Presumptive is not positive

An immunoassay drug screen does not tell you that a substance is present. It tells you that something in the sample bound to an antibody in a way consistent with that substance. Those are different statements, and the gap between them has consequences.

How often this matters

A review of 173 articles, condensed to 62 containing false-positive data from 2000 onward, tabulated documented immunoassay false positives across amphetamines, opiates, benzodiazepines, cannabinoids, tricyclic antidepressants, phencyclidine, LSD and barbiturates. Its conclusion is the operational rule: immunoassay positives “are considered presumptive until confirmed by a second independent chemical technique.”

A case that shows the stakes

A published case series describes three pregnant or postpartum patients who began labetalol — an ordinary antihypertensive in pregnancy — and produced presumptive-positive fentanyl immunoassay results.

Consider what an unconfirmed fentanyl positive triggers in a pregnancy. Child protective reporting. A birth plan changed. A relationship with a clinical team damaged in a way that does not repair. All from a blood pressure medication and a screening assay behaving exactly as documented.

And the people reading them often cannot

A survey of 911 clinical and laboratory professionals across the US and Canada tested interpretation with six questions. Toxicologists averaged 4.82 out of 6 and laboratory PhDs 4.63. Physicians in pathology, emergency medicine, primary care and internal medicine “displayed concerning knowledge gaps,” largest in identifying simulated compliance, opioid metabolism and cross-reactivity. More than 30% said they would seek interpretation guidance from the internet or from peers rather than from a laboratory expert.

So the failure is not one thing. It is a presumptive result, acted on by someone without the training to read it, in a setting where the consequence for the patient is severe and hard to reverse.

The rules that follow

1. Confirm before acting. Always. This single rule prevents most of the documented harm.
2. Never let a presumptive result leave the laboratory looking like a finding. If a display shows presumptive and confirmed results the same way, the display is the defect.
3. Return results with interpretation, and give the ordering clinician a route to someone competent to explain them.
4. Do not dismiss anyone on a test result. CDC states this directly, and it applies with particular force to unconfirmed ones.
5. Know whose sample it is. Chain of custody is the other half — see custodial toxicology.

More: the urine drug testing evidence.