Limited access to expensive equipment and trained professionals can impede the diagnosis and treatment of disease. Qualitative tests that provide a “yes” or “no” answer (like at-home pregnancy tests) have been optimized for resource-limited settings, but most quantitative tests — needed to determine precise concentrations, like viral loads — are still done in a laboratory. Using a lab-on-a-chip device and a smartphone, researchers developed a method to determine the concentration of HIV RNA in a sample.