What is LOD and LOQ and How to Calculate Them

LOD and LOQ are two of the most commonly misunderstood parameters in analytical chemistry. Every method validation requires them, yet many analysts copy the formulas without truly understanding what they mean or whether their values make sense.

This guide explains both parameters clearly, shows you the ICH Q2(R1) calculation method, and gives you a worked example.

LOD — Limit of Detection

The LOD is the lowest concentration of an analyte that can be reliably distinguished from the background noise of the method — but not necessarily quantified with accuracy.

Think of it as the point where you can say "something is there" but cannot yet say "exactly how much is there."

Where σ is the standard deviation of the response (from the y-residuals of the calibration curve or from replicate injections of a blank), and S is the slope of the calibration curve.

LOQ — Limit of Quantification

The LOQ is the lowest concentration that can be measured with acceptable accuracy and precision — typically within ±20% accuracy and ≤10% RSD.

Notice that LOQ = approximately 3 × LOD. This makes sense — at the LOQ you need 10× the signal-to-noise to quantify reliably, versus 3× at the LOD just to detect.

Worked Example

Suppose your calibration curve gives a slope S = 52,400 (peak area per μg/mL) and the standard deviation of replicate blank injections σ = 185 (peak area units).

• LOD = 3.3 × 185 ÷ 52,400 = 0.0117 μg/mL
• LOQ = 10 × 185 ÷ 52,400 = 0.0353 μg/mL

Then confirm the LOQ experimentally by injecting standards at or near this concentration and verifying %RSD ≤ 10% and % recovery 80–120%.

Signal-to-Noise Method (alternative)

An alternative approach is to inject a sample at decreasing concentrations and measure the signal-to-noise ratio directly from the chromatogram:

• LOD = concentration giving S/N ≥ 3
• LOQ = concentration giving S/N ≥ 10

Both approaches are acceptable under ICH Q2(R1). The formula method is more common for routine method validation reports.