GIFSQ Tools · Module 15 — Metrology, Validation & Laboratory Accreditation · Tool 285 of 500
Estimate the limit of detection (LOD) of a quantitative method from the blank standard deviation and the calibration slope — the metrology estimator of ISO 11843 and IUPAC. Distinct from Tool 73, which covers the microbiological detection limit.
Try sample values — fills the form with the worked example below.
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LOD (mg/L)
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LOD at 3σ (mg/L)
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LOD ÷ slope (response units)
Equation: LOD = 3.3 × s₀ ÷ slope. The 3σ variant is LOD = 3 × s₀ ÷ slope.
Method: Detection-limit estimation per ISO 11843-2 (capability of detection) and the IUPAC definition: the lowest amount that can be distinguished from a blank with stated confidence. The 3.3 factor corresponds to a 5% false-positive and 5% false-negative rate. This is the METROLOGY/statistical estimator for chemical methods — distinct from Tool 73 (Detection Limit Calculator), which covers the microbiological plate-count detection limit, and from Tool 74 (microbial LOQ worksheet).
Assumptions: the blank standard deviation comes from at least 10 independent blank measurements; the response is linear near the detection limit; the slope is the calibration slope in the same response units as s₀.
Limitations: an LOD from a single day underestimates the long-run limit; matrix effects and a non-linear low end are not captured; a reported LOD must be confirmed by spiking at the estimated level.
Rounding: LOD to 3 decimal places; intermediates unrounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Method: Detection-limit estimation per ISO 11843-2 (capability of detection) and the IUPAC definition: the lowest amount that can be distinguished from a blank with stated confidence. The 3.3 factor corresponds to a 5% false-positive and 5% false-negative rate. This is the METROLOGY/statistical estimator for chemical methods — distinct from Tool 73 (Detection Limit Calculator), which covers the microbiological plate-count detection limit, and from Tool 74 (microbial LOQ worksheet).
Assumptions: the blank standard deviation comes from at least 10 independent blank measurements; the response is linear near the detection limit; the slope is the calibration slope in the same response units as s₀.
Limitations: an LOD from a single day underestimates the long-run limit; matrix effects and a non-linear low end are not captured; a reported LOD must be confirmed by spiking at the estimated level.
Rounding: LOD to 3 decimal places; intermediates unrounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: Blank SD 0.02 AU with slope 0.5 AU per mg/L: LOD = 3.3 × 0.02 ÷ 0.5 = 0.132 mg/L (3σ variant 0.120 mg/L).
Enter values above to see results.
Confirm the estimate by analysing a sample spiked at the LOD — it should be detected in at least 95% of replicates. Tool 285 of 500 — GIFSQ
