A PFAS result reported below the laboratory's reporting limit is not the same as a result of zero. It means the method did not produce a value the laboratory would report as a number at that level. A Circle of Blue report on PFAS in drinking water, dated October 23, 2026, carries the headline point that contamination may be underestimated. Its summary opens at that same boundary: "When PFAS concentrations in drinking water fall below laboratory reporting limits" and the sentence is cut off there. The summary is partial, so this article does not describe the study's methods, sample set, or the size of any gap. Readers should work from the full piece before drawing conclusions.
Two numbers sit behind every non-detect
Every censored result carries at least two numbers. The first is the reporting limit, the lowest level the laboratory will report as a quantified value under its method, calibration, and blank performance. The second is the threshold the result is compared against, whether that is a regulatory level, a permit limit, a treatment target, or a household's own concern. A non-detect answers a question only when the reporting limit sits at or below that threshold. When the limit is higher, the result shows only that the concentration is below what the method could report. Reporting limits differ between laboratories and between methods, so the same sample can carry different limits from two labs. The gap between the two numbers is the first thing to read.
What belongs in the file
Record the reporting limit next to every result, not in a lab report appendix that nobody opens. Record the threshold the result was checked against, along with the date that threshold was in force, since the rule or permit in effect on the sampling date governs. Where the limit exceeds the threshold, flag the result as unable to demonstrate compliance at that level rather than filing it as a clean pass. Keep the lab's qualifier codes intact, because they often carry the only information about values between the detection and reporting limits. Finally, decide before data arrives how non-detects will enter any average or trend. Treating them as zero, as the limit, or as a fraction of the limit changes the output, and that choice should be written down with the data rather than inherited from a spreadsheet default.
Where the rule decides
How a non-detect counts for compliance is a matter for the governing rule, permit, or agency guidance, not for the laboratory. Utilities and consultants should check the text in force at the time of sampling and avoid assuming that a practice from one program carries over to another. Where the rule is silent, the file should say so, and the question should go to the regulator in writing before a summary is issued. Describing a result as below detection in a public report is a different statement from describing the water as free of the compound, and the wording should match what the method and the rule actually support.
For households and treatment decisions
A household test with a high reporting limit may not answer the question the household is asking, and a clean line on the report can look more reassuring than the method supports. Ask the laboratory for the reporting limit and compare it with the compounds and levels that matter for the decision at hand. Treatment claims need the same check. A unit's documentation should name the compounds it has been tested against and the concentrations used, and that list should be read against the household's own results rather than a general brand statement. Households weighing options can compare notes with local RO specialists, but the conversation goes better when it starts from the compound list and the reporting limit rather than from the unit's marketing.
What this does not settle
The study's headline suggests that PFAS contamination may be underestimated in some drinking water data. The summary available here is incomplete, and it does not describe how the researchers handled censored results, which waters they examined, or how large any underestimate might be. Those details matter for any use of the finding in a permit, a rate case, or a consumer notice. Until the full report has been read against the method and the sample record, the finding is best treated as a prompt to examine how non-detects are recorded and interpreted, not as a measure of any particular system.