Analyses for diagnosis and monitoring of pathological conditions often rely on blood samples, partly due to relative ease of collection. However, many interfering substances largely preclude the use of whole blood itself, necessitating separation of plasma or serum. We present a feasibility study demonstrating potential use of fresh or frozen whole blood to detect soluble biomarkers using an enzyme-linked immunosorbent assay (ELISA)-based method. Good correlation between levels of soluble CD25 in plasma and whole blood of healthy individuals or Alzheimer’s patients was established. These results provide a basis for development of a novel biosensor approach for disease-associated biomarker detection in whole blood.