Quality & Testing
LOD vs LOQ: What Not Detected Means in a Laboratory Report
A laboratory report that says not detected has not demonstrated that the measured substance is completely absent. To understand the result, read the stated analytical limit, its definition, the method and the report’s qualifications together. EPA’s regional guidance explicitly warns against automatically treating non-detects as zero. Its environmental risk-assessment procedures are context-specific; they are not instructions for interpreting every laboratory report. Source: EPA guidance on data near detection limits.
This general report-reading guide explains detection and quantification terminology. It does not document testing of Peptides Centre Canada materials, establish product quality or safety, or provide medical advice. For the broader document checklist, see how to read a certificate of analysis.
LOD and LOQ answer different questions
The limit of detection (LOD) concerns the lowest amount or concentration that a specified analytical procedure can detect with reasonable certainty. It is tied to the procedure and its statistical basis, not simply to the number of decimal places displayed. Source: IUPAC Gold Book, limit of detection.
The limit of quantification (LOQ) concerns the lowest level at which the method can determine an analyte with acceptable performance. Detection and acceptable quantitative measurement are therefore different capabilities. Eurachem also cautions that a calculated LOQ should be experimentally confirmed when it is a critical performance characteristic. Source: Eurachem, Planning and Reporting Method Validation Studies, 2025, pages 11–12.
A result can contain evidence of an analyte without supporting an unqualified numerical concentration. Read any estimate or qualifier in the laboratory’s own terms. Do not replace a qualified result with either an exact value or zero when copying it into a spreadsheet. EPA’s regional guidance provides one context-specific example of estimated results between analytical limits; its qualifier codes should not be assumed universal. Source: EPA reporting guidance.
Method limits are not just instrument specifications
An instrument detection limit and a limit established for the whole analytical method need not describe the same capability. Eurachem distinguishes repeating the final instrumental measurement from evaluating the full procedure, including sample preparation. It also notes that different sample matrices may need separate assessments. A matrix is the material surrounding the analyte in the sample. Source: Eurachem validation-planning supplement, page 11.
The EPA’s Clean Water Act method-detection-limit framework illustrates why this matters: its revised approach includes method blanks as well as spiked samples, and considers background contamination from laboratory supplies and equipment. It aims to represent laboratory performance over time, not only a favourable instrument check. This is a specific US analytical framework, not a universal Canadian laboratory requirement. Source: EPA method detection limit questions.
Read the limit beside “not detected”
Illustrative example only: two fictional reports show “ND” for the same named analyte. One lists a reporting limit of 0.5 mg/L; the other lists 5 mg/L. Those labels alone do not establish equivalent analytical evidence. Neither report says the concentration is exactly zero. The example is not a test result, a specification or a safety threshold.
Before comparing the reports, ask what each laboratory means by “reporting limit,” whether the sample type and method scope match, and whether both results use the same units and reporting basis. Retain each limit and qualifier alongside the result. A smaller printed limit, without its supporting context, is not a complete quality comparison.
Terminology varies between analytical programmes. For example, EPA’s SW-846 guidance discusses a lower limit of quantitation, calibration and verification through preparation and determination. Do not assume that LOD, LOQ, LLOQ and a report’s reporting limit are interchangeable labels. Source: EPA SW-846 quality-control questions.
Six questions to send back with an unclear report
Use this practical reading checklist, not as a method-validation protocol:
- Which analyte, sample identifier and sample type does this result describe?
- What does this report mean by ND and by each listed limit?
- Which method and version were used, and does the stated limit cover the complete procedure?
- What are the units and reporting basis, and are sample-specific adjustments reflected?
- What qualifiers, interference notes or quality-control limitations accompany the result?
- Is the method’s reporting capability suitable for the particular research question being asked?
Keep the laboratory’s explanation with the original report. Our lot-traceability guide addresses connecting documents to material records; our measurement-uncertainty guide explains related distinctions between uncertainty, resolution and calibration.
The useful conclusion is not “ND means nothing is there.” It is “the report must be read within the limits of the method and the question it was designed to answer.”
