On-chip sample preparation by controlled release of antibodies for simple CD4 counting

We present a simple system for CD4 and CD8 counting for point-of-care HIV staging in low-resource settings. Automatic sample preparation is achieved through a dried reagent coating inside a thin (26 μm) counting chamber, allowing the delayed release of fluorochrome conjugated monoclonal antibodies after the filling of the chamber with whole blood by capillary flow. A custom-built image cytometer is used to capture fluorescence images representing more than 1 μl of blood. The thin layer of blood in combination with the large image area allows the use of whole blood from a finger prick without the need for dilution, lysis or cell enrichment. Automatic cell counting of CD4 + and CD8 + T-lymphocytes correlates well with results obtained by flow cytometry. The delayed release of antibodies from a gelatine layer upon filling a counting chamber with whole blood simplifies CD4 + T-cell counting by rendering active mixing and precision pipetting unnecessary.