Inter- and intra-laboratory variability of CD4 cell counts in Swaziland

Background. Analytical variability in CD4 enumeration is well known, but few studies from southern Africa have quantified the inter- and intra-laboratory variability in CD4 count measurements. In addition, the possible impact of time lapse after sample collection on CD4 reliability is not well understood. Methods. A cross-sectional study was conducted at Royal Swaziland Sugar Corporation Hospital and three laboratories, Lab A (comparator), Lab B (national reference) and Lab C (rural hospital). Blood from HIV-infected individuals was collected using routine venepuncture into separate specimens for each of the three laboratories. The samples were further subdivided at each laboratory: one was run at 12 hours and the second at 24 hours after venepuncture. The results of absolute CD4 count and CD4 percentage testing were compared within (intra-laboratory) and between (inter-laboratory) laboratories. Results. Among 53 participants, the mean CD4 count at 12 hours was 373 cells/ìl, 396 cells/ìl and 439 cells/ìl, and at 24 hours 359 cells/ìl, 389 cells/ ìl and 431 cells/ìl, for laboratories A, B and C, respectively. The coefficient of intra-laboratory variation was 4%, 8% and 20% for CD4 count for laboratories A, B and C, respectively. Comparing 12- and 24-hour measurements, the mean difference (bias) within the laboratories between the two time points (and limits of agreement, LOAs) was 14 (-46 to 73), 8 (-161 to 177) and 7 (20 to 33) cells/ìl for labs A, B and C, respectively. Comparing Lab A versus Lab B, lab A versus Lab C and Lab B versus Lab C, the inter-laboratory bias for the CD4 count at 12 hours was -32, -64 and -38 cells/ìl, respectively. The corresponding LOAs were -213 to 150, -183 to 55, and -300 to 224, respectively. At 24 hours, the biases and LOAs were similar to those at 12 hours. Conclusions. CD4 counts appeared reliable at all three laboratories. Lab B and Lab C were clinically interchangeable with the comparator laboratory, Lab A, but not between themselves. Time to measurement does not affect the interlaboratory agreement within 12 and 24 hours.