What 'Non-Detect' Really Means on Your Lab Results

Lab reports often list a column of results where many entries say 'ND' or 'Non-Detect.' It's natural to read that as 'zero' and move on. That's not quite what it means — and the difference matters when you're deciding whether to act on a borderline result.

What 'non-detect' actually means

Every lab method has a detection limit — the lowest concentration the equipment can reliably measure. If your sample's concentration is below that limit, the lab can't say it's zero. They can only say 'we couldn't see it at the level we tested for.' That gets reported as ND. The actual value could be true zero, or it could be just below the detection limit.

Why detection limits vary

• Different labs use different methods with different sensitivity

• More sensitive methods cost more and take longer

• For some contaminants (PFAS), detection limits have improved 1,000x in the last decade

• A 'non-detect' from 5 years ago might detect today

How to use ND results

1. Look at the detection limit (DL) or method reporting limit (MRL) listed on the report

2. Compare the DL to the MCL or health-based limit

3. If the DL is well below the limit (e.g., 10x lower), the ND is reassuring

4. If the DL is close to or above the health-based limit, ask the lab if they can run a more sensitive method

A practical example

Suppose your PFOA result is 'ND' with a detection limit of 4 parts per trillion. The new EPA MCL for PFOA is 4 ppt. That ND tells you you're under the limit, but only barely — your actual concentration could be anywhere from zero to just under 4. If anyone in the house is pregnant, an infant, or you're trying to make a treatment decision, that's a strong reason to ask for a more sensitive analysis.

The bottom line

ND is good news but with an asterisk. Always check the detection limit, always compare it to the health-based threshold, and don't hesitate to ask the lab for clarification. Reputable labs will tell you exactly what they could and couldn't see.