Limits of Detection and Quantification: What "Not Detected" Means

Why these limits exist

Every measurement method has a floor. Below a certain amount, a laboratory instrument cannot pick out the compound from background noise, and slightly above that, it can see the compound but cannot give a trustworthy number. Lab reports express those floors as the limit of detection (LOD) and the limit of quantification (LOQ, also called the quantitation limit). Reading them correctly prevents two mistakes: treating "not detected" as zero, and treating a very small number as exact.

Definitions

The International Council for Harmonisation guideline on validating analytical procedures, as published by the European Medicines Agency (the Q2(R1) text, which later versions have revised), defines these terms as follows. The detection limit is the lowest amount of analyte in a sample which can be detected but not necessarily quantitated as an exact value. [1] The quantitation limit is the lowest amount of analyte which can be quantitatively determined with suitable precision and accuracy. [1]

"Not detected" is not "absolute zero"

"Not detected" (ND) means the method did not see the compound at or above its detection limit. It does not prove the compound is absent. A different method, a more sensitive instrument or a larger sample could find a trace. The same logic applies to "<LOQ": the compound may be present and visible, but the number is not reliable enough to print. Each lab and method has its own limits, so ND on one report may not mean the same as ND on another. Check the limits printed next to the result.

How limits are estimated

The ICH text describes one approach as comparing signals from samples with known low concentrations against blank samples, and says a signal-to-noise ratio of 3 or 2:1 is generally considered acceptable for estimating the detection limit. [1] That is a technical detail; the useful takeaway is that the limit is an estimate set by the method.

Why this matters in cannabis testing

• Cannabinoid reports. A minor cannabinoid listed as ND is simply below the method's floor for that sample. NIST researchers also described chromatographic interferences that can inflate delta-9 THC, which is a reminder that identification at low levels is not always straightforward. [2]
• Hemp compliance. The US hemp rule requires that analytical tests be sufficiently sensitive for the detectability requirements and that measurement of uncertainty be estimated and reported. [3] This is a US rule for hemp plans, read 2026-10-22; it is not legal advice, and other jurisdictions differ.
• Contaminants. For pesticides, metals and microbes, limits matter even more because the tested amounts are tiny. See pesticides in cannabis products and heavy metals.
• Methods. Instrument and technique choices change limits; see testing methods.

Reading checklist

1. Find the LOD and LOQ printed on the report (they may differ by compound).
2. Check whether "ND" or "<LOQ" appears and what limit sits beside it.
3. Compare reports only when the limits are comparable.
4. Remember that a clean report is one sample on one date. See how to read a cannabis COA.