VARIABILITY IN LEUKOCYTE SUBSET MEASUREMENTS AMONG 5 LABORATORIES IN VANCOUVER

Cell counts and percentages of CD4 are widely used in the prognostic a nd clinical management of HIV-infected patients, and as surrogate outc omes in clinical trials involving HIV-infected individuals. Considerab le variability in CD4 counts has been documented due to physiologic an d methodologic factors. While studies of variability of CD4 counts amo ng American and French laboratories have been reported in the literatu re, no published data are available for Canadian laboratories. This pa per describes the results of a study to determine the variability of l eukocyte subsets among 5 laboratories in Vancouver, British Columbia. Samples were collected in a prospective fashion from 52 HIV-negative p atients from July 1991 to November 1993. Coefficients of variation (CV ) were calculated for leukocyte subset percentages and absolute cell c ounts among laboratories. Our results demonstrate that the variability in leukocyte subsets among 5 Vancouver laboratories was lower than or comparable with published findings. The variability remained stable o ver the time period of the study, although 4 of the 5 laboratories par ticipated in quality assurance programs. This suggests a plateau in th e impact of this program. Since the variability among laboratories is less than the variability attributable to physiologic factors, further research efforts to reduce this variability should focus on physiolog ic sources.