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

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.

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.

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).

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.