Below detection is not zero: how to read DL, QL and “not detected”

Analytical reports often use short phrases—“not detected,” “below detection limit,” “below quantitation limit”—that sound more definitive than they are. For anyone comparing research materials or reviewing a certificate of analysis, the first question is not simply whether a number appears. It is whether the method was capable of detecting or quantifying the analyte at the level relevant to the question.

DL and QL answer different questions

The International Council for Harmonisation’s Q2(R2) guideline defines the detection limit, or DL, as the lowest amount that can be detected but not necessarily quantified as an exact value. It defines the quantitation limit, or QL, as the lowest amount that can be measured with suitable accuracy and precision. Read the official ICH Q2(R2) guideline.

That produces three useful zones:

The wording “may support” is intentional. Specificity, accuracy, precision, sample preparation and matrix effects still matter. A lower limit is one performance characteristic, not a complete evaluation of the method.

“Not detected” needs a method and a limit

A useful “not detected” statement should be read together with the analyte, analytical procedure, units and stated DL. Without that context, two laboratories can use the same words while having very different capabilities.

Imagine two reports examining the same hypothetical impurity. Laboratory A has a stated DL of 0.10% and Laboratory B has a stated DL of 0.01%. If both report “not detected,” they have not necessarily made equally sensitive observations. Each result is bounded by its own procedure and lower limit. This fictional example illustrates interpretation only; it is not a comparison of any actual laboratory or product.

How lower limits may be established

ICH Q2(R2) describes several approaches for evaluating lower-range limits. These include visual evaluation using samples with known concentrations, signal-to-noise comparisons, calculations based on response variability and calibration slope, and direct evaluation of accuracy and precision near the lower range.

For procedures with baseline noise, the guideline notes that signal-to-noise ratios around 3:1 and at least 10:1 are generally considered acceptable for estimating DL and QL, respectively. Those ratios are method-validation concepts, not universal conversion factors to apply to a report that does not disclose its approach.

The guideline also recommends presenting the limit and the method used to determine it. When a limit is estimated, confirmatory work near that limit may be appropriate. The FDA’s current Q2(R2) guidance page identifies the same harmonised guideline for analytical-procedure validation. See the FDA Q2(R2) guidance page.

A five-question reading checklist

1. What analyte is the limit for? A DL for one impurity does not automatically describe the capability for a different analyte.
2. What are the units? Percent area, mass fraction, concentration and absolute amount are not interchangeable.
3. Which method and sample preparation were used? The limit belongs to the complete procedure, not just the instrument name.
4. How was the limit established? Look for the stated approach and whether the report distinguishes DL from QL.
5. Does the limit answer the actual research question? A method can be valid for one intended purpose while being insufficiently sensitive for another.

This checklist complements our guide, Is the Analytical Method Fit for the Question?, which looks more broadly at identity, purity and quantity claims.

Record the reporting convention

When transferring a result into a spreadsheet or evidence summary, preserve the laboratory’s original qualifier. Do not replace “<0.05%,” “below QL” or “not detected at DL 0.01%” with a bare zero. A bare zero erases the method’s lower boundary and can distort later comparisons or calculations.

A compact record can include: analyte, reported result, qualifier, units, method, DL, QL, report date, lot or sample identifier and source document. If a field is not supplied, mark it “not stated” rather than guessing.

Scope matters

ICH Q2(R2) is written for validation of analytical procedures used in regulated pharmaceutical contexts. This article uses its clearly defined analytical terms as a research-literacy reference; it does not claim that every catalog report is governed by that guideline or that a result has been independently verified by bwell.

Sources and editorial method

1. ICH Q2(R2): Validation of Analytical Procedures, final version adopted November 1, 2023.
2. U.S. FDA: Q2(R2) Validation of Analytical Procedures.

Sources checked October 13, 2026. Prepared with AI assistance using the linked primary guidance. The interpretation table, checklist and hypothetical example are editorial explanations from bwell; they do not represent independent laboratory testing or product verification.