Platelet Function Testing Is Commonly Performed in Patients with Known Confounding Factors

Introduction: Commonly used platelet function tests include Platelet Function Analyzer -100 Closure Time (PFA-100), used as a screening test, and platelet aggregation testing by Light Transmission Aggregometry (LTA), which can help differentiate between various types of platelet function defects and guide further testing. In this study, we assessed the indications for platelet aggregation testing at our center, and the effect of factors that may interfere with the outcomes of testing.

Methods: This was a retrospective electronic medical records based study in which we included all patients in our healthcare network who had platelet aggregation testing done using LTA between August 2008 and August 2013. Routine descriptive measures were used for frequencies and proportions. McNemar test was used for comparison of paired nominal variables and Wilcoxon signed-rank test was used for comparison of paired continuous variables. P value < 0.05 was considered significant.

Results: Four hundred ninety seven patients had platelet aggregation testing performed during the 5-yr study period. Among these, 315 (63%) patients had antecedent screening with PFA-100 with 157 (31%) showing at least one abnormality. Two hundred forty nine (50%) patients underwent some form of further testing after LTA, including VWF analysis (42%), platelet flow cytometry (19%) and/or electron microscopy (1%). Two hundred fifty six (51%) patients had at least one factor previously known or suspected to interfere with platelet aggregation testing (platelet count <100,000, hematocrit <25% or medications), and 205 (80%) of these patients had abnormal results in response to one or more agonists. Eighty three of these patients had repeat aggregation testing performed - 26 were re-tested after correction of 1 or more suspected factor (s), while 57 were re-tested without correction. The former group showed a significant improvement with subsequent testing, both in the average number of abnormalities in the LTA panel (P = 0.01) and in the number of patients with abnormal aggregation to ≥2 agonists (P = 0.02), while the outcomes of platelet function testing in the latter group did not significantly change on subsequent testing (P = 0.15 and 0.21 respectively for the above two parameters). Three hundred forty five (69%) patients had a current or past history of abnormal bleeding. Of these, 81% had history of spontaneous bleeding while 29% had surgical or procedure-related bleeding. The most common sites of spontaneous bleeding were skin (47%) and mucosa (45%). Eight percent of patients had a history of spontaneous bleeding in deeper sites. Fourteen percent had a family history of abnormal bleeding. The most common indication for testing was documented abnormal bleeding (62%) - recent or past. Twenty five percent underwent testing for peri-operative prediction of bleeding risk and 13% to monitor efficacy of anti-platelet therapy.

Conclusions: A majority of the patients in our study did not have abnormalities in PFA-100 testing despite abnormal results on subsequent platelet aggregometry. Fifty one percent of patients who underwent aggregometry were on medications that may affect platelet function or had other factors known or suspected to interfere with aggregation testing. Eighty percent of these patients had abnormal results. Repeat testing after correction of the known factor(s) led to a significant improvements in the results of LTA. The most common indication for platelet aggregation testing was a current or past history of abnormal spontaneous bleeding and the most common sites of bleeding in these patients were the skin and mucosal surfaces.