false positive

A false positive is a compound that looks like a hit but isn't — the assay says 'active!' while the molecule is doing nothing real to the target. It is the screening equivalent of a smoke alarm going off because someone made toast: the alarm fired, but there is no fire.

False positives arise from many mechanisms that fool the readout rather than engage the biology. A compound may quench or emit fluorescence and distort an optical signal, absorb light at the detection wavelength, be insoluble and form aggregates that nonspecifically gum up the protein, contain a reactive impurity, chelate a metal the enzyme needs, or interfere with the detection chemistry itself. None of these is genuine, target-specific activity.

Because primary screens test each compound once at a single concentration, false positives are unavoidable and often outnumber real hits. The whole apparatus of hit confirmation, counter-screens, orthogonal assays, and PAINS filtering exists to expose and discard them. Treating an unconfirmed primary hit as real is one of the most common and costly mistakes in early drug discovery.