The limit of detection concerns recognising an analyte at a low level; the limit of quantification concerns measuring its amount with acceptable performance. These capabilities depend on the analyte, method and sample context. A small number printed beside LOD or LOQ needs a definition and supporting evidence before it can be compared with another method's number.
Keep the two analytical questions separate
Detection asks whether the measurement can distinguish a low analyte level from the relevant background with the intended confidence. Quantification adds a requirement for acceptable measurement performance at that level. A distinguishable signal can still be too variable or biased for a dependable numerical amount.
| Term | Question it addresses |
|---|---|
| Detection limit | At what low level can the method reliably detect the analyte under its definition? |
| Quantification limit | At what low level can the method measure amount acceptably? |
| Reporting threshold | At what level does the reporting procedure require a stated result? |
| Specification limit | What result is acceptable for the material or intended decision? |
A reporting threshold or specification limit is not created simply by calculating the instrument's sensitivity. Those limits serve different purposes. Keep their units and sample bases aligned when assessing whether an analytical method can answer the required question.
Understand what a calculation-based estimate assumes
ICH Q2(R2) includes a linear-response approach using DL = 3.3σ/S and QL = 10σ/S, where σ describes response variability and S is the calibration slope. It also describes other approaches, including direct assessment of accuracy and precision at the lower range.ICH — Q2(R2), lower range limits (opens in a new tab)
For an original illustration, let σ be 2 response units and S be 100 response units per ng/mL. The estimates are DL = 0.066 ng/mL and QL = 0.20 ng/mL. These values follow the stated model; they are not measured performance figures for a peptide.
The unit check is useful: response units divided by response units per ng/mL leaves ng/mL. Using a standard deviation already expressed in concentration units and dividing by the slope again would introduce a second, incorrect conversion.
Distinguish instrument capability from the whole method
Eurachem's 2025 validation guide distinguishes limits estimated by presenting material directly to an instrument from limits covering the complete measurement procedure. Sample preparation and the final calculation belong in the latter assessment.Eurachem — Method versus instrument detection limits (opens in a new tab)
Suppose the final measured solution has an illustrative quantification limit of 0.20 ng/mL and the original sample undergoes a tenfold dilution. On the original-sample concentration basis, that corresponds to 2.0 ng/mL under the assumed simple dilution model. Reporting 0.20 without explaining the basis would overstate the original-sample sensitivity.
This rescaling does not validate recovery or matrix behaviour. If those effects are important, they must already be addressed by the full method. An arithmetic conversion cannot supply missing experimental performance evidence.
Check the evidence near the claimed limit
ICH calls for the approach and supporting data to accompany lower-limit claims, including appropriate confirmation of estimated quantification limits. The point is to connect an estimate with actual performance near the level it describes, rather than relying on extrapolation alone.ICH — Q2(R2), lower range limits (opens in a new tab)
A useful comparison records the analyte, matrix, preparation, signal definition, estimation approach and acceptance criteria. Different conventions or performance requirements can produce different limits without a contradiction. Smaller numbers are only meaningfully better when the claims are comparable.
For an individual result reported as not detected or below a limit, the certificate-reading guide explains the wording. Here, the underlying question is whether the stated analytical capability was established for the task being performed.
Sources and further detail
- ICH — Q2(R2), lower range limits (opens in a new tab)
Final guideline with 2025 error correction, section 3.2.3. The equations are one estimation approach, not universal substitutes for validation or proof that a supplied method complies.
- Eurachem — Method versus instrument detection limits (opens in a new tab)
Fitness for Purpose of Analytical Methods, third edition (2025), section 5.3. The original dilution example assumes no additional unaddressed process effects.
Sources checked 19 September 2026. Worked examples are illustrative unless a supplied report is explicitly identified. This article has not undergone independent scientific peer review.