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.
