A Blank Lab Line Is a Limit, Not a Clean Result

If you run a private well, your water report may have a line that reads "not detected" next to a chemical name, or a number with a "less than" sign in front of it. It is easy to read that as a clean result. A recent piece from Circle of Blue, PFAS Contamination in Drinking Water May Be Underestimated, New Study Suggests, published October 23, 2026, takes up the gap between what a lab can report and what may be in the water. The summary opens by noting that when PFAS concentrations in drinking water fall below laboratory reporting limits, they are hard to see in the results.

That phrase is the key to reading your own report. A laboratory does not measure a sample and simply find nothing. It works with a floor, a smallest amount its method is set up to report as a number. Below that floor, the lab may say the compound was not detected, or it may report a value marked as less than the limit. Either way, the blank is a statement about the lab's reporting threshold for that test on that day. It does not say the water contains zero of that compound.

Why the floor matters for small systems

Private wells are tested less often than public systems, and often only when a homeowner asks or when a real estate sale or a new baby prompts a test. That means a single report may be the only record a household has. If that report shows a blank, the blank becomes the answer for years unless someone asks what it meant.

The reporting limit can also differ between labs, between methods, and between test dates. Two reports for the same well can look different even when the water has not changed much, simply because the lab used a different floor. Comparing a blank from one year with a number from another year can therefore mislead you. A number under one limit and a blank under a higher limit are not the same kind of result.

Questions to put to the lab

You do not need to be a chemist to ask a lab for the information that makes a report readable. Ask for the following on paper, and keep copies with the rest of your well records:

First, ask for the reporting limit for each compound on the report, not only the detection line. If the lab cannot state it, the report is missing the number you need to judge a blank.

Second, ask which method was used. Methods differ in what they are built to catch, and the method name should appear on the report or be available from the lab on request.

Third, ask whether a "not detected" entry is a final answer or a result that was below the reporting limit. Labs use different wording, so the same practical meaning can appear in different words from one report to the next.

Fourth, confirm the sampling date and the date the lab analyzed the sample. A sample collected months earlier does not describe the water running from your tap today, and that gap matters more when a test is close to a threshold.

Keep the blank in context

A blank line should prompt a question, not a conclusion in either direction. It does not prove the water is contaminated, and it does not prove the water is free of a compound. What it does is describe the limits of one test. When the reporting limit is higher than the level you care about, the report simply cannot tell you whether that level is present.

If you have questions about what a result means for the people in your household, those questions belong with a qualified health professional or with the agency that oversees private well testing in your county or state. A water report is a useful record, but it is not a medical opinion, and no single test should be read as one.

The practical habit is simple. Keep every well report in one folder. Write down the reporting limit, the method, and the sampling date beside each compound you care about. When a new report arrives, compare the limits before you compare the numbers. A blank with a clear limit tells you something specific. A blank with no limit tells you very little, and that is the line to question first.