Use of formalin-fixed, propidium iodide-stained human leukocytes as a standard for enumerating CD4+ T lymphocytes in a single-platform assay.

A new, inexpensive method is described that enables lymphocytes to be enumerated very precisely. Normal leukocytes were simultaneously stained and fixed with a propidium iodide-paraformaldehyde solution. The preparation obtained (CellBeads) was used as an internal standard for cell enumeration by flow cytometry and was stable at 4 degrees C for at least 60 days. Unlike synthetic beads, the CellBeads behaved similarly to normal cells during red blood cell lysis and cell washing procedures. When known numbers of CellBeads were added to whole-blood samples and the numbers of CellBeads and lymphocytes were determined, highly reproducible and accurate enumerations were obtained-far more so than when synthetic beads were used. This inexpensive method is suitable for routine use.