What is a false positive on a lab test?

A false positive is a test saying something is wrong when it is not - and it is not a rare malfunction but a mathematical certainty of testing. No test is perfect: every screening test trades off catching real cases against flagging healthy people, and MedlinePlus's lab guidance notes results are always read against clinical context for exactly this reason. The counterintuitive part: when the condition being screened for is rare, most positive results can be false - screen a million low-risk people and even a 99-percent-accurate test produces armies of false alarms alongside the true cases. This is why screening programs are designed around confirmation, and why an abnormal first result is a hypothesis, never a diagnosis. Where false positives come from: lab error and sample mishandling; cross-reacting substances - biotin supplements famously distort thyroid and cardiac troponin tests; recent meals and hydration for chemistry panels; medications; and normal biological variation catching you on an odd day. The protective habits: an unexpected abnormal gets repeated or confirmed with a different test before any treatment; tell the lab what you take, supplements included; follow the prep rules exactly; and match the result against how you feel - a dramatic result in a person who feels fine deserves skepticism before it deserves panic. If two tests disagree, the repeat usually settles it. Pymander is a free AI doctor, 24/7 by text. Share the surprising result, and it can help you plan the confirmation.