Overestimation of Automated Platelet Counts by Blast Fragments in Acute Hypercellular Leukemias: A Retrospective Study Comparing Impedance, Optical (PLT-O), Fluorescent (PLT-F), and CD41/CD61 Flow Cytometry Methods

Introduction

Acute leukemias with hyperleukocytosis (> 100 × 10⁹/L) can produce cytoplasmic blast fragments that interfere with platelet counts, notably in impedance-based methods, potentially masking severe thrombocytopenia and increasing hemorrhagic risk. While fluorescent platelet counting (PLT-F) is promoted as platelet-specific, its accuracy in the presence of blast fragments remains uncertain.

Methods

This retrospective study analyzed 269 blood samples from 87 patients with hypercellular acute leukemia. Blast fragments were identified on blood smears. Platelet counts by impedance were compared to optical (PLT-O) and fluorescent (PLT-F) methods when available. Flow cytometry (FC) quantification by CD41+/CD61+ staining was performed in selected cases.

Results

Blast fragments were observed in 26% of cases, predominantly in AML-M5 and AML-M1 subtypes. In the absence of blast fragments, PLT-I, PLT-O, and PLT-F showed comparable results. However, in samples with blast fragments, PLT-I frequently overestimated platelet counts compared to PLT-O and PLT-F. PLT-F counts were closer to FC quantification but still overestimated platelet numbers in 6 of 16 samples with FC results, particularly in cases of severe leukocytosis. Notably, PLT-F failed to trigger abnormal scattergram flags in all but one of the discrepant cases. PLT-O provided results comparable to PLT-F in most cases with blast fragments but also demonstrated limitations in select cases.

Conclusion

PLT-O and PLT-F methods both have limitations in hypercellular acute leukemias with blast fragments. FC remains the most reliable approach when blast fragments are present. Routine blood smear evaluations are essential for detecting interferences and ensuring accurate thrombocytopenia assessment in these high-risk patients.