National external quality assessment scheme for lymphocyte immunophenotyping in Belgium

In 2000, the Belgian Scientific Institute of Public Health introduced a voluntary external quality assessment scheme for lymphocyte immunophenotyping. This paper provides an analysis of the first six surveys. Specimens consisted of fresh EDTA-anticoagulated whole blood and were sent by overnight mail. The 41 participants were surveyed for methodology and were asked to report white blood cell count, percentage of lymphocytes, and percentages and absolute numbers of CD3(+), CD4(+), CD8(+), and CD19(+) cells. Median intralaboratory coefficients of variation were 1.0, 1.3, 1.7, and 3.2% for CD3(+), CD4(+), CD8(+), and CD19(+) cell percentages, respectively. Interlaboratory variability was consistently lower than 6.5% for CD3(+) and CD4(+)CD3(+) cell percentages, and lower than 9.5% for CD8(+)CD3(+) cell percentages. Median coefficients of variation for the absolute values were higher, ranging from 10.1% for CD4(+)CD3(+) cells to 16.5% for CD19(+) cells. The percentage of CD4(+)CD3(+) and CD8(+) CD3(+) cells was in several samples significantly lower than the percentage of total CD4(+) and CD8(+) cells. The number of laboratories measuring total CD4(+) and CD8(+) cells decreased by 30% during the programme. Between-laboratory variability remained stable over time. Analysis of individual laboratory performance indicated that some laboratories markedly improved their results.