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Reading a laboratory report

How to Read a Well Water Lab Report and Know What It Tells You

Lab reports can feel dense because the result, unit, flag, and sample details may sit in different columns. Start with the bottle the report describes, not the biggest number on the page. This guide helps you check the paperwork and preserve the laboratory's wording. Meaning and safety questions still go to the certified laboratory or your state or local health authority.

Prepared by Well Water ClarityLast reviewed September 17, 2026How we prepare guidance
  • Observe. Begin by reading the sample ID, collection point, collection date and time, and raw-or-treated status, and check each against your own label before you look at any result.
  • Test. Confirm that the laboratory tested the analyte you meant to order and reported the sample you meant to send.
  • Compare. Set reports side by side only when the sample points, analyte names, units, and collection details match, and keep every result with its own printed unit.

These checks help you read the right row in the right context. Observing the report and comparing the paperwork support the laboratory's record, but they do not replace its written directions, the notes printed on the report, or guidance from the health authority that serves your area.

Start with the sample, not the number

A report follows one container of water from a named sample point. Before looking at a result, find the sample or well identifier, collection date and time, location, and raw-or-treated status. Check those details against your label or collection notes. If they do not match, stop and call the laboratory.

The sample point changes what the report describes. Water collected after a kitchen filter is not the same sample as water from a tap that bypasses treatment. Keep the plumbing path with the report so you do not later mistake one for the other.

Once the sample identity is clear, read each row without separating the result from its unit, flag, and report notes.

A laboratory analyst examining a tray used to check a water sample for bacteria
A laboratory report records what one sample at one location showed. The collection date, sample point, and units on the form matter as much as any value in the table.
Important exceptions

When to act before reading the numbers

A few conditions are urgent, and they come before all the reading tips on this page. If any of these applies, switch to bottled water or another source recommended by local officials and call your local or state health department, environmental agency, or the emergency service or well professional that handles your situation. Do not wait to finish the guide.

  • Officials issued an advisory for your area or your water.
  • Someone may be ill and a water issue could be involved, especially an infant, someone pregnant, or a person with a weakened immune system.
  • Floodwater reached the well or the area around it.
  • Sewage or chemicals could be involved, including runoff, a spill, or a failed septic system.
  • Your report shows coliform bacteria present. A coliform result, and especially a result with E. coli, is one where you stop explaining and start calling. Health agencies treat E. coli in well water as a sign that human or animal waste may have reached the water. Stop or adjust water use as your local authority directs and follow its sampling advice. See the EPA guidance for private wells after a flood and ask your health department about retesting.

These are the exceptions. For an ordinary report with no advisory, no illness, no flood and no sewage or chemical concern, read on with the structure-based method in this guide.

Read each result in order: sample, date, location, substance, units

Use the same order every time. It keeps a familiar-looking number from pulling your attention away from a wrong sample point, date, or bottle.

1. Sample and well identifiers

Find the sample number and the well or client identifier the laboratory assigned. If you sent several bottles, each sample usually gets its own identifier. Confirm the one on the report matches the bottle and paperwork you submitted. If it does not, call the laboratory before using the report.

2. Collection date and time

Compare the collection date and time on the report with your own notes. A mismatch is worth a phone call. The date also helps you keep reports in order and connect a result with weather, well work, plumbing changes, or treatment maintenance.

3. Sample location: raw or treated water

Look for a location label such as a kitchen tap, outdoor spigot, wellhead tap, or stated point before or after treatment. A raw-water sample and a treated-water sample answer different questions. Never assume an outdoor faucet bypasses treatment, and never assume a kitchen tap shows raw water. If the location is unclear, use the guide to sampling before or after a softener and confirm the plumbing route before the next collection.

4. The analyte: what was actually measured

Each row names one substance or property, often called an analyte or parameter. Read that name exactly. One row does not answer a question about substances that were not included in the test. If an abbreviation is unclear, ask the laboratory what it stands for before treating the row as meaningful.

5. The result and its unit

Copy the result exactly as printed, including its unit and any symbol or word beside it. A result without its unit is incomplete. If the report uses a word such as present, absent, or non-detect instead of a number, keep that wording intact rather than translating it yourself.

Keep the unit attached so you compare results correctly

The unit tells you how the laboratory expressed a result. Common labels include mg/L (milligrams per liter), ug/L or µg/L (micrograms per liter), ppm (parts per million), ppb (parts per billion), grains per gallon, and pCi/L (picocuries per liter). Some rows, such as pH or bacteria findings, use different kinds of scales or wording.

