Pitfalls in Interpreting Platelet Function Tests in Thrombocytopenic Patients Referred by a Hematologist for Diagnostic Testing: Results from a Single Center Prospective Study.

A recent survey of clinical laboratories indicated difficulties interpreting diagnostic platelet function testing when samples have reduced platelet counts. This is problematic considering the importance of such testing to establish diagnoses for some thrombocytopenic (TCP) disorders (e.g. Bernard Soulier Syndrome, BSS).

Study goals: To evaluate findings and approaches suitable for interpreting platelet function results in TCP patients, a single center study was undertaken.

Methods: Prospectively collected data on consecutively tested patients, referred for platelet function studies in Hamilton between May 1997–2005, was reviewed to identify individuals with TCP (defined as platelets <150x10⁹/L). Medical records and laboratory databases were reviewed to obtain diagnostic information and laboratory results, including data for controls simultaneously tested at the same platelet count. Aggregation testing was done using platelet rich plasma (PRP), and most testing (80%) included: ADP, epinephrine, collagen and ristocetin, arachidonic acid and a thromboxane analague (U46619). % aggregation responses were compared to reference ranges (for samples with 250x10⁹platelets/L) and diluted control results.

Results: 17% (146/841) of individuals referred for testing had TCP. Information from medical records (available on 119/146 TCP patients) indicated diagnoses among patients included: TCP with platelet function defect (PFD; 61%), ITP (21%; 2 with associated PFD or acquired BSS), asymptomatic TCP (8%), TCP 2° to liver disease (4%), TCP 2° to known or suspected myelodysplasia (3%; 3 with PFD), lymphoma (n=1), von Willebrand disease (2%; types: 2B, n=2; 2M, n=1 with liver disease), Gaucher’s disease (n=1) and phospholipid antibody syndrome (n=1). TCP platelet disorders included: BSS (n=2; 1 acquired), asymptomatic MHY9 related disorder (n=1), autosomal dominant (AD) TCP with predisposition to AML (n=1), undefined AD TCP with PFD (n=15), and disorders with other/uncertain inheritance (n=23). Median (range) platelet counts (x10⁹ /L) of patients were 99.5 (11–149) for CBC, and 159 (9–268) for PRP tested. 2% had micro TCP and 10% macro TCP. % aggregation data showed an influence of platelet count on responses in accumulated data from control tests, and for 4 controls tested with the full agonist panel at standardized reduced platelet counts. Most % aggregation responses for control samples with ≥150x10⁹ platelets/L were within reference ranges, except with ADP and epinephrine. False positives were seen with low dose ristocetin. Many patients with TCP and PFD had reduced responses whereas those without PFD had responses similar to controls. The data from controls was important for diagnostic interpretations.

Conclusions: Laboratories need to be cautious in interpreting platelet function tests when samples have reduced platelet counts. A strategy for interpreting findings, using accumulated information from controls tested at low platelet counts, is helpful. Currently, many individuals referred by hematologists for platelet function testing with TCP have PFD due to undefined problems, many of which appear to be inherited.