Understanding your test result: positive, negative and what happens next
A laboratory result is not a verdict — it is a measurement, produced by a defined process, with safeguards at every stage. This guide explains what the terms mean and what happens between the sample and the final report.
The two-stage process
Screening first, confirmation second
Laboratory testing happens in two stages. The first is an immunoassay screen — a fast, sensitive test designed to catch anything that might be present. Because screening trades some specificity for speed, anything it flags is called non-negative, not positive. The second stage, applied only to non-negative screens, is confirmation by mass spectrometry — an analytical technique that identifies the specific substance and measures its concentration. Only a confirmed result is reported as positive.
This separation matters. Screening tests can cross-react with legitimate medicines; confirmation resolves exactly what is present. A result that skipped confirmation could not be relied on — which is why non-negative screening results are confirmed by a more specific method at an accredited laboratory before they are reported.
Cut-off levels: why they exist
Every substance has a defined cut-off — a concentration below which the result is reported negative. Cut-offs are not arbitrary. They are set so that a positive from trace or passive exposure, such as being in a room where cannabis was smoked, is very unlikely, while still detecting genuine use. Cut-offs are based on recommendations from international guideline bodies. They can differ between guidelines and laboratories, and by substance and sample type. The cut-offs that apply to your result are those in the analysing laboratory’s current published cut-off schedule.
This page is a general guide. It is not legal, medical or technical advice. Laws, guidance and laboratory methods change, so check the current requirements for your own case. Detection windows are indicative. They vary with the substance, dose, how often it is used and the individual. Last reviewed .
What a result can and cannot show
- A negative result is not proof of no use. It shows only that nothing was found above the cut-off.
- A result shows use. It does not show impairment.
- Timing from hair is approximate, not exact.
- Results are read alongside the case history.
What a positive result does and does not say
A confirmed positive says that a specific substance was present above the cut-off in the sample provided. It does not by itself say when the substance was taken, how much was taken, or whether the donor was impaired at any particular moment — those questions depend on the sample type and its detection window. A urine result reflects recent days; a hair result reflects a pattern over months. Interpreting a result properly means reading it against the window of the sample that produced it.
Medical review
In workplace programmes, review by a suitably qualified professional, such as a medical review officer (MRO), is available on request once a confirmed laboratory result is back. The reviewer considers the confirmed laboratory finding alongside medication declared at collection and any relevant medical history. Where a laboratory positive is explained by a legitimate prescription, the review says so, protecting the donor from an unfair outcome and the instructing party from acting on an incomplete picture.
The B sample: a built-in second opinion
Where samples are collected under chain of custody, the collection normally includes a second, separately sealed sample. If the donor disputes a confirmed result, that B sample can be analysed at an independent accredited laboratory. The right of challenge is not a weakness in the system — it is part of what makes the system fair and the original result credible.
How results are reported, and to whom
Results are reported in writing to the instructing party named at collection — the solicitor, court, social services team or employer who commissioned the test. For court work, results are presented in a format prepared for proceedings, with interpretation and, where required, expert witness support — see medico-legal services. Results are handled in confidence and are never released beyond the instruction.
Common questions
Results — your questions answered
What does a non-negative result mean?
Non-negative means the initial screening test detected something that requires laboratory confirmation. It is not a positive result. A proportion of non-negative screens are not confirmed — for example where a legitimate medicine cross-reacts with the screening test — which is exactly why confirmation exists.
Can prescribed medication cause a positive result?
Yes. Codeine-based painkillers, some cold and flu remedies and a range of prescribed medicines can produce genuine laboratory positives. Prescribed medicines such as medicinal cannabis, diazepam or dexamfetamine can confirm as positive, some CBD products can contain THC, and poppy seeds can produce a morphine result. Tell the collector about anything you take. Declared medication is recorded at collection, and results benefit from review by a suitably qualified professional before anyone acts on them.
What is a cut-off level?
A cut-off is the concentration threshold below which a result is reported as negative. Cut-offs are set so that a positive from trace or passive exposure, for example being near someone smoking cannabis, is very unlikely. They are based on recommendations from international guideline bodies and can differ between guidelines and laboratories.
Can I challenge a result?
Where samples are collected under chain of custody, a second, separately sealed sample is normally retained at collection. The donor can request analysis of that B sample at an independent accredited laboratory. Ask us at the outset how a second-sample request works for your instruction.
Who sees my result?
Results are released only to the instructing party named at collection — the solicitor, employer or court that commissioned the test — and handled in line with data protection law. Results are never published or shared beyond the instruction.
Questions about a result or an instruction?
Talk to us in confidence. For court and workplace instructions, we will recommend the right test and explain exactly how the result will be reported.