How to read a lab report

The two stages

A laboratory does not run one test on a specimen. It runs a screen, and then — only where the screen calls for it — a confirmation.

Stage 01 — ScreeningThe screen looks at a whole category of drugs at once — amphetamines, opiates, benzodiazepines. It tells you whether something in that category is present. It does not tell you which one.

Stage 02 — ConfirmationThe confirmation names the drug. It tests each substance in the category separately and reports exactly which one is there, and how much.

That second step matters because a category is not a drug. A positive amphetamines screen could be any of several substances, and some everyday prescription and over-the-counter medications can set a category off on their own. Only the confirmation says which substance is actually present.

What a negative result does and does not establish

A negative result means that no substance on the panel was detected at or above the laboratory's cutoff in this specimen. A negative result is not proof that no drug use occurred. It states only that no drug use was detected. The distinction matters.

The laboratory does not report levels below cutoff; a specimen below cutoff is reported as negative. A negative result on a dilute specimen is still reported as negative — see the dilute section below.

A hair or PEth report

Hair results are organised by category. Every category the panel covers appears, whether or not anything was found.

Every category carries a screening limit. Nothing screened positive, so nothing went on for confirmation — the quantitation and confirm limit columns stay empty.

What changes when a category is positive

Switch the view above. When a category screens positive, the report opens it up and lists every substance in that category — positives and negatives alike, each with its own confirmation limit. Here amphetamines is positive, and the breakdown shows the substance present is MDMA. The other four are negative.

That breakdown is the point of the second stage. "Amphetamines positive" names a category. It does not say which substance, and the category row alone never will.

Confirmation limits are set per substance, and they are not always the same within a category. On a cocaine panel, for instance, some substances confirm at 50 pg/mg and others at 100. The limit that applies is always the one on the same row as the result.

A PEth report reads the same way

PEth uses the same five columns and the same parent-and-substance structure. The difference is that PEth measures one substance, so there is a single entry rather than a category holding several.

On a negative PEth, only the top row appears, carrying the screening limit — exactly as a negative hair category does.

Note that both limits here are 20 ng/mL, where the hair panel's amphetamines category screens at 500 and the substances beneath it confirm at 100. The reason is printed at the foot of the report: all screen methods are immunoassay unless otherwise noted. Both PEth rows are labelled LCMSMS, so the screen and the confirmation are the same method and carry the same limit. On the hair panel the category rows carry no label — those are immunoassay — while the substances beneath them are LCMSMS or GC/MS. Two different methods, two different limits.

A urine report

Urine reports place both limits on the same line — but they come from two different tests.

Initial test level is the screening cutoff. MS confirm test level is the confirmation cutoff.

The initial test is the screen. It covers a family of substances at once — the amphetamine line, for instance, answers for amphetamine and methamphetamine together. The MS confirm test is the second stage, and it answers for one substance at a time.

Two different tests, measuring two different things, printed side by side in the same units. A result is read against the confirmation level on its own line. The initial level describes only what it took to route the specimen onward.

Some lines carry the same figure in both columns — codeine and morphine at 2000, phencyclidine at 25. Matching numbers do not make them the same test. They are two thresholds that happen to be set at the same value.

A sweat patch report

The patch comes from a different laboratory in a different format, and it carries a result state the others do not: present.

The report is in two blocks. Initial test lists each substance screened, with its cutoff. Confirmation lists what was confirmed, with a measured value and its own cutoff. There is no category-and-substance indentation — the two stages are simply two lists.

Between 2.0 and 9.9 the metabolite is reported as present. At 10.0 and above it is reported as positive.

What decides a cocaine result

Two lines matter, and they are not equal. MS Cocaine is the drug itself. MS Cocaine/Metab is the metabolite — benzoylecgonine. The result turns on the metabolite.

Values reported as "greater than"

A quantitation may appear with a greater-than sign, such as >10 pg/mg.

Every instrument has a validated range over which it measures accurately. When the amount in a specimen goes above the top of that range, the exact figure cannot be measured without diluting the sample and running it again.

Once a result is definitively positive, the exact number above that point does not change the determination. So the laboratory reports the top of its validated range with a greater-than sign, rather than stating a figure it did not measure.

The sign means the amount present was above the number shown.

Why cutoffs exist

A cutoff is not an arbitrary line. It is the level at or above which a laboratory will report a substance as present, and it is set deliberately.

Instruments detect far more than a report acts on. Modern laboratory equipment can find quantities far smaller than any amount a testing programme is designed to respond to. Detecting a trace is not the same as finding a reportable amount, and the cutoff is what separates the two.

