Whole-blood aggregometry: are there any limits with regard to platelet counts?

Comparison of multiple electrode aggregometry with lumi-aggregometry for the diagnosis of patients with mild bleeding disorders

Essentials There is a clinical need for new technologies to measure platelet function in whole blood. Mild bleeding disorders were evaluated using multiple electrode aggregometry (MEA). MEA is insensitive at detecting patients with mild platelet function and secretion defects. More studies are required to investigate MEA in patients with a defined set of platelet disorders.

SUMMARY

Background Multiple electrode aggregometry (MEA) measures changes in electrical impedance caused by platelet aggregation in whole blood. This approach is faster, more convenient and offers the advantage over light transmission aggregometry (LTA) of assessing platelet function in whole blood and reducing preanalytical errors associated with preparation of platelet-rich plasma (PRP). Several studies indicate the utility of this method in assessing platelet inhibition in individuals taking antiplatelet agents (e.g. aspirin and clopidogrel). Objective Our current study sought to evaluate the ability of MEA in diagnosing patients with mild bleeding disorders by comparison with light transmission lumi-aggregometry (lumi-LTA). Methods Forty healthy subjects and 109 patients with a clinical diagnosis of a mild bleeding disorder were recruited into the UK Genotyping and Phenotyping of Platelets study (GAPP, ISRCTN 77951167). MEA was performed on whole blood using one or two concentrations of ADP, PAR-1 peptide, arachidonic acid and collagen. Lumi-LTA was performed in PRP using several concentrations of ADP, adrenaline, arachidonic acid, collagen, PAR-1 peptide and ristocetin. Results Of 109 patients tested, 54 (49%) patients gave abnormal responses by lumi-LTA to one or more agonists. In contrast, only 16 (15%) patients were shown to have abnormal responses to one or more agonists by MEA. Conclusions In this study we showed that MEA is less sensitive in identifying patients with abnormal platelet function relative to lumi-LTA.

Platelet aggregation is dependent on platelet count in patients with coronary artery disease

INTRODUCTION

Platelet function testing in whole blood is widely used to evaluate the effect of antiplatelet agents, but it is not known whether results are affected by whole blood parameters. This study investigated the importance of platelet count, haematocrit, red blood cells (RBC), and white blood cells in whole blood platelet aggregometry.

MATERIALS AND METHODS

We included 417 patients with coronary artery disease on aspirin mono-therapy and 21 aspirin-naïve healthy individuals. Blood sampling was performed one hour after aspirin ingestion. The antiplatelet effect of aspirin was evaluated using the VerifyNow® Aspirin assay and multiple electrode aggregometry (MEA, Multiplate®) induced by collagen (1.0 μg/mL) and arachidonic acid (1.0 or 0.75 mmol/L). Measurements of whole blood parameters were performed to evaluate the three major cell lines in circulating blood.

RESULTS

In patients, platelet count correlated significantly with platelet aggregation (MEA(collagen), p<0.0001; MEA(arachidonic acid), p<0.0001; VerifyNow®, p=0.03). Haematocrit and RBC correlated inversely with MEA induced by collagen (p(haematocrit)<0.001; p(RBC)=0.07) and with VerifyNow® (p(haematocrit)<0.0001; p(RBC)<0.0001), but not with MEA induced by arachidonic acid (p(haematocrit)=1; p(RBC)=0.87). White blood cells correlated significantly with platelet aggregation (MEA(collagen), p<0.001; MEA(arachidonic acid), p<0.0001; VerifyNow®, p=0.05). Similar associations were observed in aspirin-naïve healthy individuals.

CONCLUSIONS

Whole blood aggregometry is dependent on all major cell lines in whole blood. Importantly, platelet aggregation is significantly associated with platelet count even within the normal range.

