Choosing a chemical testing laboratory: why the accreditation scope rather than the accreditation decides whether your test is covered, what has to be named in the request for two quotations to compare, and how to read a report that carries an uncertainty

Commissioning the work

1. Name the analyte, the matrix and the method. The same compound in water, in soil and in a formulated product is three different tests with three different preparations and three different detection limits. A request naming only the analyte will be quoted against whatever the laboratory assumes.
2. Check the accreditation scope, not just the certificate. Accreditation is granted for specific methods on specific matrices over specific ranges. The scope document is public and it is where you confirm the laboratory is accredited for your test, rather than for a test that sounds like it.
3. State the detection limit you need. A limit of quantitation below your decision threshold is the point of the exercise. A result reported as less than a limit that sits above your specification answers nothing, and this is discovered after the invoice more often than before.
4. Agree sampling, or accept that the result is about the sample only. A result describes what was tested. Where the sample was taken, by whom and how it was preserved decides what it represents, and a laboratory that did not sample cannot answer for that half of the measurement.
5. Fix turnaround and the rush policy in writing. Standard turnaround, the rush premium and what happens if a result fails quality control and needs a repeat. The repeat clause is the one nobody reads and the one that decides the real timeline.

Sampling, preservation and holding times

Many methods have a holding time that starts at collection, and a sample analysed outside it produces a qualified result at best. Ask for the container, preservative and holding time per analyte before sampling rather than after.

Record the collection time on the container and on the chain of custody. A sample with no collection time cannot be shown to be within its holding time, whatever the laboratory does afterwards.

Reading the report

The report should carry the method, the preparation, the limits of detection and quantitation, the quality control results for the batch, any qualifiers and the measurement uncertainty. A bare table of numbers is not reviewable.

Read the qualifiers. A result flagged for a failed spike recovery or an exceeded holding time is telling you something specific, and those flags are routinely ignored by people reading only the number column.

When a subcontracted test appears

Laboratories subcontract analyses outside their scope, which is legitimate and must be stated on the report. Ask in advance which tests would be subcontracted and to whom, because it adds transit, a second schedule and a second set of records.

Where the subcontracted test is the one that matters, consider going to that laboratory directly. The margin is real and so is the extra week.

Common questions

How do I check a chemical testing laboratory can do my test?

Read its scope of accreditation, which names the methods, matrices and ranges it is accredited for. A laboratory holding accreditation is not thereby accredited for everything it can physically run.

What has to be in a testing request?

The analyte, the matrix, the method or the specification you are testing against, the detection limit you need, the number of samples and the turnaround. Without those, two quotations are not comparable.

What does the measurement uncertainty on a report mean?

The range within which the true value is expected to lie. It matters most when a result sits near a specification limit, because a result inside the limit with an uncertainty that straddles it is not a clear pass.

Can a laboratory advise on what to test?

Many will, and it is worth asking early. A conversation before the samples are taken frequently changes the method, the preservation and the number of samples, and it is much cheaper than repeating the exercise.

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