Selection criteria: positive urine drug test that does not confirm
Causes for a false positive urine screening test:
(1) heterophile antibody in a patient with nephritic syndrome (normally there is very little chance of immunoglobulin being in the urine)
(2) taking a medication that can cross-reacts with the target drug (for example, decongestants may be detected by a test for amphetamines)
(3) level of drug near the cutoff (the urine screen may detect the drug but the concentration may be below the cutoff of the confirmatory test)
(4) contamination of the urine sample (for example, pharmaceutical worker pulled from the production line)
Causes for an unintentional true positive urine screening test:
(1) taking an alternative medication secretly laced with a controlled substance
(2) naturally occurring compound that cross-reacts with the target drug (for example, poppy seeds and urine test for opiates)
