Estimation of the lower limits of manual and automated platelet counting

Most evaluators of automated or manual methods for platelet counting focus on characteristics such as imprecision, linearity, and carry over. The limits of the analytical procedure are usually not assessed. The limits of the different techniques are neither discussed in the literature nor do manufacturers of analytical systems supply these data. A new procedure is presented to assess the performance of the manual as well as the automated platelet count. This procedure allows, with defined statistical confidence (eg, 95%), the determination of (1) the limit of platelet detection (LD) at which signals of platelets can be discriminated from the system noise; (2) the lower limit of quantification (LLQ), at which a certain imprecision is not surpassed; and (3) the power of definition (PD) that defines the number of values that can be discriminated in a certain interval. For each value, the PD allows calculation of the two adjacent (lower and higher) values that are significantly (P > or = 0.95) different. For the manual count, LD was found to be 1.6 x 10(9) plt/L and the LLQ 6.9 x 10(9) plt/L. For the automated count with the Technicon H1, LD was 4.3 x 10(9) plt/L and LLQ 13.8 x 10(9) plt/L (CVmax = 15%). The PD in the range 20 to 100 x 10(9) plt/L is 8 for the automated and 7 for the manual count.