Affordable CD4 + T cell counts by flow cytometry : II. The use of fixed whole blood in resource-poor settings

We tested the feasibility and precision of affordable CD4 + T cell counting in resource-poor settings using a recently standardised fixative, TransFix™ in whole blood (WB) by flow cytometry (FCM). The precision of the assays was established under optimal conditions for single-platform FCM such as the volumetric CytoronAbsolute and the bead-based FACSCan. Fresh WB samples from HIV-seropositive and seronegative patients were tested in Tanzania and South Africa, fixed and sent to the UK for reanalysis 7 days later. Correlation, bias and limits of agreements were analysed by linear regression and the Bland–Altman test. Absolute CD4 + T cell counts remained stable for at least 10 days when TransFix was added to WB in 1:10 dilution at 20–25 °C, and for 7 days when added in 1:10 or 1:5 dilution to samples stored to mimic ‘tropical’ conditions at 37 °C. Higher temperatures such as 42 °C were tolerated for only short periods since the recovery had decreased to 63% by day 3. The reproducibility of lymphocyte subset analysis remained unchanged by TransFix with coefficient of variations