Preserving whole blood in formalin extends the specimen stability period for manual cell counts for fish

Total blood cell counts for fish are challenging, especially due to cell fragility. Thrombocyte aggregation, cell distortion, and lysis can occur within hours of collection; therefore, hemocytometer counts may not be possible for large sample numbers. Preserving whole blood in formalin offers a simple way to extend the specimen stability period for cell counts.

The objectives of this study were to characterize and compare the analytical performance of the Natt-Herrick method for fish blood cell counts using freshly collected and formalin-fixed specimens.

Specimens from 11 elasmobranch species (n = 36) were compared for WBC counts. WBC, RBC, and thrombocyte counts from 50 striped bass, Morone saxatilis, were compared including 2 time points for the formalin-preserved cells (appr. 1-week and 1-month storage). Coefficient of variation (CV), bias, and total error (TEcalc ) were calculated. TEcalc was compared with allowable total error (TEa ) defined by the Clinical Laboratory Improvements Amendment (CLIA) for human blood cell counts.

The CV and TEcalc for fresh and fixed WBC and thrombocyte counts met CLIA limits. In both the fresh and preserved cells, the RBC CV and TEcalc were nearly 3 times higher than the TEa .

Preserving freshly collected blood in 10% formalin is a reliable method to maintain cell morphology for manual counts for up to 1 month post collection. This is especially useful for field studies, where laboratory access is limited. Further evaluation is needed to determine the clinical usefulness of the manual RBC count in fish.