GIFSQ Tools · Module 4 — Microbiology Methods & Laboratory Operations · Tool 74 of 500
Calculate the limit of quantification (LOQ) from blank measurements and the calibration slope, with the detection limit shown alongside for comparison.
Try sample values — fills the form with the worked example below.
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LOQ signal (blank + 10 s)
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Limit of quantification (CFU/g)
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Detection limit, 3.3 s ÷ slope (CFU/g)
Equation: LOQ signal = blank mean + 10 × s, where s is the standard deviation of the blank. LOQ (concentration) = LOQ signal ÷ slope. Detection limit for comparison: LOD = 3.3 × s ÷ slope.
Method: IUPAC definition of the limit of quantification and ISO 11843-2: the lowest amount that can be quantified with acceptable precision and accuracy — about three times the detection limit.
Assumptions: the blank measurements are normally distributed; the calibration is linear through the low range; the slope is constant near the quantification limit.
Limitations: the 10 s rule is a convention, not a guarantee of accuracy — confirm the LOQ experimentally by spiking at the LOQ level and checking recovery and precision. Results between the LOD and the LOQ are detected but not quantifiable.
Rounding: LOQ and LOD to 3 decimal places; the signal to 4 decimal places. The factors 10 and 3.3 are applied to unrounded values.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Method: IUPAC definition of the limit of quantification and ISO 11843-2: the lowest amount that can be quantified with acceptable precision and accuracy — about three times the detection limit.
Assumptions: the blank measurements are normally distributed; the calibration is linear through the low range; the slope is constant near the quantification limit.
Limitations: the 10 s rule is a convention, not a guarantee of accuracy — confirm the LOQ experimentally by spiking at the LOQ level and checking recovery and precision. Results between the LOD and the LOQ are detected but not quantifiable.
Rounding: LOQ and LOD to 3 decimal places; the signal to 4 decimal places. The factors 10 and 3.3 are applied to unrounded values.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: Blank mean 0.05, blank standard deviation 0.02, slope 0.5 signal per CFU/g. LOQ signal = 0.05 + 10 × 0.02 = 0.2500; LOQ = 0.2500 ÷ 0.5 = 0.500 CFU/g; the detection limit is 3.3 × 0.02 ÷ 0.5 = 0.132 CFU/g.
Enter values above to see results.
Results between the LOD and the LOQ are reported as detected, not quantified. Never report a number below the LOQ as a quantitative result. Tool 74 of 500 — GIFSQ
