesis common to the antiphospholipid syndrome.

Dipyridamole inhibits red cell-induced platelet activation

We have recently shown that red blood cells can induce spontaneous platelet aggregation (SPA) in whole blood ex vivo, which could be inhibited by dipyridamole. Since this drug, at therapeutic doses is not an effective inhibitor of platelet aggregation in platelet rich plasma, the inhibition of platelet interaction with the red cell was thought to be the mechanism of its action. Values for the percentage fall in the single platelet count due to SPA in whole blood after 3 and 6 min rollermixing were: control 15 +/- 2.2 and 42 +/- 2.9; 6 microM dipyridamole 6 +/- 1.1 (P less than 0.001) and 31 +/- 2.6 (P less than 0.01); 12 microM dipyridamole 2 +/- 0.9 (P less than 0.0005) and 22 +/- 2.3 (P less than 0.0005) (mean +/- SEM, n = 10). Electron microscopic observation revealed that the aggregation involves an initial platelet adhesion to the red blood cell; the adherent platelets then become activated and serve as foci for the growing aggregates. Dipyridamole appeared to inhibit the initial platelet adhesion to the red cell (the principal trigger mechanism for SPA) which may mimic the initiation of thrombosis in some situations in vivo. The inhibitory effect of dipyridamole on the platelet-red cell interaction suggests that this drug has antithrombotic potential in situations where red blood cells have a trigger role in platelet activation and may explain why the drug has been more effective in some situations than in others.

Reproducibility of two in vivo tests of platelet function

We correlated repeat determinations of platelet aggregate ratios (PAR) and plasma beta-thromboglobulin (BTG) in the right and left arms of thirty-four subjects in order to measure test reproducibility and to evaluate the effects of venipuncture. The reproducibility of PAR on the same blood samples was relatively low but similar to that obtained when the right arm and left arm values were compared, thus indicating little variation secondary to venipuncture. For BTG, the reproducibility on identical blood samples was quite high, but fell significantly when plasma samples from the two sides were correlated. This finding and increased BTG values when obvious blood collection problems occurred, indicated that plasma BTG levels are quite sensitive to venipuncture technique.

Platelet function in uncomplicated insulin-dependent diabetic patients at rest and following exercise

There are conflicting reports of platelet function abnormalities in diabetic patients without vascular complications. We have studied in vitro platelet aggregation, using platelet rich plasma and whole blood techniques, in 18 patients with uncomplicated insulin-dependent diabetes and a matched group of 24 non-diabetic subjects. In addition we measured plasma beta-thromboglobulin levels in these groups, as an index of in vivo platelet activation, and compared the indices of in vitro and in vivo platelet function before and after maximal bicycle exercise. Before exercise plasma beta-thromboglobulin levels and platelet sensitivities to ADP, collagen or adrenaline, as assessed by both methods of platelet aggregation, were the same in diabetic and control subjects. Both groups showed similar increases in beta-thromboglobulin levels and in platelet sensitivity to all agonists in whole blood following exercise. Using platelet rich plasma there were no changes in platelet sensitivity in either group after exercise. In non-diabetic subjects, increases in noradrenaline levels after exercise correlated with increases in platelet sensitivity to adrenaline in whole blood. This was not observed in the diabetic group. Abnormalities of platelet function, using the techniques described here, are not present in diabetic patients who do not have clinical evidence of vascular disease.