Limit of detection (LOD)

The limit of detection is the lowest concentration of a substance that an analytical method can reliably distinguish from a blank sample containing none.

Every instrument has background noise, and the limit of detection is the point at which a signal is confidently above that noise. It is specific to the method, the instrument, the sample type and the laboratory, so the same analyte can have different detection limits on different certificates. Below this level the method cannot say whether the substance is present.

On a certificate of analysis the limit is often printed in a column beside each result. A result reported as below the limit of detection means the method saw nothing it could distinguish from zero, and the practical meaning depends entirely on how low that limit was. For THC, a product reported at 0.0% THC has been measured below the method's threshold, which is a statement about the sensitivity of the test as much as about the product. A lower limit is a more demanding test.