A PFAS result that reads "not detected" is easy to file and easy to misread. The reported value comes from two things at once: the water that was sampled and the method the laboratory used to measure it. Before a utility, a dealer, or a household treats a non-detect as a clean finding, it helps to understand where the reporting limit sits and what it does to the record.
A reporting limit is the lowest concentration a laboratory will report as a quantified value under its procedure and quality controls. Below that level, a result is typically flagged as a non-detect, a below-reporting-limit value, or an estimate, depending on the laboratory and the method. The flag describes the floor of the measurement. It does not state the true concentration in the water, which may be lower, or higher if the sample was compromised before analysis. These are different claims, and a careful record keeps them apart.
What the recent coverage raises
Circle of Blue, in an article dated October 23, 2026, reported on a new study suggesting PFAS contamination in drinking water may be underestimated. The headline's own wording places the measurement question at the center. The summary notes that when PFAS concentrations "fall below laboratory reporting limits," the report moves into territory where the number alone cannot settle the matter. The summary is cut off before it states what the study concluded about those samples, so this article does not restate that conclusion. Readers should consult the full piece and the study it describes before drawing any conclusion about the size of the gap.
Three things a non-detect can mean
The first meaning is a true absence at the method's resolution: the compound is not present above the level the method can see. The second is presence below the reporting limit, where a real but small amount exists and the method cannot quantify it with confidence. The third is a result that never reflected the water at all, because of a problem in collection, preservation, shipping, or preparation. Only the third is a sampling failure. The first two are properties of the method. A record that merges all three into a single "ND" entry loses the information needed to decide whether a result is reassuring or merely silent.
Keep the limit beside the value
For any PFAS dataset, the reporting limit for each compound should appear in the same row as its value. Limits differ between laboratories and between methods, and they can change within one laboratory when its procedure is revised. A run of non-detects from one period, set against detections from another, may not measure the same thing. A trend line built on that mix can show a rise that is partly a change in the measurement floor, or a flat line that hides a real shift. The safer habit is to record the method and the limit on every row and to flag any comparison that crosses a change in either.
Questions to put to the lab and the file
Operators, equipment buyers, and consultants should ask a few plain questions before a non-detect shapes a decision. What was the reporting limit for each compound on each sampling date? Was the sample handled under a documented chain of custody from collection to analysis? Did the laboratory report estimated values between the detection limit and the reporting limit, and if so, how does the summary table treat them? Were field blanks and duplicates run, and do they show contamination or variability that the primary result cannot reveal? Each answer changes how much weight a non-detect can carry.
What a single result cannot settle
A single non-detect does not establish safety, and a single detection does not establish a source. Both conclusions depend on the sampling design, the timing relative to any treatment change, and the consistency of the method across the period under review. Where the record is thin, the accurate statement is that the question remains open at the method's current resolution. That is less satisfying than a clean number, but it is the claim the data can support. Questions about health effects belong with the local health authority and a physician, not with a laboratory flag.