Three measurements that should not be silently swapped
A guide to reading the method, not reference ranges or testing instructions.
| Measurement | Context that must stay attached |
|---|---|
| Blood or saliva cotinine | Specimen, assay and sample time; do not import a urine cut-point. |
| Urinary cotinine | Free or total cotinine, dilution and any creatinine adjustment; do not treat concentration as a cigarette count. |
| Rapid cotinine strip | The device's stated reporting category and limits; not a complete quantitative account of passive exposure. |
Nicotine exposure and cigarette smoking are different questions
Cotinine can follow nicotine from cigarettes, other nicotine-containing products, nicotine replacement products or secondhand smoke. This does not make those exposures equivalent in composition or health effects. It means the marker alone cannot distinguish them.
Consider a fictional report that says only ‘cotinine detected’. Without the specimen, assay, timing and relevant nicotine sources, changing that sentence to ‘this person secretly smoked a cigarette’ adds a conclusion that was not measured. A clinician or researcher may use additional context and measurements; this page supplies no accusation rule or home testing plan.
The same unit does not make two samples comparable
Blood, saliva and urine are different measurement settings. A report may measure free cotinine or total cotinine, which includes a conjugated form. Urine concentration also depends on dilution; some research adjusts for creatinine. A value cannot be moved between those settings simply because both reports use ng/mL.
The 2019 SRNT update, published in 2020, also distinguishes laboratory assays from rapid strips. The evaluated strips gave limited qualitative or semiquantitative information and were not reliable tools for quantifying low secondhand-smoke exposure. A strip's result is not the same as a precise laboratory concentration; the review is not certification of every test currently sold.
A detection window is not a countdown to passing
Nicotine is processed more quickly than cotinine, which is one reason cotinine is widely used in exposure studies. But a detectable result depends on the starting exposure, metabolism, sample time and method sensitivity. The study or laboratory's cut-point has a stated purpose; it is not a universal health or honesty boundary.
A one-time measurement cannot establish uninterrupted abstinence over a much longer period. ‘Below the reporting limit’ is also not synonymous with zero exposure. These limits are reasons to preserve context, not instructions to time a sample, dilute urine, change medication or try to evade a test.
Keep interpretation with the service responsible for the test
For an existing test, ask the ordering clinician or laboratory what was measured, which specimen and method were used, what the reporting boundary means for its purpose, and which exposure context is needed. Agree who can receive the report. Do not collect someone else's sample secretly or turn a biomarker into permission to punish or monitor them.
In England, NHS Better Health links to stop-smoking support; elsewhere use the relevant local health service. Support for stopping and interpretation of a particular laboratory report are different services. Do not post reports, identifiers, medication details or smoking histories here or in a public link; this page has no result entry field.
What to keep in mind
Sources
The central claims on this page were checked against the sources below.
- Society for Research on Nicotine and Tobacco / Nicotine & Tobacco Research: Biochemical Verification of Tobacco Use and Abstinence: 2019 Update, published 2020 — cotinine, specimen differences and assay limits
Sources checked: 2026-10-09
- Centers for Disease Control and Prevention: Biomonitoring detects exposure, not a diagnosis or a product-specific health result
Sources checked: 2026-10-09
- Centers for Disease Control and Prevention: Cotinine, analyte, concentration and detection limit
Sources checked: 2026-10-09
- NHS Better Health: England: stop-smoking support information
Sources checked: 2026-10-08
General measurement education only. No personal result interpretation, diagnosis, clearance estimate, testing order or surveillance guidance. Samples and reports require consent and appropriate privacy.