Difference Between LOD and LOQ

FOOD ANALYSIS : DATA QUALITY

Figure 1: Detection comes first and quantification comes second. The gap between them is the zone where the analyst can say the substance is present but cannot say how much.

Introduction

LOD and LOQ are the two lowest limits of an analytical method, and they answer different questions. The limit of detection, LOD, is the lowest amount or concentration of a substance that the method can detect, but not necessarily measure with acceptable certainty. The limit of quantification, LOQ, is the lowest concentration that can be measured with acceptable precision and accuracy under the stated conditions of the test. LOQ is always higher than LOD, because measuring a number needs more confidence than simply seeing that something is there.

Quick comparison

Table 1: LOD compared with LOQ

Scientific explanation

Definition

Limit of detection, LOD: the lowest amount or concentration of an analyte that can be detected but not necessarily quantified with acceptable certainty by an analytical method. Limit of quantification, LOQ: the lowest concentration of the analyte that can be quantified with acceptable precision and accuracy under stated experimental conditions. Analyte: the substance being measured. Blank: a test run with no sample, to measure what the reagents and the instrument themselves contribute.

Working principle

Every instrument gives a small signal even for a blank, and that signal wobbles from run to run. LOD is set where the signal is clearly above that wobble: by the blank approach, the mean blank signal plus 3 times its standard deviation, which matches a signal to noise ratio of about 3 to 1. LOQ is set higher, where the signal is steady enough to measure: the mean blank plus 10 times the standard deviation, which matches a signal to noise ratio of about 10 to 1. Because quantification needs greater confidence and less variability than mere detection, LOQ is always higher than LOD, and it is often about 3 to 10 times the LOD.

Apparatus

Both limits belong to the method and the instrument together, not to either alone: the same instrument gives different limits for different foods, because the food matrix changes the blank noise. No separate instrument is needed: the limits are worked out from blank runs and calibration standards on the instrument that runs the test.

Measurement procedure

Run blank samples and work out the mean and the standard deviation of the blank signal. Set LOD at the mean blank plus 3 times the standard deviation, and LOQ at the mean blank plus 10 times the standard deviation. Check the limits with standards near those levels, and confirm that results at the LOQ repeat with acceptable precision and accuracy. Report a result below the LOD as not detected, and a result between the LOD and the LOQ as detected but not quantified, without a number.

Factors affecting readings

Blank noise is the first factor: a noisy blank pushes both limits up. The slope of the calibration line matters too: a steep slope separates small amounts more clearly than a shallow one. The food matrix matters: a complex food raises the blank and the interference, so the limits for a contaminant in a spice can be higher than in water. The limits must be set for each food and each method, not borrowed from another food.

Formulas and calculations

LOD = mean blank plus 3 times the standard deviation of the blank. LOQ = mean blank plus 10 times the standard deviation of the blank. By signal to noise, LOD matches a ratio of about 3 to 1 and LOQ a ratio of about 10 to 1.

The FTN books give no worked food example with numbers for LOD and LOQ, so no worked example is given here rather than an invented one. The rule to carry is the ratio: if a method has an LOD of 1 unit, its LOQ sits near 3 to 10 units, and a result between the two is reported as detected, not as a number.

Industrial applications

• Contaminant testing uses LOD and LOQ for mycotoxins, heavy metals, pesticide residues, and dioxins in feed and food.
• A laboratory that screens for aflatoxin reports a lot as not detected when the result sits below the LOD, and holds the lot for a quantitative method when the screen says the toxin is likely present.
• Regulatory limits are set above the LOQ, so that a laboratory can measure the amount it must enforce.
• Method validation reports both limits, so a buyer can judge whether the method is fit for the level that matters.

Advantages and limitations

LOD: it lets a laboratory screen fast and cheaply and say whether a substance is present. Its limit is that it gives no number: a result at the LOD cannot be reported as an amount, and it cannot be compared with a limit that needs a number.

LOQ: it gives a number that can be compared with a regulatory limit or printed on a report. Its limit is that it is higher than the LOD, so a substance present below the LOQ cannot be measured, only reported as detected, and the limits must be set again for each food and each method.

FAQ

References and further reading

• Skoog, D. A., West, D. M., Holler, F. J., and Crouch, S. R., Fundamentals of Analytical Chemistry: limit of detection, lowest amount distinguishable from noise, limit of quantification, lowest amount quantifiable with acceptable precision (consulted through Google Books).
• Wenzl, T., Haedrich, J., Schaechtele, A., Robouch, P., and Stroka, J., Guidance Document on the Estimation of LOD and LOQ for Measurements in the Field of Contaminants in Feed and Food, EUR 28099 EN, Publications Office of the European Union, 2016: estimation of LOD and LOQ from blank samples, calibration approach, and signal to noise.
• FTN Food Analysis, Unit 7: Acidity, pH, safety testing and analytical data (notes on disk): blanks, standards, and the rule that a screening result is confirmed by a quantitative method.
• Food and Agriculture Organization of the United Nations, Pesticide Registration Toolkit, Terms and definitions: limit of detection and limit of quantification (JMPR definitions).