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Affordable CD4⁺-T-Cell Counting by Flow Cytometry: CD45 Gating for Volumetric Analysis

HIV Immunology, Department of Immunology and Molecular Pathology, Royal Free and University College Medical School, London, United Kingdom,¹ Department of Molecular Medicine and Hematology, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa²

Received 14 January 2002/ Returned for modification 19 March 2002/ Accepted 22 April 2002

The flow cytometers that are currently supported by industry provide accurate CD4⁺-T-cell counts for monitoring human immunodeficiency virus disease but remain unaffordable for routine service work under resource-poor conditions. We therefore combined volumetric flow cytometry (measuring absolute lymphocyte counts in unit volumes of blood) and simpler protocols with generic monoclonal antibodies (MAbs) to increase cost efficiency. Volumetric absolute counts were generated using CD45/CD4 and CD45/CD8 MAb combinations in two parallel tubes. The percentage values for the various subsets were also determined within the leukocyte and lymphocyte populations utilizing a fully automated protocol. The levels of agreement between the newly developed method and the present industry standards, including both volumetric and bead-based systems using a full MAb panel for subset analysis, were tested by Bland-Altman analyses. The limits of agreement for CD4 counts generated by the volumetric methods using either CD45/CD4 (in a single tube) or the full Trio MAb panel (in three tubes) on the CytoronAbsolute flow cytometer were between -29 and +46 cells/mm³ with very little bias for CD4 counts (in favor of the Trio method: +8 CD4⁺ lymphocytes/mm³; 0.38% of lymphocytes). The limits of agreement for absolute CD4 counts yielded by the volumetric CD45/CD4 method and the bead-based method were between -118 and +98 cells/mm³, again with a negligible bias (-10 CD4⁺ lymphocytes/mm³). In the volumetric method using CD45/CD8, the strongly CD8⁺ cells were gated and the levels of agreement with the full Trio showed a minor bias (in favor of the Trio; +40 CD8⁺ cells/mm³; 5.2% of lymphocytes) without a significant influence on CD4/CD8 ratios. One trained flow cytometrist was able to process 300 to 400 stained tubes per day. This workload extrapolates to a throughput of >30,000 samples per year if both CD45/CD4 and CD45/CD8 stainings are performed for each patient or a throughput of >60,000 samples if only CD45/CD4 counts are tested in a single tube. Thus, on the basis of the high efficiency and excellent agreement with the present industry standards, volumetric flow cytometers with automated gating protocols and autobiosamplers, complemented by generic CD45, CD4, and CD8 MAbs used in two-color immunofluorescence, represent the most suitable arrangements for large regional laboratories in resource-poor settings.

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