Biological and methodological variation of lymphocyte subsets in blood of human adults

Although lymphocyte populations are often monitored over time, information about the biological variation over time is limited. Three-colour-flow cytometry was used to investigate the biological and methodological variation of lymphocyte populations in blood. Fifteen healthy individuals (11 females and 4 males) were longitudinally monitored for 2–8 years.Blood samples were drawn monthly when possible. In total, 493 observations were included. Absolute counts and proportions were determined for T-cells (CD3+), T-helper cells (CD3+ CD4+), cytolytic T-cells (CD3+ CD8+), B-cells (CD3− CD19+) and NK-cells (CD3− CD16+/56+).As to variation over the year, ANOVA testing showed only a minor monthly variation for absolute counts of the CD8+ population (p < 0.05) for October compared with June and July, whereas no significant differences were found for the other populations or in the proportions of lymphocyte subsets. Although lower than the longitudinal variation, the methodological variation, expressed as coefficient of variation (CV %), was in a similar range as the variation over time, indicating that the normal biological variation should not be overestimated, while the methodological inter-assay should be taken into consideration in longitudinal studies or monitoring of patients.