Most methods for quality control of white blood cell (WBC) depletion in blood products are based on flow cytometric techniques. Nearly all commercial kits are based on propidium iodide staining of the DNA and subsequently counting those DNA based events as residual WBC. Here, we could show that a substantial proportion of those events are derived from nucleated red blood cells and therefore not specific for WBCs (e.g. in erythrocyte products 30%). We developed a flow cytometric method for residual WBC counting applying simultaneous DNA- and WBC-specific surface staining to enable this.