Detection limits and “not detected” results

Key terms

Why “not detected” isn’t zero

Every method has a floor below which it can’t see. A substance may be present below that floor. So “not detected” means “less than the detection limit”, and the useful question is how low that limit is. A report that says “ND” without stating the limit leaves you unable to judge the result; see what “not detected” means and what detection limits mean.

When a result is meaningful

Figures are illustrative.

Detection in microbiology

For microbiology, results are often “absent in 25 g” or “not detected in 10 g”. Here the sample size matters: a larger tested amount makes “absent” more meaningful. Counts below a threshold may appear as “< 10 cfu/g”, which is the method’s lower reporting limit; see microbiological testing.

Why detection limits vary

Limits depend on the method, the instrument, the sample type and how much sample was used. ICP-MS typically reaches much lower limits for metals than older methods; see ICP-MS explained. Complex materials like shilajit can raise limits because of interference from the sample matrix.

How to read ND results

1. Find the LOD or LOQIt should be stated beside the result or in the methods.
2. Compare with the legal limitWell below is meaningful.
3. Check the unitsmg/kg, µg/kg or per serving.
4. Ask if anything is missingA good brand or lab can explain.

Detection limits and uncertainty

Results just above the detection limit carry more relative uncertainty than higher results. That is normal, and it is one reason laboratories state measurement uncertainty; see reading a COA.

Why detection limits matter for legal compliance

To show a product meets GB supplement limits, a method’s detection limit needs to be well below the maximum. Mercury is the most demanding case: with a 0.1 mg/kg limit, a method that can only detect down to 0.2 mg/kg can’t demonstrate compliance, even if it reports “not detected”.^[1]

Reading “less than” results

When a result is shown as “< 0.05 mg/kg”, the true value could be anywhere from zero to just under 0.05 mg/kg. For calculating daily exposure, some people use the limit value as a worst case, which gives a conservative estimate. That is a sensible approach when deciding whether a product suits long-term use.

Detection limits in pesticide screens

Pesticide screens test for many substances at once, each with its own reporting limit. A summary saying “no pesticides detected” should be backed by the list of substances tested and their limits. Ask for these if they aren’t provided.

Questions to ask about detection limits

1. What was the LOD or LOQ for each metal?
2. How does it compare with the GB limit?
3. Which pesticides were included in the screen?

Putting it into practice

When you see “not detected” on a report, look for the detection limit beside it or in the methods section, and compare it with the GB limit for that metal. If it is well below, the result is reassuring; if it is close to or above the limit, or missing, ask the brand for clarification. For microbiology, check the sample size behind “absent”. These small checks turn a reassuring-looking line into an informed judgement.

In short

“Not detected” is only as useful as the detection limit behind it. Always ask how low that limit is, and compare it with the relevant legal maximum before taking reassurance from the result. For mercury in particular, where the GB limit is just 0.1 mg/kg, a sensitive method makes all the difference. When in doubt, ask the brand or laboratory to state the limit for each result, so you can judge it for yourself rather than relying on a single “ND”. A clear limit turns “ND” into real information.

The bottom line

“Not detected” means below the method’s detection limit, not zero. It is reassuring only when that limit is stated and well below the legal maximum. Check both before taking comfort from an ND result.

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