In a screening result, what does a 'not detected' result mean and what caveats should be considered?

In a screening result, what does a 'not detected' result mean and what caveats should be considered?

• A. It confirms the analyte is completely absent in the specimen.
• B. The analyte is below the assay’s LOD/LOQ or not present, but absence is not guaranteed and results can be affected by timing, dilution, or matrix effects.
• C. It indicates a measurement error requiring immediate reanalysis.
• D. The analyte exceeded the upper reportable range.

Answer: (B)

A not detected result means the analyte is at or below the test’s ability to observe it—that is, below the assay’s detection and/or quantitation limits. It doesn’t guarantee the substance is completely absent. Factors like when the sample was taken relative to exposure (timing), any dilution during handling, and matrix effects from the specimen type can all push a real, low-level presence below the instrument’s threshold or affect signal in ways that prevent detection. In practice, you interpret not detected as “likely below the assay’s LOD/LOQ or not present,” but with caveats: a detection could be missed if levels are very low, if the sample degraded, or if there are substances in the matrix that interfere with the measurement. If there’s still concern about possible exposure, consider a more sensitive confirmatory test or a different specimen and timing to rule out a false negative.