A smart preparation strategy for point-of-care cellular counting of trace volumes of human blood

Blood counting is one of the most commonly ordered clinical assays, and is often part of the basis for initial diagnosis and screening for disease. While substantial prior research has shown the ability of portable instruments to accurately produce blood counts through image- or flow-based cytometry, these methods require complex sample preparation using either costly commercial imaging chambers or complicated reagents. To address these issues, in this paper we developed a method to prepare trace volumes of whole blood aimed at portable blood counting. The strategy is based on pre-storing dry-form reagents and fabricating a specifically designed cell counter. In order to obtain total cell counts for red blood cells, platelets, and 3-part differentials of white blood cells, two parallel counting chambers with different depths are made from cost- and environmentally friendly materials using soft lithography. As little as 1 μl of whole blood is prepared with pre-stored reagents in centrifuge vials, whereas red blood cells are sphered and white blood cells are stained at the same time. Driven by the capillary force, prepared blood samples enter the hydrophilic chambers automatically. Monolayers of cells are formed when the blood dilution factors and the chamber depths are co-optimized. Combined with our previous custom-built instrument and automated analysis algorithm, the sample preparation strategy allows producing counting results with excellent agreement to a gold-standard clinical hematology instrument. The success of this preparation method may further advance applications of our technology for global use in low-resource settings where central hematology laboratories are not accessible.