Multiple methods for platelet enumeration. Observation of a newly introduced bias

After the introduction of Coulter S-Cal, a bias became apparent between platelet counts obtained from instruments calibrated with this material and those obtained from a Clay Adams Ultra-Flo. Statistical methods were used to compare platelet counts obtained from the Coulter S-Plus IV, Ortho ELT-800, Clay Adams Ultra-Flo 100, and phase microscopy. At a P value of 0.01, paired t analysis revealed statistically significant biases between the Ultra-Flo and each of the other methods. Significant biases were also found between phase microscopy and each of the other methods, although these were of a smaller magnitude. The results indicate the necessity for users of multiple platelet counting methods to conduct comprehensive interinstrument evaluations, particularly when altering methods of calibration.