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.

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.

Specimen validity

A urine report carries a second section describing the specimen itself rather than what was found in it.

Both creatinine and specific gravity sit inside the acceptable range. The specimen is suitable for testing and no validity finding is reported.

Dilute takes two findings, not one

Switch the view above and watch the table gain a row. In the within-range view there is no specific gravity line at all — the report prints it only once creatinine has failed. That is why creatinine is the figure people quote: on most reports it is the only one of the two they ever see.

In the dilute view creatinine has fallen to 14.8, under its floor of 20 mg/dL, and a specific gravity line appears at 1.0020, under its own acceptable range of 1.003 to 1.020. Both values sit outside their parameters, and that is what produces the finding.

Note that specific gravity is given as a band rather than a floor. A value above 1.020 is out of range too, and points at a different problem than a dilute specimen.

Either one alone does not. A creatinine below range with a specific gravity inside range is not a dilute specimen, and neither is the reverse. The two have to agree before the laboratory reports one. The criteria are the same on regulated and non-regulated testing.

The laboratory makes this determination, and states it 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.

pH is a different question

pH stays inside range in both views above, and it plays no part in a dilute finding. It asks whether the specimen falls within the range normal urine occupies. A value outside that range points toward something having been added to the specimen rather than toward concentration, and can lead the laboratory to report the specimen as invalid — meaning it could not be reliably tested at all.

Dilute, adulterated, and invalid are three separate findings. A dilute specimen was still tested and still produced results.

A dilute result describes the concentration of the specimen. It is a finding about the sample, not about a substance, and the report says nothing about how the specimen came to be that way.