Detection Limits in Water Testing: Why Non-Detect Isn’t Zero

You open your water test report and find ND or a less-than sign beside the contaminant that worried you. Non-detect means the lab did not find the substance above the lowest level it can report. It does not mean none of the substance is present.

Detection limits in water testing determine how low a method can see. At ETR Laboratories, we want you to read every ND alongside the limit reported with it. This guide covers those limits, the flags that mark uncertain numbers, and how to compare a reporting limit with a legal limit.

Our guide to how to read your water test report line by line explains the other columns and terms.

Key Takeaways

• Non-detect, or ND, means the lab did not find a substance at or above its reporting limit.
• The true amount could be zero or a small amount below that limit.
• The reporting limit is usually higher than the method detection limit.
• A J-flag commonly marks an estimated result below the reporting limit.
• An ND is most useful when the reporting limit is low enough to answer the question you are asking.
• Compare reporting limits and units before comparing results from different laboratories.

What Non-Detect Actually Means

Non-detect means the laboratory did not find the substance at or above the reporting limit for that test. The true concentration could be zero, but it could also be present at a level below what the laboratory reports as a firm number. That is why an ND should always be read together with its reporting limit.

Every analytical method has a floor. Below a certain concentration, the laboratory cannot confidently report a substance as a measured number.

The Minnesota Department of Health explains that, for compliance purposes, a result shown as less than the reporting limit is treated as non-detect.

For a homeowner, however, the next question is how that reporting limit compares with the level you care about.

Detection Limit vs Reporting Limit: The Terms on Your Report

Several related terms can appear on laboratory reports, and they do not all mean the same thing.

Term	What It Means
Method detection limit (MDL)	The lowest measured concentration that can be distinguished from background with the statistical confidence defined by the method.
Limit of detection (LOD)	The lowest concentration that can be distinguished from zero or background.
Limit of quantitation (LOQ)	The lowest concentration that can be measured with acceptable accuracy and precision.
Practical quantitation limit (PQL)	A practical level at which the laboratory can reliably quantify a substance during routine testing.
Reporting limit (RL)	The lowest concentration the laboratory reports as a numerical result.

Terminology can differ between methods, regulatory programs, and laboratories, so the definitions provided with your own report should come first.

The Minnesota Department of Health notes that a reporting limit is generally around two to five times the MDL. That is a rule of thumb from that source, not a universal ratio for every method or laboratory.

Our guide to water quality parameters labs measure provides more terminology, while how VOCs are tested shows how analytical methods and limits work together.

How Does a Lab Set a Method Detection Limit?

A method detection limit is not chosen arbitrarily.

EPA’s procedure for determining the method detection limit uses a statistical process to establish the minimum measured concentration that can be distinguished from method-blank results with 99% confidence.

A method blank is clean material that goes through the same analytical process as the customer sample. Laboratories also use calibration standards to make sure the instrument’s response is tied to known concentrations.

The reporting limit is then established above the detection floor. The Minnesota Department of Health notes that the reporting limit generally cannot be lower than the laboratory’s lowest calibration standard.

At ETR, calibration standards and blanks are part of the quality-control process described in how ETR tests your water. Our article on what makes a water lab certified explains how laboratories review quality systems.

What Does a J-Flag Mean?

A J-flag commonly marks an estimated concentration between the method detection limit and the reporting limit.

In other words, the laboratory detected a signal, but the concentration was below the level at which it normally reports a fully quantified result. The Minnesota Department of Health notes this type of qualifier in its guidance on drinking water reports.

Other common notations include:

• ND: The substance was not reported as detected above the laboratory’s applicable limit.
• < followed by a number: The result is below the stated reporting limit.
• J beside a number: The value is estimated and carries more uncertainty than a fully quantified result.

Qualifier conventions can vary between laboratories, so check the definitions on the report itself.

If a J-flag appears next to a contaminant that matters to your decision, ask the laboratory what the qualifier means for that method and whether another sample would add useful information.

How Does a Reporting Limit Compare With the Legal Limit?

This is where detection limits become especially useful.

A maximum contaminant level, or MCL, is a regulatory drinking water limit. A reporting limit tells you how low the laboratory reports a result. They are different numbers with different jobs.

EPA’s National Primary Drinking Water Regulations list the MCL for arsenic as 0.010 mg/L, or 10 parts per billion. That same standard is referenced on ETR’s arsenic water test page.

