Abstract
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/μl, 396 cells/μl and 439 cells/μl, and at 24 hours 359 cells/μl, 389 cells/ μl and 431 cells/μl, 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/μl 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/μl, 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.
 
 S Afr J HIV Med 2012;13(2):59-63.
Highlights
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
Thirty-five million people are infected by HIV globally, twothirds of whom live in sub-Saharan Africa.[1]
Determination of eligibility for Antiretroviral therapy (ART) initiation relies heavily on CD4 enumeration, and CD4 results are monitored as the major indicator of response to treatment over time
Summary
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. This study was undertaken at HIV clinics at the Royal Swaziland Sugar Corporation Hospital in Swaziland and three laboratories, Lab A, Lab B and Lab C (identity of the laboratories deliberately not disclosed). Lab B was the Swazi national reference laboratory based in the capital city, 250 km away from the study setting, and had a turnover of 4 000 CD4 enumerations per Demographic characteristics Gender (n) Male Female.
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