Platelet function following trauma. A multiple electrode aggregometry study

Platelets play a central role in coagulation. Currently, information on platelet function following trauma is limited. We performed a retrospective analysis of patients admitted to the emergency room (ER) at the AUVA Trauma Centre, Salzburg, after sustaining traumatic injury. Immediately after admission to the ER, blood was drawn for blood cell counts, standard coagulation tests, and platelet function testing. Platelet function was assessed by multiplate electrode aggregometry (MEA) using adenosine diphosphate (ADPtest), collagen (COLtest) and thrombin receptor activating peptide-6 (TRAPtest) as activators. The thromboelastometric platelet component, measuring the contribution of platelets to the elasticity of the whole-blood clot, was assessed using the ROTEM device. The study included 163 patients, 79.7% were male, and the median age was 43 years. The median injury severity score was 18. Twenty patients (12.3%) died. Median platelet count was significantly lower among non-survivors than survivors (181,000/μl vs. 212,000/μl; p=0.01). Although platelet function defects were relatively minor, significant differences between survivors and non-survivors were observed in the ADPtest (94 vs. 79 U; p=0.0019), TRAPtest (136 vs. 115 U; p<0.0001), and platelet component (134 vs.103 MCEEXTEM - MCEFIBTEM; p=0.0012). Aggregometry values below the normal range for ADPtest and TRAPtest were significantly more frequent in non-survivors than in survivors (p=0.0017 and p=0.0002, respectively). Minor decreases in platelet function upon admission to the ER were a sign of coagulopathy accompanying increased mortality in patients with trauma. Further studies are warranted to confirm these results and investigate the role of platelet function in trauma haemostatic management.

The influence of low platelet count on whole blood aggregometry assessed by Multiplate

The Multiplate, a whole blood (WB) platelet function test, has shown promising results identifying patients on antiplatelet therapy at increased risk of rethrombosis. In the present study, the influence of low platelet count on platelet aggregation was analyzed and compared with aggregation results in an artificial matrix, platelet-rich plasma (PRP). Heparinized and citrated blood was diluted with autologous plasma to platelet concentrations 200 to 25 × 10(9)/L in WB samples (n = 10) and 200 to 100 × 10(9)/L in PRP samples (n = 7). The platelet aggregation was investigated by the ADP-, ASPI-, COL-, and TRAP-test. The WB responses decreased at platelet concentration of ≤100 × 10(9)/L (all P < .03), except for heparin-TRAP (50 × 10(9)/L, P = .008) and citrate-ASPI (150 × 10(9)/L, P = .03). In general, WB samples demonstrated higher aggregation than PRP samples at platelet concentrations 200 to 100 × 10(9)/L (P < .05). In conclusion, platelet concentration of <150 × 10(9)/L may influence Multiplate which should be considered in clinical settings. Furthermore, the findings emphasize the importance of evaluating haemostasis in its natural matrix, WB.

BAPA, a synthetic dual inhibitor of Factor Xa and Thrombin, extends the storage-time to a maximum of 12 hours in ADP- and 24 hours in arachidonic acid-induced impedance aggregometry

Impedance aggregometry is mainly limited by the time-dependent instability of platelets in whole blood samples, caused by the influence of ex-vivo anticoagulants. The synthesis of a new anticoagulant, Benzylsulfonyl-D-Arg-Pro-4-amidinobenzylamide (BAPA), has recently been introduced. Data from recent studies suggest stable results for up to 32 h. In view of this data we were interested in the results of ADP- and arachidonic acid (AA)-induced measurements after a prolonged storage time depending on the use of citrate or BAPA. Blood samples from 24 healthy individuals were anticoagulated with either citrate or BAPA. ADP- and AA-induced measurements were carried out between 2 h and up to 48 h after blood samples were taken. Results of ADP-induced measurements remained stable for 8 h in citrated and for a maximum of 12 h in BAPA-anticoagulated samples. AA-induced measurements yielded stable results up to 12 h in citrated and up to 24 h in BAPA anticoagulated blood. These data demonstrate that the storage time for whole blood impedance aggregometry can be prolonged when BAPA is used as an anticoagulant. However, in contrast to previous studies implicating considerably longer potential storage times we could not see a clear overall purpose in using BAPA. Further studies including blood samples of patients and the comparison of BAPA to other anticoagulants are necessary to define the potential role of BAPA anticoagulated samples.

Platelet monitoring follow-up in a pregnant patient with HELLP syndrome

Monitoring the course of platelet function in HELLP (haemolysis, elevated liver-enzymes and low platelets) syndrome is important for clinical decision-making. We present a primigravid woman developing HELLP syndrome at 29 weeks and 6 days. Platelet function was monitored by multiple electrode aggregometry (MEA), platelet function analyzer (PFA-100®), platelet count and mean platelet volume (MPV) over an 11-day period. MPV and PFA-100® seem better predictors for platelet function than platelet levels.