Electronic counting of dog leucocytes. Discrepancies arising from calibration with Coulter standard 4C and with the haemocytometer

The size distributions of leucocytes in canine blood and in standard 4C are markedly different. The use of 4C to calibrate Coulter counters may result in the selection of a threshold setting for canine leucocytes which is too high. Repeated hand counting may be used as a method of calibration, but regular discrepancies occur between hand and electronic counts which are attributable to the differing lytic actions of the diluents used, acetic acid having a more marked effect than commercial Zapoglobin. Canine leucocytes did not show significantly increased lysis when subjected to Zapoglobin at approximately four times the standard concentration, but did do so on exposure to the standard concentration for longer than five minutes. The degree of discrepancy between hand and electronic counts varied in individual dogs suggesting that there is an inconstant leucocyte subpopulation which behaves differently in response to different lytic agents.