Change from dual- to single-platform reporting of CD4/CD8 values: experience from a small district general hospital...

Accurate and reliable CD4 and CD8 counts are essential for monitoring HIV disease progression or successful therapy. CD4 and CD8 counts can be determined on a flow cytometer by either single- or dual-platform technology. Dual-platform technology uses a haematology analyser to obtain a total white cell count and lymphocyte absolute count. CD4 and CD8 absolute values are then calculated from the CD4 and CD8 percentage positive results obtained from the flow cytometer. Single-platform technology uses latex beads of a predefined concentration, which are added to the blood sample immediately before flow cytometric analysis, thereby removing the need to use an additional analyser. Recent recommendations propose that single-platform technology should be the gold standard for CD4 measurement because it offers better inter-laboratory coefficients of variation (CVs). Before changing to single-platform technology in our department, CD4 and CD8 absolute counts, determined on 20 healthy volunteers, were used to establish new normal ranges for single-platform technology (Coulter epics XL), permitting absolute value data for dual-platform and single-platform technologies to be compared. Data obtained with single-platform technology was significantly higher for both CD4 and CD8 (P=0.001 and P=0.003, respectively). For CD4, mean single-platform value was 0.993 x 10(9)/L (+SD = 0.510-1.376) and dual-platform value was 0.920 x 10(9)/L (+SD = 0.500-1.340). For CD8, single-platform value was 0.483 x 10(9)/L, (+SD = 0.207-0.756) and dual-platform value was 0.457 x 10(9)/L (+SD = 0.222-0.692). Thus, the differences between dual- and single-platform absolute CD4 and CD8 results were small (8% and 6%, respectively) but significant. It is important, therefore, that clinicians closely monitoring CD4 and CD8 values and are informed of any laboratory changes.