For many blood-based assays, blood cells need to be removed from samples before the analysis can be done. But for other assays, the blood cells themselves are the whole point. One such measurement is the hematocrit—the ratio of the volume of red blood cells to the total blood volume. A hematocrit value above normal is a sign of dehydration, whereas a value below normal can mean anemia. Conventional hematology analyzers can be too complex and expensive to use in settings with limited resources. So Charles Mace and coworkers at Tufts University have designed and tested thermometer-style paper-based microfluidic devices for measuring the hematocrit, the first time such devices have been used for cell assays (Lab Chip 2016, DOI: 10.1039/c6lc00895j). Mace’s team makes the devices from 190-μm-thick paper with 15-μm-wide pores. Red blood cells applied to the paper travel a distance that is proportional to the hematocrit. Blood with a high hematocrit travels only a short distance, whereas blood with a low hematocrit travels much farther. To achieve a linear response to the hematocrit, the researchers treated the device with ethylenediaminetetraacetic acid, which stabilizes the blood, and NaCl, which improves cell transport. The devices are inexpensive—just 3 cents each.