quantification limit

Quantification Limit (LOQ — Limit of Quantification) is the lowest concentration of an analyte that can be measured with acceptable accuracy and precision — higher than the detection limit, LOQ is the concentration at which quantitative results become reliable, typically defined as 10σ of the blank.

LOQ Calculation

• 10σ Method: $LOQ = 10 \times \sigma_{blank}$ — ten times the standard deviation of blank measurements.
• ICH Method: $LOQ = 10 \times \sigma / m$ where $sigma$ is blank SD and $m$ is calibration slope.
• Signal-to-Noise: $LOQ$ at $S/N = 10$ — sufficient signal for quantitative reliability.
• Accuracy/Precision: At the LOQ, accuracy should be within ±20% and precision (CV) should be ≤20%.

Why It Matters

• Reporting: Results below LOD are reported as "not detected"; between LOD and LOQ as "detected but not quantified"; above LOQ as quantitative values.
• Specifications: The LOQ must be below the specification limit — cannot reliably determine if a sample passes if LOQ > spec.
• Method Selection: If LOQ is too high, a more sensitive method is needed — drives instrument selection.

Quantification Limit is the reliable measurement floor — the lowest level at which quantitative results have acceptable accuracy and precision.