Impact of Heat Block Drying on Morphologic Preservation and Staining of Peripheral Blood Smears

INTRODUCTION: Morphologic evaluation of the peripheral blood smear is critical for diagnosis and ruling out certain hematologic disorders. While air-drying is standard, some laboratories may use heat blocks to expedite drying. Excessive heat may induce morphologic artifacts, but there is a lack of systematic literature on this practice. METHODS: Thirteen residual clinical samples representing normal and pathologic conditions were prepared in triplicate and dried under three conditions: ambient air, 37°C, and 45°C heat blocks. Smears were stained and evaluated by two trained pathology residents using a 100-cell WBC differential, semiquantitative grading of RBC forms (0-3+), and subjective smear quality scoring (0-5). Discrepancies were reconciled after unblinding. RESULTS: Average smear quality was 3.5 (SD 1.1) with no significant difference across drying conditions (p > 0.05). WBC differential counts were unaffected by heat treatment. All diagnostically relevant morphologic findings (e.g., blasts, hypersegmented neutrophils, hereditary RBC forms) were identified in all conditions. However, smears dried at 45°C exhibited spurious morphologic changes, including increased schistocytes (5 cases), spherocytes (5 cases), and elliptocytes (3 cases). CONCLUSIONS: Heat block drying at 37°C yields morphology that approximates air drying but would require internal validation prior to implementation. In contrast, drying at 45°C possibly introduces clinically significant artifacts, risking false-positive interpretations especially for hemolysis. Laboratories should avoid drying at high heat, internally validate any heat block use, and pursue standardized smear preparation protocols to minimize preanalytical variability.