An ND can mean different things depending on the reporting limit:

• Reporting limit well below the MCL: An ND gives useful evidence that the contaminant was not found near the regulatory limit.
• Reporting limit close to the MCL: An ND establishes that the result is below the reporting threshold but gives less information about how far below the MCL it may be.
• Reporting limit above the MCL: An ND may not be sensitive enough to answer whether the sample meets the regulatory benchmark.

That is why the number beside the ND matters.

Lead deserves similar care. EPA lists a health goal of zero for lead, while federal drinking water requirements use an action level rather than an MCL in the same sense as arsenic. An ND is more informative when the reporting limit is sufficiently low for the question being evaluated.

For more on regulatory limits and health goals, see our guide to MCL vs MCLG.

PFAS: Where Detection Limits Carry the Most Weight

PFAS provides an especially clear example because concentrations are discussed in parts per trillion rather than parts per billion.

EPA’s PFAS drinking water information lists limits for PFOA and PFOS at 4.0 parts per trillion. One part per billion equals 1,000 parts per trillion, so a method designed to report only at much higher concentrations would not be useful for evaluating a result near that level.

ETR’s PFAS water test screens 18 compounds, with analysis performed through an independent accredited laboratory. Because the relevant concentrations are so small, ask about the current reporting limits for PFOA and PFOS before ordering.

Our guide to PFAS limits for wells provides more context for private well owners.

Why Do Two Labs Report Different Limits?

Two laboratories can test for the same substance and still report different limits.

Several factors can explain the difference:

• Different analytical methods and instruments have different sensitivities.
• Laboratories may use different calibration ranges.
• Regulatory programs can require particular reporting limits.
• The sample itself can interfere with the analysis.
• A sample may need to be diluted because of matrix interference, which can raise the effective reporting limit.

That last point matters because water is not always a simple laboratory matrix. High concentrations of other substances can interfere with an instrument’s ability to measure the target compound at its usual sensitivity.

Compare limits before comparing prices. A lower-cost test is not necessarily useful if its reporting limit is too high for the level you need to evaluate.

Our guide to red flags on a water test result covers missing or unsuitable limits and other things to check when comparing reports.

What Should You Check Before You Trust an ND?

When you see ND or < on a laboratory report, use this five-step check:

1. Find the reporting limit on the same line. That tells you how low the laboratory was prepared to report the substance.
2. Check the units. Make sure the result, reporting limit, and benchmark use the same scale before comparing them.
3. Compare the reporting limit with the relevant standard. An ND is more informative when the limit is comfortably below the level you need to evaluate.
4. Look for qualifiers. A J-flag or another notation may indicate that the laboratory detected something but considers the number estimated.
5. Ask the laboratory if the limit answers your question. If it does not, ask whether another method or lower reporting limit is available.

For help with the other parts of the document, return to our guide on how to read your water test report line by line.

Frequently Asked Questions

Does ND mean there is none in my water?

No. ND means the laboratory did not report the substance at or above its applicable reporting limit. The actual amount could be zero, but it could also be present below that limit. Look at the reporting limit beside the result to understand how low the test could meaningfully report.

What is the difference between an MDL and a reporting limit?

The method detection limit is a statistically determined level at which the method can distinguish a signal from background. The reporting limit is the lowest level the laboratory normally reports as a numerical result and is generally higher than the MDL.

What does a J-flag mean on a water test?

A J-flag commonly means the reported concentration is estimated. Under the Minnesota Department of Health framework used here, it refers to a value between the method detection limit and reporting limit. Because qualifier conventions can vary, check your laboratory’s definitions and ask whether retesting would be useful.

Can a lab lower its reporting limit?

Sometimes. A different method, instrument setup, calibration range, or less sample dilution may allow a lower reporting limit. That is not possible for every analyte or sample, so ask the laboratory before ordering whether the available limit is low enough for your purpose.

Check the Limit Before You Order

Before ordering a water test, ask what reporting limit the laboratory uses for the contaminant you are concerned about and compare it with the benchmark you need to evaluate.

This matters especially for testing at very low concentrations, such as PFAS, where results are measured in parts per trillion. If the reporting limit is not low enough, an ND may not answer the question you actually have.

ETR Laboratories can explain the reporting limits used for the parameters you are considering. If you are still choosing what to test, our panel guide can help you compare options, while our guide to reading your water test report covers the rest of the results.

You can also call ETR at (800) 344-9977 and ask about the reporting limit for a specific parameter before placing an order.