It seems logical: You notice something funny on your skin, but you don’t want to take the time to go to a doctor to see whether it’s cancerous, so you check it out instead by taking a picture with your phone and submitting it to an online app. If the app gives you the all-clear, you can quickly get on with your life, right? But a study from the University of Pittsburgh Medical Center finds that such skin-check apps are unreliable. In three out of four apps reviewed, roughly 30 percent of marks were dismissed as unconcerning, when they were really potentially fatal skin cancers, Joel A. Wolf and his colleagues found.

The authors didn’t identify the apps, but said they found one that was trustworthy: A site that sent the pictures directly to a board-certified dermatologist accurately diagnosed melanoma 98 percent of the time. The others examined skin photos with an automated algorithm. The application that involved doctors was also the most expensive at $5 per lesion analyzed, and took about 24 hours, compared with nearly instantaneous results with the automated apps.