Original Research

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

Ganizani Mlawanda, Paul Rheeder, Jacqui Miot
Southern African Journal of HIV Medicine | Vol 13, No 2 | a139 | DOI: https://doi.org/10.4102/sajhivmed.v13i2.139 | © 2012 Ganizani Mlawanda, Paul Rheeder, Jacqui Miot | This work is licensed under CC Attribution 4.0
Submitted: 15 December 2012 | Published: 07 June 2012

About the author(s)

Ganizani Mlawanda, School of Health Systems and Public Health, Faculty of Health Sciences, University of Pretoria, and Royal Swaziland Sugar Corporation Medical Services Hospitals, Mhlume, Swaziland, Swaziland
Paul Rheeder, School of Health Systems and Public Health, Faculty of Health Sciences, University of Pretoria
Jacqui Miot, School of Health Systems and Public Health, Faculty of Health Sciences, University of Pretoria


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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.

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