These labels are part of the result. Do not remove a unit, replace it from memory, or compare results printed in different units. Ask the laboratory whether the reports can be compared. For your records, copy the analyte, result, unit, and any flag exactly as printed.

Units also help reveal transcription mistakes. If a handwritten note leaves the unit out, return to the original report instead of guessing. If the unit on a new report differs from an older report, flag the difference when you call the laboratory.

Understand what non-detect means and what it does not

A report may show ND (not detected), BDL (below detection limit), or a less-than symbol beside a value. This means the laboratory did not report the substance at or above the stated reporting limit for that method and sample. It is not proof that the substance is absent.

Find the reporting or detection limit printed on the report and keep it with the ND result. Laboratories may use several related terms for these limits, so ask what the label on your specific report means. Do not turn ND into a pass, a safety conclusion, or a zero in your records.

If the report does not show the limit, or if you do not know whether the method was suitable for your question, call the laboratory. Ask it to explain the method, the reporting limit, and whether another analysis or sample is needed. That is a laboratory-method question, not something to settle from a general conversion chart.

Compare results only against the value your report supplies

A report may include a comparison value, guidance value, reference value, or flag beside a result. Use the value and label printed on the report or supplied by the certified laboratory or the appropriate state or local program. Do not substitute a number remembered from a search result, a neighbor, an older report, or another state.

Comparison labels can serve different purposes. Some relate to health, while others address taste, odor, staining, corrosion, or equipment performance. A public-water standard also may not carry the same regulatory meaning for a private well. Ask the laboratory what the comparison column means before treating it as a household decision.

A result and a comparison value are pieces of the report, not a complete safety judgment. If the applicable comparison is missing or unclear, record the question and call the laboratory or your state or local health program. Do not create your own comparison or use the page to decide that the water is safe or unsafe.

Tell health flags apart from taste, staining, and equipment notes

Not every row on a report answers the same kind of question. Some reports group findings by health significance, appearance or taste, and plumbing or treatment performance. Others place every result in one table and use flags or notes to show why a row matters.

Use the report's own headings and flags rather than assigning categories from memory. A substance can affect more than one part of the household, and the same label may be handled differently under state or local guidance. If a row is not clearly classified, ask the laboratory whether it is being flagged for health, aesthetic, or operational reasons.

This separation keeps treatment discussions in the right order. A staining or equipment concern may still be worth solving, but it should not be presented as a health conclusion. A health-related flag needs interpretation from the laboratory or the appropriate health program before you compare treatment.

Take your water well results and safety questions to the lab or health authority

Your part is to identify the sample and read the row accurately. The certified laboratory or the appropriate public health authority can explain what the finding means for your location and household. Keep the sample details, analyte, result, unit, reporting limit, flags, and comparison column in front of you when you call.

Do not label the water safe or unsafe from one row or from a number found elsewhere. If a result concerns you, ask the laboratory what the row reports, which local guidance applies, and whether it recommends another sample or another test.

When you speak to the laboratory, useful questions include:

  • What does this row describe, and what was the reporting limit for this method?
  • Which comparison value applies to this result for a private well in my area?
  • Was the sample raw or treated, and how does that affect what I can conclude?
  • Should I retest, sample at a different location, or test additional analytes? See what to test each year and how to collect a correct sample.
  • Can you recommend or refer me to my local or state health authority for interpretation?

Compare two reports fairly when the samples differ

Two reports can look similar while answering different questions. Before treating a change on paper as a change in the well, check whether the samples are genuinely comparable. Start with the sample location. A raw-water tap, a kitchen tap after treatment, and an outdoor faucet on an unknown plumbing branch are different locations, even when all of them receive water from the same well.

Put the reports side by side and work from the headers down. Circle or note each field that differs, but do not try to explain the difference. First match the sample IDs to your own collection records. Then match the tap descriptions, collection dates and times, and raw-or-treated labels. If one report leaves a field blank, mark it as unknown and ask the laboratory. A blank is not permission to assume the samples match.

Next compare the collection notes. Look for the same analyte name, the same unit, and the same raw-or-treated status. Check whether the laboratory used the same method and whether the reporting limit changed. A new laboratory, a different method, or a different reporting format can change how a result is printed without proving that the water itself changed.

Use the laboratory's full analyte name rather than shortening it in your notes. Keep qualifiers, symbols, and footnotes with the row they belong to. Check the next page when a row carries a footnote marker, since the explanation may not sit beside the table. If either report uses an abbreviation or flag you do not recognize, write that down as a question. The comparison is ready for a laboratory call when the paperwork is organized, not when you have supplied your own explanation.

