What a Molecular Result Does and Does Not Prove | AI Learning Guide

The conclusion block is the rule to carry forward. A molecular test reports that a target was present in the specimen. It does not report that the organism was alive, that it is susceptible to anything, or that it is the cause of the illness. So the result has to be read next to the culture and next to the patient. A positive result in someone with a matching syndrome and signs of inflammation supports causation. A positive result in someone with no matching findings may be colonization or contamination. And a negative result in someone who looks infected does not rule infection out; it means the tested targets were not found, which should push you to widen the differential rather than to relax.

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Where false positives come from

Contamination: amplified product from earlier reactions, or nucleic acid from reagents or the environment, entering a later reaction
Detection of a colonizing organism that is present but not causing the current illness
Detection of non-viable organisms left over after effective antibiotic treatment

Where false negatives come from

Inhibitors in the specimen, such as blood, mucus, or certain transport media, suppressing the polymerase reaction
Organism burden below the limit of detection, which is common in paucibacillary specimens
A pathogen that is not among the panel's targets, which will never be detected regardless of quantity

How to read the result

A molecular result reports presence of a target, not viability, not susceptibility, and not causation. Interpret it together with the culture result and the clinical findings. A positive result in a patient with a compatible syndrome and evidence of inflammation supports the organism as the cause; a positive result without compatible findings may reflect colonization or contamination; a negative result in a patient who looks infected does not exclude infection and should prompt reconsideration of the differential rather than reassurance.