LOD/LOQ (DL/QL) per ICH Q2(R2)

LOD/LOQ — Sensitivity Parameters

The lowest levels a method can reliably detect (DL) and quantify (QL) — computed by all three ICH Q2(R2) approaches, and confirmed at the QL so the number you report is defensible.

ICH Q2(R2) §3.2.3 Impurity Methods Three Approaches QL Confirmation

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Industry Use

How DL/QL are applied in pharmaceutical laboratories

The detection limit (DL) is the lowest analyte level that can reliably be detected; the quantitation limit (QL) is the lowest level that can be quantified with acceptable accuracy and precision. ICH Q2(R2) uses the terms DL and QL — LOD and LOQ are the common synonyms. Q2(R2) offers several approaches; this calculator implements three, and computing more than one is the best defense of the value you report.

Where this is used:

Impurity and related-substances methods (reporting thresholds vs QL).
Cleaning-verification methods (the LOQ must sit below the acceptance limit — see the rinse and TOC guides).
Any method validated for use near its sensitivity floor.

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The Three Approaches

Per ICH Q2(R2) §3.2.3

1 — Signal-to-noise (Q2(R2) §3.2.3.2)

DL at S/N ≈ 3:1 · QL at S/N ≥ 10:1

2 — From the calibration (Q2(R2) §3.2.3.3)

DL = 3.3·σ / S · QL = 10·σ / S

σ = the residual standard deviation of a low-range calibration line, or the standard error of its intercept; S = the slope.

3 — Blank response

σ = SD of ≥ 6 independent blank responses → same 3.3σ/S and 10σ/S

The classical blank-based (Kaiser-style) estimate of σ, fed into the same formulas.

Whatever the approach, confirm the QL: Q2(R2) expects the quantitation limit to be validated by analyzing samples at that level — typical firm criteria: RSD ≤ 10% and recovery 80–120% at the QL. The formulas alone are estimates; the confirmation is what makes the number defensible.

ICH Q2(R2) §3.2.3.2 / §3.2.3.3 USP <1225> Kaiser blank convention

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Variable Definitions

All input parameters and their meaning

Symbol	Variable	Units	Description
DL (LOD)	Detection limit (result)	µg/mL	Lowest level reliably detected — identification, not quantitation.
QL (LOQ)	Quantitation limit (result)	µg/mL	Lowest level quantifiable with acceptable accuracy and precision. Must be confirmed experimentally.
σ	Response standard deviation	response units	From low-range calibration residuals, the intercept's standard error, or ≥ 6 blank responses — in the SAME response units as the slope (see Assumptions).
S	Calibration slope	response per µg/mL	From linear regression of the low-range calibration; steeper slope = higher sensitivity.
S/N	Signal-to-noise ratio	—	Peak signal against baseline noise; DL ≈ 3:1, QL ≥ 10:1 (approach 1).

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Step-by-Step Tutorial

Worked example: approaches 2/3 combined, with QL confirmation

Scenario: HPLC impurity method, calibration 0.05–1.0 µg/mL, slope S = 9,380 area units per µg/mL (R² = 0.9998). Ten blank injections give a response SD of σ = 14 area units at the impurity retention time. Note σ and S share the same response scale — mixing peak-height noise with peak-area slope produces nonsense values.

Compute the DL.

Calculation
DL = 3.3 × 14 / 9,380 = 0.0049 µg/mL
Compute the QL.

Calculation
QL = 10 × 14 / 9,380 = 0.0149 µg/mL → round to 0.015 µg/mL for confirmation
Confirm the QL experimentally: six replicates at 0.015 µg/mL.

Calculation
0.0141, 0.0152, 0.0147, 0.0158, 0.0149, 0.0144 µg/mL
Mean = 0.01485 (recovery 99%) · RSD = 4.1%
Firm criteria: RSD ≤ 10% ✓ · recovery 80–120% ✓
Report: DL 0.005 µg/mL; QL 0.015 µg/mL (confirmed). Results below DL: not detected. Between DL and QL: detected, not quantifiable. At or above QL: quantifiable.

✓ DL = 0.005 µg/mL · QL = 0.015 µg/mL (confirmed: RSD 4.1%, recovery 99%)

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Assumptions and Limitations

What the calculation presumes — and what it does not cover

Consistent response units: σ and S must come from the same response units and the same method conditions.
Low-range linearity: the calibration must be linear near the floor; a curved floor inflates σ.
Drift: DL/QL move with instrument condition; re-establish per your SOP and after major maintenance.
Fitness for purpose: the reported QL must be at or below the level the method needs to quantify (specification, reporting threshold, or cleaning limit).

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References

Primary sources for this guide

ICH Q2(R2), Validation of Analytical Procedures (2023), §3.2.3 — DL/QL approaches. database.ich.org
USP General Chapter <1225>, Validation of Compendial Procedures.

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