A Higher PFAS Number Can Come From a Better Lab: Check the Method Before You Compare Years

A PFAS result can rise from one year to the next while the water itself stays much the same. That sounds odd, but it follows from how laboratories report what they measure. Before a rising number reads as a warning, a reader should check whether the test underneath it changed.

What the lab is allowed to say

Every drinking water test has a floor. A laboratory can report a concentration only when it can measure that concentration with enough confidence, and the lowest level it will report is called a reporting limit. Below that level, a report often shows the compound as not detected, shows a value flagged as below the limit, or leaves a qualifier in place of a number. Two labs can look at the same tap and produce different-looking reports if their floors differ. A lab that lowers its floor can find compounds that a higher floor would have hidden. Nothing in the water has to move for the report to change.

A recent piece on drinking water shows why this matters beyond a single sheet. Circle of Blue carried an article by Alissa de Chassey, which originally appeared on Inside Climate News, under the headline PFAS Contamination in Drinking Water May Be Underestimated, New Study Suggests, dated October 23, 2026. The summary of that piece points to the situation where PFAS concentrations "fall below laboratory reporting limits," a boundary that every test has. For a reader, the lesson is not a verdict on any one result. A missing or low value depends on the floor beneath it.

Check four things before you compare years

Pull the two lab reports side by side, then look for these four items.

First, the reporting limit. Find the number printed next to each compound. If the limit dropped between years, a newly detected value may reflect a better floor rather than new contamination.

Second, the laboratory and the method. A change in lab or analytical method is a change in the instrument and the procedure, even when the sample came from the same tap.

Third, the sampling point and the timing. A sample taken at the treatment plant, at a distribution point, and at a home tap are different measurements. A sample taken in a different season, or after a line was flushed, is also a different snapshot of the same system.

Fourth, the qualifiers. Look for notes that say not detected, estimated, or below a limit. A blank cell in a table is not the same as a zero, and a footnote can change how a number should be read.

What a higher number can and cannot tell you

When the floor, the method, and the sampling point all match, a rise is worth taking seriously. It may mean the water changed. When one of those three differs, the honest reading is that the two numbers answer slightly different questions. That is not a reason to dismiss a result, and it is not a reason to assume the earlier result was clean. It is a reason to ask the utility or the laboratory, in writing, what changed between the two reports.

Utilities usually keep this paperwork, and state drinking water programs often hold copies as well. A written request for the full lab report, the reporting limit, and the sampling protocol is a routine step for a customer or a board member. Ask for the same three items for both years so the comparison is between like things rather than between a new method and an old one.

Small systems deserve the same care. A town with a handful of sampling points may see a single new result change its public picture, so the sampling calendar and the lab identity matter as much as the number itself. A board packet that explains the method change next to the result is more useful than one that shows only a chart.

Read the trend line as a claim

A trend is a claim that two measurements can be compared. Often they can, and then the change matters a great deal. Sometimes they cannot, and the line on a chart reflects the laboratory's progress rather than the tap's condition. The useful habit is simple: before accepting a rise or a fall, confirm that the floor, the lab, and the place you sampled stayed the same. Only then does the number tell you what it appears to say.