False positive drug test – causes and handling
Why do false positive drug tests occur? Common substances that cause cross-reactions and how the employer handles unclear results in a legally sound way.
A false positive drug test occurs when the screening analysis indicates the presence of a substance that is not actually in the sample — or that is there for legitimate medical reasons. For employers, it is crucial to know how these are handled in a legally sound way. Never act on a rapid test without confirmation.
Why do false positives occur?
Rapid tests are based on antibody binding. If a medicine has a similar chemical structure, the antibody can bind and give a reading, even if it is not the drug the test is looking for. This is called cross-reaction and is the most common cause.
- Technical errors during sample collection or analysis (rare with a quality-assured procedure)
- Contamination during sample collection or transport
- Incorrect interpretation of cut-off levels
- Mixed-up samples due to a deficient chain of custody
Common substances that give false positives
Clonazepam and 7-aminoclonazepam
Clonazepam (Iktorivil, Rivotril) is broken down into 7-aminoclonazepam (7-ACL). Both can register on benzodiazepine screening. With a prescription, it is not a "false" positive — it is a legitimate medical explanation that the MRO physician approves.
Pseudoephedrine and cold medicines
The most common cause of a false positive amphetamine test. Sudafed, cough medicines and nasal drops containing pseudoephedrine can give a measurable reading for 24–48 hours.
ADHD medication
Vyvanse, Attentin, Concerta and Strattera register on amphetamine screening because they are amphetamine derivatives. Prescription = legitimate medical explanation.
Codeine and tramadol
Codeine is metabolised to morphine and gives a positive reading on opiate tests. Tramadol can register on certain screening tests.
Sertraline and certain antidepressants
Sertraline (Zoloft) has documented cross-reactions in benzodiazepine screening. Confirmation analysis rules out false positives.
Confirmation analysis is mandatory
Confirmation analysis with GC-MS or LC-MS/MS identifies specific substances without relying on antibody binding. Confirmed results are immune to cross-reactions. Provsvaret's laboratory analysis confirms all positive rapid tests before reporting.
MRO review in the event of a confirmed positive
Even when confirmation shows that the substance really is present, it may come from legitimate medication. The MRO physician contacts the test subject and asks about prescriptions. Where legitimate prescriptions are confirmed, the result is reported as "negative – medical explanation". Patient privacy is protected — the employer does not receive details about the medicine.
What should employers do?
- Never act on a rapid test — confirmation analysis is mandatory
- Make sure the lab has an MRO process
- Train managers in how a positive result should actually be interpreted
- Have procedures that require confirmation + MRO review before any action
- Document the process — your defence in a Labour Court dispute
What should employees do?
- Inform their immediate manager about long-term medication in line with your alcohol and drug policy
- Be prepared to show prescriptions if the MRO calls
- Mention dietary supplements and over-the-counter medicines too
- In the event of an unclear result — ask for confirmation analysis
In summary
A positive rapid test is not proof. It is a preliminary indication that must be confirmed. Provsvaret never reports positive results without confirmation + MRO review. Contact us for a review of your current process.
Want to know more?
Contact us for personalized advice on drug testing and how we can help your organization.
