The CD4 count is the best surrogate marker for monitoring HIV. The reference method for assessing CD4 counts (flow cytometry, FCM), as expensive technique, is not widely used in non-OECD countries. To make HIV-monitoring available to more patients in these countries, we modified a commercially available density-based negative selection assay (RosetteSep TM ) to make it applicable for low-cost cell enumeration. For evaluation (Step 1), blood taken from 25 HIV patients and 29 healthy donors was assayed with the modified negative selection method (MNS) and compared with FCM. For validation (Step 2), this method was performed in blind quintuplicates on 12 HIVþ blood samples according to FDA guidelines. Association of MNS with the FCM is given by regression models for both steps:Step 1: slope = 1.091, intercept = À46.5.Step 2: slope = 1.074, intercept = À38.3 (Step 2). The imprecision of MNS assessed during Step 2 was 21.2% (intraserial) and 18.8% (interserial). The results suggest that MNS is capable of providing an approximate CD4 count. At a cost of 0.30, it is affordable to patients living in resource-restrained areas. The technique has the potential to deliver an accurate, precise, low-cost test to monitor HIVþ patients. ' 2007 International Society for Analytical Cytology