References UK clinical guidance from the NHS, the British HIV Association (BHIVA), and the British Association for Sexual Health and HIV (BASHH). Last updated: April 2026.
STI tests are highly accurate, but no medical test is perfect. False positives (a positive result when you don’t have the infection) and false negatives (a negative result when you do) can happen, though they’re uncommon with modern testing methods.
If you’ve received unexpected test results, either a positive when you thought you were clear, or a negative when you’re still experiencing symptoms, this guide explains how often errors occur, what causes them, and what to do next.
Want reliable results? Order a laboratory-processed home STI test
How Accurate Are STI Tests?
Modern STI tests, particularly nucleic acid amplification tests (NAATs) used for chlamydia and gonorrhoea, are extremely accurate:
| STI Test | Sensitivity (catches true positives) | Specificity (avoids false positives) | False Negative Risk | False Positive Risk |
|---|---|---|---|---|
| Chlamydia (NAAT) | 97-99% | 99.5%+ | Very low | Very low |
| Gonorrhoea (NAAT) | 96-99% | 99%+ | Very low | Low (cross-reactivity possible) |
| HIV (4th gen) | 99.7%+ | 99.5%+ | Very low (after window) | Low (confirmatory test required) |
| Syphilis (EIA/TPHA) | 99%+ | 98-99% | Low | Low (biological false positives exist) |
| Herpes (swab) | 90-95% | 99%+ | Moderate (depends on swab timing) | Very low |
| Herpes (blood/IgG) | 95-97% | 94-98% | Low | Low-moderate |
In plain terms: if a NAAT test says you have chlamydia, it’s almost certainly right. If it says you don’t, it’s almost certainly right too, as long as you tested after the correct window period.
What Causes False Negatives?
A false negative means the test missed a real infection. The most common causes:
Testing too early (window period)
This is the number one cause of false negatives. Every infection needs time to reach detectable levels after exposure:
- Chlamydia/gonorrhoea: Test from 2 weeks after exposure
- Syphilis: Test from 4-6 weeks
- HIV: 4th generation test from 4 weeks; conclusive at 90 days
- Herpes (blood test): From 12 weeks for reliable antibody detection
If you test within the first few days after exposure, most tests will come back negative even if you are infected. This isn’t a test failure. It’s a timing issue.
Wrong test for the location
A urine chlamydia test will not detect throat or rectal chlamydia. If you’ve had oral or anal sex, you need site-specific swabs. This is a common reason for missed infections.
Herpes swab timing
A herpes swab test is most accurate when taken from a fresh, unbroken blister. If the sore has already begun healing, crusted over, or was swabbed incorrectly, the test may miss the virus.
Sample quality
An incorrectly collected urine sample (too little urine, first void not collected, or too dilute) can reduce test sensitivity. Follow the collection instructions carefully.
What Causes False Positives?
A false positive means the test flagged an infection that isn’t actually there. This is less common than false negatives but does happen:
Cross-reactivity (gonorrhoea)
Some NAAT tests for gonorrhoea can cross-react with non-pathogenic Neisseria species (harmless bacteria found in the throat). This is why a positive gonorrhoea result, particularly from a throat swab, may be sent for confirmatory testing.
Biological false positives (syphilis)
Certain conditions can trigger a false positive syphilis result, including pregnancy, autoimmune diseases (lupus), recent vaccinations, and some chronic infections. A positive syphilis screening test is always followed by a confirmatory test (TPHA/FTA-ABS) to rule this out.
HIV screening
A reactive (positive) HIV screening test is never reported as final without a confirmatory test. If a rapid test or ELISA comes back reactive, a second test using a different method is performed. Confirmed false positives on HIV testing are extremely rare with modern two-step protocols.
Herpes blood test (IgG)
Herpes blood tests detect antibodies, not the virus itself. In populations with low herpes prevalence, the positive predictive value can be lower, meaning a borderline positive result may not always indicate true infection. Results with index values close to the positive cutoff should be interpreted with caution.
What to Do If You Get an Unexpected Result
Unexpected positive
- Don’t panic. Most positive results are accurate, but confirmatory testing is available for syphilis, HIV, and gonorrhoea.
- Retest if the result seems inconsistent with your sexual history. A second test using a different method can confirm or rule out the result.
- Talk to a clinician. A sexual health clinic can interpret results in context and advise on next steps.
- If confirmed, get treated. Most STIs are easily treatable, and early treatment prevents complications.
Unexpected negative (but you have symptoms)
- Check the timing. Did you test within the window period? If so, retest after the appropriate interval.
- Check the test type. Did you test the right body site? Throat and rectal infections need specific swabs.
- Consider other causes. Your symptoms may be caused by a UTI, thrush, bacterial vaginosis, or a skin condition rather than an STI.
- Retest. If symptoms persist, test again or visit a sexual health clinic for a full examination.
Frequently Asked Questions
Can antibiotics cause a false negative STI test?
Yes. If you took antibiotics recently (for any reason, not just STIs), they may have partially suppressed the bacteria, reducing the test’s ability to detect it. Wait at least 2 weeks after finishing antibiotics before retesting for chlamydia or gonorrhoea.
How often do chlamydia tests give false positives?
NAAT tests for chlamydia have a specificity above 99.5%, meaning false positives occur in fewer than 1 in 200 tests. If you test positive, it’s overwhelmingly likely to be a true positive.
Can a UTI cause a false positive STI test?
No. UTIs and STIs are caused by different organisms. A UTI will not trigger a false positive on a chlamydia, gonorrhoea, or syphilis test. However, UTI symptoms (burning, frequency) can mimic STI symptoms, which is why testing is important for distinguishing between them.
Should I get tested at a clinic or use a home test?
Both are reliable. Home test kits use the same laboratory NAAT technology as clinic-based tests. The difference is convenience. Home kits are posted to you and returned by post. Clinics offer same-day results in some cases and can do visual examinations. Read more: STI testing options
Related reading
- How Accurate Are Rapid HIV Tests?
- Anonymous STI Testing: What It Means
- Are HIV Tests Accurate If You're on PrEP, PEP or ART?
- Are STI Blood Tests Accurate?
- Before You Get Intimate: Why You Should Ask About STI Testing
Sources
- NHS — Sexual health conditions
- BASHH — UK clinical guidelines for STI management
- British HIV Association (BHIVA) — HIV treatment guidelines
- Terrence Higgins Trust — sexual health information
- UK Health Security Agency — STI surveillance data
Steve Page is a recognised expert on Sexually Transmitted Diseases (STDs) and STD treatments, having published numerous articles in peer-reviewed journals and presented his research at conferences around the world. He has an in-depth understanding of the latest medical research on STDs, and is an advocate for the development of new treatments and protocols to improve the health of those affected. In addition to his research, he has dedicated his career to understanding the causes and symptoms of STDs, as well as how to best treat those impacted.
