A Non-Detect Is Not a Clean Bill: Read PFAS Results Across Several Years

A PFAS result on a water report often arrives in one of two forms: a number, or a line saying the compound was not detected above a stated reporting limit. The second form is easy to read as a clean bill, and that reading can mislead a household trying to decide whether to ask more questions, keep watching, or consider a filter. A reporting limit is the lowest level a laboratory can measure with confidence using a particular method. Anything below that line is reported as a non-detect. That label describes the instrument, the method, and the sample handling as much as it describes the water itself.

A recent Circle of Blue report on PFAS in drinking water, dated October 23, 2026, starts from this problem. Its summary notes that "When PFAS concentrations in drinking water fall below laboratory reporting limits" the results need careful reading. The headline itself says contamination may be underestimated, which is a reminder that a non-detect is a statement about what a test could see, not a guarantee about what is present.

Why the limit matters from year to year

Laboratories update methods, replace instruments, and sometimes change contract labs. Each change can move the reporting limit. A page from one year might show a limit that is higher or lower than the limit printed on the next year's page. A non-detect at a higher limit does not carry the same meaning as a non-detect at a lower one. If the limit dropped, a later detection may be the first time a smaller amount became visible, which can look like a sudden rise even when the water has not changed in a meaningful way. If the limit rose, an older detection could be hidden in a newer report that looks cleaner.

The practical habit is simple. Put each year's result next to that year's reporting limit, method name, and laboratory name. A row that reads "not detected" means something different when the limit beside it is far above the one from the year before. Without the limit, the row cannot be compared with anything.

Build a multi-year file

Start by asking the utility for the full laboratory report, not only the summary table that often appears in the consumer confidence report. The full report usually names the method, the reporting limit for each compound, the sample date, and where the sample was taken. Sampling location matters. A sample from a treated entry point to the distribution system answers a different question than a sample drawn at a house. Write that location down beside each result.

Keep a simple table with columns for the sample year, sample date, location, each compound reported, the result, the reporting limit, the method, and the laboratory. Note how the report marks a non-detect, whether with "ND," a less-than sign, or a blank. Those marks differ between reports, and a blank can mean the compound was not tested at all. Save the original PDFs in the same folder so you can check the table against the source later.

Questions to bring to the utility

Once the table is built, the gaps usually show where to ask. Useful questions include which method was used each year, whether the reporting limit changed, whether the laboratory changed, and what sampling schedule applies to the system. It is also reasonable to ask whether samples are taken at more than one point in the system, and whether the utility plans to test additional locations. A utility can answer these questions without special access. They are record questions, the same kind of questions a homeowner might ask about any other test.

If the answers are unclear, the file itself becomes the question. A utility that can show the method, the limit, and the sample location for each year gives a reader something concrete to compare. A utility that cannot produce those details gives the reader a reason to ask again in writing.

Where treatment fits

A household that wants to reduce what comes out of its own tap has several options, and each depends on the compounds involved and the product's certification. Read the certification language closely and check that it names the compounds you care about. Compare it against your own test results and the utility's full report, not against the summary alone. A homeowner comparing point-of-use options might start by asking reverse osmosis specialists serving the St. Louis area what their units can and cannot remove, then check those answers against the certification paper.

Questions about personal exposure, or about what a result means for a particular person, belong with a doctor or a local health office. This article is about reading the paper trail a water system leaves behind, so that the next report can be understood in context rather than taken at face value.

Related coverage: Top Water Treatment Missouri.