What 'Not Detected' Means on a Lab Report
Why 'not detected' never means zero, and what a certificate must state for the result to mean anything.
Does 'not detected' mean zero?
No. It means the analyte was below the method's limit of detection. Every analytical method has a threshold below which it cannot distinguish signal from noise, and that threshold is never zero. A certificate reporting 'not detected' should state the limit, because not detected at 50 parts per million and not detected at 2 parts per million are very different claims.
Reading the phrase as absence rather than as below-threshold is one of the three classic misreadings of a laboratory report.
- Limit of detection and limit of quantitation
- The limit of detection is the lowest concentration distinguishable from background noise. The limit of quantitation is the lowest concentration that can be measured with acceptable accuracy and precision.
- The quantitation limit is always higher than the detection limit. A result between the two means the analyte is present but the number attached to it is unreliable, which is why such results are usually reported as detected below quantitation limit rather than given a value.
Frequently asked questions
- What is the difference between LOD and LOQ?
- The limit of detection is the lowest concentration at which the method can tell the analyte is present at all, conventionally where signal is about three times background noise. The limit of quantitation is the lowest concentration at which a number can be assigned with acceptable accuracy and precision, conventionally around ten times noise. Between the two, presence is real but the measured value is not trustworthy.
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REPLACE: Analytical Lead
Head of Analytical Chemistry · PhD Analytical Chemistry
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