When laboratory tests can mislead even when they appear plausible.

A laboratory test has three phases, pre-analytical, analytical and post-analytical. The purpose of this review is to highlight an issue concerning the analytical phase of one of the most widely deployed groups of in vitro diagnostic tests using a common technology - namely immunoassay.Immunoassay entails an inherently high error rate and, therefore, has the potential for inaccurate and misleading results susceptible to misinterpretation and/or diagnostic misapplication by clinicians. An approach based on Bayesian inference (without mathematics or equations) - illustrated by examples - is presented; this may help clinicians in discerning potentially erroneous results even when they appear plausible and not unreasonable.Essentially, false positive results are most likely to occur when the disease prevalence/incidence is low. False negative results become more prominent when the prevalence/incidence of disease increases. When concern is raised, available follow-up laboratory tests should be initiated to establish with confidence the diagnostic reliability or unreliability of such results. © Royal College of Physicians 2017. All rights reserved.