Here, Sackmann et al. describe a new technique that can be used to diagnose asthma from only a single drop of whole blood. The authors have developed a handheld microfluidic device that sorts neutrophils by pumping the blood cells through P-selectin-coated microchannels. The selected neutrophils are then exposed to a hydrogel that contains a neutrophil chemoattractant, and their movement is measured using time-lapse microscopy and automated tracking software. Using this method, the authors found that neutrophils from patients with asthma showed significantly reduced chemotaxis velocities compared with neutrophils from control patients. They suggest that the decreased chemotaxis velocity of neutrophils from patients with asthma may reflect the increased adhesion of these cells to the P-selectin molecules that coat the microchannels. Importantly, the technique is easy to perform and obtains a rapid diagnostic result.