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Objective: The purpose of this study was to compare platelet concentration in cavalier King Charles spaniels (CKCS) measured by different methods commonly used in veterinary hospitals and commercial laboratories.

Methods: Blood samples obtained from 41 (CKCS) were analysed by impedance cell counter, laser cell counter and microscopic estimation. Quantitative buffy coat analysis was performed only on 17 samples, selected from (CKCS) that had low platelet counts detected by cell counters. Platelet counts, platelet estimations and platelet parameters using these different methods were compared.

Results: The median platelet number was lower when estimated using impedance cell counter (1363×109/l) with respect to laser cell counter (1723×109/l), microscopic estimation (2383×109/l) or quantitative buffy coat analyser (2923×109/l) (P<0·01). Although impedance cell counter, laser cell counter and microscopic estimation were positively correlated, there was no acceptable agreement among methods. (CKCS) with macrothrombocytes in blood smears had significantly lower counts on impedance cell counter, laser cell counter and microscopic estimation. The percentages of (CKCS) with platelet count < 1003×109/l were 34·1 per cent (impedance cell counter), 26·8 per cent (laser cell counter), 22·0 per cent (microscopic estimation) (not statistically different) and 5·8 per cent (quantitative buffy coat analyser) (P<0·05).

Clinical Significance: (CKCS) with macrothrombocytosis have low platelet counts on impedance cell counters, laser cell counters and microscopic estimation. (CKCS) with low platelet counts may have a normal platelet crit detected by a quantitative buffy coat analyser and thus a normal circulating platelet mass.