Total and differential white blood
cell (WBC) counts are vital
metrics used routinely by clinicians to aid in the identification
of diseases. However, the equipment necessary to perform WBC counts
restricts their operation to centralized laboratories, greatly limiting
their accessibility. Established solutions for the development of
point-of-care assays, namely lateral flow tests and paper-based microfluidic
devices, are inherently limited in their ability to support the detection
of WBCsthe pore sizes of materials used to fabricate these
devices (e.g., membranes or chromatography papers)
do not permit passive WBC transport via wicking.
Herein, we identify a material capable of the unimpeded transport
of WBCs in both lateral and vertical directions: a coffee filter.
Through in situ labeling with an enzyme-labeled affinity
reagent, our paper-based cytometer detects WBCs according to their
immunophenotype. Using two cultured leukocyte lines (Jurkat D1.1 T
cells and MAVER-1 B cells), we demonstrate the specific, colorimetric
enumeration of each target cell population across the expected physiological
range for total lymphocytes, 1000–4000 cells μL–1. Additionally, we highlight a potential application of this type
of device as a screening tool for detecting abnormal cell counts outside
the normal physiological range and in subclasses of cell types, which
could aid in the identification of certain diseases (e.g., CD4+ T lymphocytes, an important biomarker for HIV disease/AIDS).
These results pave the way for a new class of paper-based devicesthose
capable of controlled white blood cell transport, labeling, capture,
and detectionthus expanding the opportunities for low-cost,
point-of-care cytometers.