Two limits, two measurement questions
LOD is a limit of detection; LOQ is a limit of quantitation. Detection concerns distinguishing a signal appropriately from background. Quantitation concerns reporting an amount with suitable performance for the method’s intended use.
The FDA Q2(R2) guidance provides analytical-validation context. It is cited for measurement concepts, not as a declaration that a research-material report complies with pharmaceutical requirements or must use a universal numeric limit.
An example that prevents a common reading error
Illustrative method limits: LOD 0.02 mg/g and LOQ 0.05 mg/g for a defined target. A report marked not detected below 0.02 mg/g does not establish exactly 0.00 mg/g. A detected signal below 0.05 mg/g may be described as below quantitation under the method rather than assigned a precise amount.
These figures are invented examples, not results or recommended limits. The report’s own method and reporting conventions govern the actual interpretation. Ask whether the units refer to the original sample or to a prepared measurement solution.
| Report language | Example basis | What it does not establish |
|---|---|---|
| Not detected | Below stated LOD of 0.02 mg/g | Exact zero |
| Detected, below LOQ | Below stated LOQ of 0.05 mg/g | A precise quantified amount |
| Numeric result | Within method scope and reporting basis | All other targets are absent |
| Limit missing | Interpretation unresolved | Universal meaning of ND |
A limit belongs to a target and a method
A detection limit for one analyte cannot be transferred to every potential impurity. It can depend on matrix, preparation and detection conditions. Likewise, an instrument capability statement is not necessarily the method limit for the sample as received.
Ask how any dilution or preparation affects the reported limit on the original-material basis. A very small number in a brochure is not enough to show that the report can answer a project-specific question.
Use the limit in the decision, not just the label
If the laboratory needs evidence below a particular level, it should check whether the method can meaningfully address that level. A report saying not detected can remain inadequate if its reporting limit is above the relevant requirement.
Do not impose an invented acceptance threshold from this article. Have the responsible reviewer define the actual requirement and interpret the result with its reporting basis. Preserve the issuer’s notation instead of replacing ND or below LOQ with zero in a spreadsheet.
Questions buyers ask
Can I enter ND as zero in my comparison sheet?
Do not silently convert it. Preserve the reported notation and limit so the information is not overstated.
Does one method limit apply to every impurity?
No. Identify the target, method, matrix and preparation basis for the stated limit.
Your next step
Request the target-specific detection and quantitation limits with units and sample basis. Have the laboratory assess whether that scope meets the project question.
Related reading: result and specification distinctions; batch records.
AI-assisted educational guide prepared from the cited sources. No testing of a supplied batch is reported here. For research-material purchasing, not medical guidance or instructions for human use. Confirm the actual offered material and available evidence before ordering.