Detection and quantification ask different questions
The limit of detection, LOD, relates to the ability to distinguish a signal from the background according to a defined criterion. The limit of quantification, LOQ, refers to the level above which an amount can be reported with appropriate performance. They are not simply two names for the same threshold. Eurachem’s guides develop these distinctions within analytical validation and uncertainty. Eurachem: validation and measurement guides
ND usually means “not detected”, but you need to read the laboratory’s key. Other reports use wording such as “less than LOD” or “less than LOQ”. The particular abbreviation matters less than its definition. A useful document explains what each symbol means rather than leaving the reader to guess.
A ruler that cannot see millimetres
Imagine a teaching ruler that only distinguishes centimetres. If an object is smaller than you can resolve with it, you do not conclude that it does not exist. You conclude that this tool cannot describe it in the same way. The comparison has limitations, but helps explain why “not measurable with this method” does not mean “an exactly zero amount”.
Take a fictional example: laboratory A reports ND with an LOD of 0.02 mg/g. Laboratory B reports ND with an LOD of 0.005 mg/g. Both use the same two letters, but they are not communicating the same detection capability. The second can investigate lower levels of that analyte, always within its method’s validated conditions.
When there is a signal, but no reliable figure
There may be a range in which a method detects a signal without being able to quantify it with the required performance. That is why “detected below the LOQ” should not become an invented number or disappear from the account. Reporting should preserve the distinction between detected presence and a quantified amount.
Health Canada recommends that limits reported on certificates properly reflect a method’s capability. It also highlights the importance of the matrix. Citing excellent sensitivity obtained under ideal conditions is not enough if it does not correspond to the type of sample being analysed. This technical reference does not itself establish Spanish requirements. Health Canada: limits on certificates of analysis
Comparing without losing context
First check the analyte, units, matrix and definition of the abbreviation. Then look at the stated limit. Two ND results are not automatically comparable if one refers to a substance and another to a family, or if the units differ. An absent row does not mean ND either: it may mean that no test was performed.
The most useful reading keeps the complete phrase: “not detected using this method and this limit in this sample”. It is longer than “zero”, but far more informative. When a brand maintains that precision, readers can assess the evidence without the workings being hidden. Well-communicated uncertainty brings clarity, not confusion.
Take this with you
ND needs context: method, analyte, sample and limit. Without them, two letters can conceal important differences.
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