What “Not Detected” Means on a Lab Report
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“Not detected,” often abbreviated ND, can sound like a statement that a substance is completely absent. In laboratory reporting, that interpretation goes too far.
An analytical method has limits. An ND result needs to be read alongside the laboratory's definitions, the substance being measured, and the method's reporting threshold.
Detection and measurement are different
The limit of detection, or LOD, describes the method's ability to detect small amounts of a substance. The limit of quantitation, or LOQ, concerns the level at which an amount can be measured with suitable performance, such as accuracy and precision.
A method may detect a substance at a concentration too low to quantify reliably. Depending on the laboratory's reporting conventions, that finding might appear as “detected below LOQ,” “<LOQ,” or another qualified result. These labels should not be treated as interchangeable without reading the report's legend.
An illustrative example
Imagine a method with an LOD of 0.001% and an LOQ of 0.004%. A signal corresponding to a level between those thresholds might support detection without supporting a dependable numerical concentration. This is an example of the distinction, not a universal rule for how every laboratory must report a result.
Why two reports may look different
Laboratories can use different methods, thresholds, and reporting rules. An ND result from one method does not necessarily conflict with a small numerical result from a more sensitive method. Sample differences can matter as well.
When interpreting ND, ask three questions: What does this laboratory mean by ND? What threshold applies to this compound? Was the relevant substance included in the test panel at all?
Use the laboratory's explanation rather than replacing ND with “zero” or “free from” in a public claim.
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