Treatment and maintenance records belong beside the reports. A filter replacement, softener service, plumbing repair, pump work, well disinfection, or change in the sampling tap can explain why two samples are not directly comparable. Weather and property events also belong in the record when they prompted testing, but they do not prove the cause of a result.

A simple comparison note should contain:

  • the date and time shown on each report;
  • the exact sample location and whether it was before or after treatment;
  • the analyte name, result, unit, and reporting limit exactly as printed;
  • the laboratory and method, when the report lists them;
  • well, plumbing, or treatment work completed between the samples; and
  • the question you want the laboratory to answer.

If any of those fields differ, mark the difference instead of forcing a trend. Ask the laboratory whether the two reports can be compared and what additional sample, if any, would answer the question. This keeps your records useful without turning a pair of laboratory forms into a diagnosis.

Keep a one-page cover note with each report. Write the reason for testing, the tap used, whether treatment was operating, who collected the sample, and any event that led to the test. Attach the laboratory's instructions and your submission form when you have them. On a future call, this small record lets the laboratory see how the sample was collected without relying on memory.

Follow a worked example to read your own report

Reading a report is a repeating habit, not a hunt for the largest number on the page. To see what that rhythm feels like, walk through one sample using neutral placeholders instead of the real terms on your own form. The point is to practice the order, not to fill in values from memory.

Start with the sample identity at the top of the page. Find the sample ID the laboratory assigned, then the collection point it printed, such as an unnamed kitchen tap, outdoor spigot, or stated point before or after treatment. Check both against the label you attached and the collection date and time on the form. Stop and call the laboratory if the sample ID or the collection point does not match what you submitted.

Next, read a single row as one group. Keep the analyte name, the result, the unit, any flag, and any footnote marker together. Read the explanation at the bottom of the page before you move to the next row, because a footnote marker rarely sits beside the value it qualifies. Copy the analyte and result exactly as printed, including whatever wording the laboratory uses for a non-numeric finding.

When you compare two reports, confirm both describe the same sample point and that each result is printed in the same unit before you treat the papers as related. Different units or different sample points are reasons to call the laboratory, not to continue reading. Finally, prepare your questions for that call: what the flag means, which comparison value applies, and whether a retest or another sample is suggested. Observation supports this sequence, but the laboratory's written directions remain the authority for how the collection was planned.

What to do when your water well report arrives

Work through this short checklist so the report is read consistently, then keep it with the date, the sample details, and any treatment changes.

  • Confirm the sample ID, collection date, and location on the report match the bottle you sent.
  • Read each row as one analyte, one value, one unit, and one comparison value.
  • Note every non-detect row, and ask for the reporting limit if it is not shown.
  • Separate health-relevant results from aesthetic, staining, and operational results.
  • Use the comparison value the report or your state and local authority provides.
  • Call the laboratory or your health authority for interpretation before drawing any conclusion about safety.
  • File the report with the sample date and location. Compare a future report to it only when the samples are comparable, meaning the same location and similar collection method.

If your report shows coliform present, or arrives at a time of advisory, illness, flooding, or a possible sewage or chemical event, treat the earlier "When to act before reading the numbers" section as your first step and contact the right authority before anything else. For choosing what to test next, see annual well testing, and to plan your next sample read how to collect a well water sample. If a result leads to a treatment conversation, wait until the laboratory or health authority has interpreted the result, then compare options with your local professionals.

If you are investigating the reason a sample was tested in the first place, pair the report with the observations that started it. The cloudy-well-water guide shows how an observation leads to a test, and the Symptom Finder can help you confirm which question to ask next. Use well maintenance guidance to keep the wellhead and infrastructure in shape while you work through results, and keep the full testing plan and the contaminants and lab results page close by as reference.

Avoid common mistakes that muddle a report

  • Comparing values in different units. Keep each result in the unit printed on its report and ask the laboratory whether the records can be compared.
  • Reading ND as a guarantee of zero. Non-detect means at or below the reporting limit. It is a statement about the method, not proof the substance is absent or the water is safe.
  • Comparing different sample locations. A raw-water result and a treated-water result are not the same question. Compare only samples you know are comparable.
  • Treating a public-water MCL as a private-well verdict. Those values are references from a regulated system, not a diagnosis of your well. Your state and local authority and your laboratory interpret your results.
  • Ranking health and aesthetic results together. Taste, staining, and operational findings answer different questions than health-relevant ones. Keep the categories separate.
  • Concluding safe or unsafe at a glance. A single result cannot carry that verdict. Say what the report shows and ask who should interpret it.

Where this guidance comes from