Everyday exposure stays out of the result. Very small quantities can arrive by routes that have nothing to do with use — secondhand smoke, poppy seeds in food. A cutoff set above those levels keeps them from producing a positive.

Results stay consistent. A cutoff is a published, fixed standard. The same specimen produces the same determination at any laboratory testing to that cutoff, which is what makes a result defensible rather than a matter of one laboratory's judgement.

Incidental exposure and alcohol testing

Alcohol raises this question more often than any other substance, because ethanol turns up in things nobody thinks of as drinking — mouthwash, hand sanitizer, some cleaning products, and a range of foods and medicines.

EtG, the urine marker for alcohol, is sensitive enough to pick that exposure up. The cutoff is what separates it from consumption. At the cutoffs used in forensic testing, low-level incidental contact — a rinse with mouthwash, hand sanitizer through the day — is unlikely to produce a quantity above the reporting threshold.

Which cutoff was used is the whole of it. A test run at a lower cutoff is more sensitive, and the incidental-exposure question is live at levels where it is not at a higher one. The cutoff appears on the report, and the answer turns on that figure rather than on the question in the abstract.

What that means for a negative

A negative means the substance was not found at or above the cutoff. Below the line, the report shows no number at all — a quantity just under the line and no quantity whatsoever look identical on the page.

So a negative is a statement about a line, not a measurement of zero.

The same line is what gives a positive its weight. A result above cutoff cleared a threshold set high enough to exclude incidental exposure and instrument sensitivity in the first place.

What specimen validity means

For a urine specimen the laboratory measures creatinine and specific gravity and checks pH and adulterants to establish that the specimen is consistent with normal human urine. The values are printed on the report. A specimen within the laboratory's ranges is reported as valid. Outside those ranges the laboratory reports it as dilute, substituted, adulterated, or invalid, and the report states which. Validity values are not a drug result and do not establish use.

Dilute

"Dilute" describes the specimen, not the substances tested. It is a finding about how concentrated the urine was — a statement about the sample, not about any drug being present or absent.

The laboratory decides, and the report says so

Dilution is a laboratory determination, stated on the report. It is not something a reader works out from the numbers. If a specimen was dilute, the report says so on its face — and if the report does not say so, it was not dilute.

Dilute takes two findings, not one

Every specimen is tested for creatinine. The minimum is 20 mg/dL. If creatinine is 20 or above, the specimen passes and nothing further is measured.

If creatinine falls below 20 mg/dL, that triggers a second screen: specific gravity. The acceptable range is 1.003 to 1.020. If specific gravity is within range, there is no dilution. If it is below 1.003, the specimen is reported dilute.

It takes both — creatinine below 20 mg/dL and specific gravity below 1.003 — for a specimen to be considered dilute.

Within range — no dilution, validity testing passes

Creatinine is above 20 mg/dL and passes. Specific gravity was not triggered, which is why it does not appear on this report.

Reported dilute — both values outside their parameters

Creatinine fell below 20, which triggered specific gravity. Specific gravity fell below 1.003. The laboratory reported the specimen dilute.

What causes it

Dilution is the result of too much water consumed prior to testing. Too much water can move a sample that would have been positive below the cutoff to negative. The excess intake can be intentional or unintentional — there is no way to be certain.

A dilute result does not change a positive. A specimen that is positive and also dilute is reported positive.

When this occurs, our advice is to repeat the test as soon as possible and moderate water intake. Generally, consuming no more than 40 oz of water prior to collection is sufficient to produce a urine sample without dilution.

pH and oxidant testing answer a different question

pH has a range; oxidants are positive or negative. Together they determine whether a sample has been tampered with or adulterated. A failure on either can lead the laboratory to report the specimen as adulterated or invalid — meaning it could not be reliably tested at all. Those are separate findings from dilute, and the report names them if they apply.

What a cancelled test means

A cancelled test means no testing was performed or testing could not be completed. The laboratory cancels a specimen it cannot test as received — a broken seal, an unsigned chain of custody, insufficient quantity, or a specimen reported invalid with no medical explanation. A cancelled test is not a result and no conclusion about use can be drawn from it. What happens next is determined by the order; in most cases a new collection is scheduled as soon as possible.

What the order has to say

Much of what makes a report readable is settled before the specimen is collected. An order should name the specific test rather than the substance — where a substance can be tested more than one way, the cutoffs and the detection window both follow from which test was run. It should also say what a dilute specimen means, because without that a dilute result carries no consequence.

The model order covers the handling of dilute, substituted, and invalid specimens, and distinguishes the laboratory report from the MRO report — an order that asks only for “the results” may not produce the laboratory report this page describes.