What does an inconclusive test result mean?

It means the lab couldn’t give a clear positive or negative result from your sample. It doesn’t mean you have an infection, and it doesn’t mean you don’t.

Labs describe unclear results in different ways:

• Equivocal or indeterminate: the result is borderline, not clearly positive or negative.
• Reactive: a first positive result on a screening test, which still needs confirming with more tests.
• Invalid or inhibitory: the test didn’t work on that sample, for example because something in the sample stopped it working.
• Insufficient: there wasn’t enough of the sample to test.

Whatever the wording, the next step is usually the same: another sample, sometimes after a short wait. In our experience, a repeat test usually gives a clear answer.

Different STIs are tested in different ways:

• Nucleic acid amplification tests (NAATs), such as PCR, look for the genetic material of an infection. They’re used for Chlamydia, Gonorrhoea, Mycoplasma and Trichomonas, on urine or a swab.
• Antibody tests look for the antibodies your body makes to fight an infection. They’re used for Syphilis, Herpes and Hepatitis C, on a blood sample.
• Antigen tests look for part of the germ itself. Lab HIV tests look for antibodies and the p24 antigen together, and Hepatitis B tests look for an antigen called HBsAg.

Borderline readings are typical of blood tests for HIV, Syphilis and Herpes. Urine and swab tests can also give unclear results, for example when something in the sample stops the test working. Read more in “PCR testing vs antibody testing vs antigen testing: what’s the difference?”

There are a few common reasons.

Testing too early

Every test has a window period, the time it takes for an infection to show up. If you test before the end of it, the result can be negative or borderline even if you have the infection. UK guidelines say:

• Chlamydia and Gonorrhoea: a test within 2 weeks of sex can miss an infection
• HIV: a lab test for antibodies and the p24 antigen is reliable from 45 days
• Syphilis: a test in the first 3 months after sex can miss the infection
• Hepatitis B: it usually takes 30 to 60 days for a test to show the infection
• Hepatitis C: antibodies can take 12 weeks or more to show

Read more in “What’s the difference between a window period and an incubation period?”

Problems with the sample

A sample may not give a clear result if there isn’t enough of it, it takes too long to reach the lab, or blood cells in it have broken down. Something in a urine sample can also stop the test working, which is more likely in pregnancy and in the third week after a period. Labs check for this, and ask for a new sample if it happens.

Borderline antibody levels

People’s immune systems respond differently, so antibody levels can sit close to the cut-off, especially early on. Low readings on Syphilis and Herpes blood tests are more likely to be false positives, and some people lose their Herpes antibodies over time.

Other conditions and medicines

False positive Syphilis results are more likely in older people, people with autoimmune conditions and people who inject drugs. If you take HIV PrEP or have recently taken PEP, an HIV test can occasionally give an unusual result, which needs more tests to sort out.

The lab or your doctor will usually ask for another sample. What happens next depends on the test:

• A reactive HIV screening test is confirmed with more tests, including on a second sample, before anyone is diagnosed with HIV.
• A borderline Syphilis result is usually checked with a repeat sample about 2 weeks later.
• A positive Hepatitis C antibody test is followed by a test for the virus itself, to show whether the infection is still active.
• A borderline Herpes antibody result can be repeated at least 3 weeks later, or checked with a different test.
• An invalid or inhibitory urine or swab test needs a new sample.

Try not to assume the worst while you wait. An unclear result isn’t a diagnosis, and if you do turn out to have an infection, most STIs can be cured and the others can be managed with treatment.

You can lower the chance of an unclear result:

• Test at the right time after sex, and repeat the test after the window period if you’re advised to.
• Follow the kit instructions carefully, and make sure you collect enough of the sample.
• For a urine test, don’t pee for at least 1 hour first, and collect the first part of the stream.
• Post a home test kit back as soon as you’ve taken the sample.
• If you take HIV PrEP or have recently taken PEP, mention it when you test.

You don’t need to fast before an STI test. Read more in “How to take a urine sample for a home test” and “How to take a blood sample for a home test”

If you need to test again, or you’d like a full check, Better2Know offers:

• Full Screen: tests for 7 infections, including Chlamydia, Gonorrhoea, HIV, Syphilis and Hepatitis B, from 28 days after sex, with results in 3 days (5 days for the home test). You can book it at one of our clinics or order a Full Screen home test.
• HIV test: a blood test for HIV 1, HIV 2 and the p24 antigen, from 28 days after sex, with results in 1 day.
• Syphilis blood test: from 9 days after sex, with results in 1 day.

If you test before 45 days, a negative HIV result needs a repeat test. A Syphilis test in the first 3 months after sex can miss the infection, so you may need a repeat test. Not sure which test you need? Our Patient Care Team is available 24/7 on 020 7099 0955.

Is an inconclusive result the same as a positive result?

No. It means the result wasn’t clear either way. A repeat test usually gives a clear answer.

Can medicines affect STI test results?

Most don’t. HIV PrEP and PEP can occasionally cause an unusual HIV result, so mention it if you’ve taken either.

Do I need to fast before an STI test?

No. Better2Know’s STI screens have no fasting or dietary requirements.

An inconclusive result is frustrating, but it isn’t a diagnosis. It usually comes down to timing, the sample or a borderline reading, and a repeat test usually settles it. Follow any advice about when to test again, and talk to us if you’re worried.

Book a Full Screen at a clinic near you or order a home test kit from 28 days after sex. Our Patient Care Team is available 24/7 on 020 7099 0955.

Not in the UK? Choose your country from the selector at the top of the page for local tests and prices.

This article originally appeared on 9 January 2026 and was last updated on 30 September 2026.

Medical review. This article was medically reviewed by Dr Steve Chapman, Doctor with a wealth of clinical experience in the field of STIs, on 3 February 2026. It is general information and not a substitute for personal medical advice.

Sources

1. UK Standards for Microbiology Investigations (Royal College of Pathologists), SMI V 37: Chlamydia and gonorrhoea infection, testing by nucleic acid amplification tests, issue 5, 12 December 2024. Available from the RCPath resource library at https://www.rcpath.org
2. UK Standards for Microbiology Investigations, SMI V 11: Laboratory diagnosis of HIV infection, 8 May 2025. https://www.rcpath.org/resourceLibrary/uk-smi-v-11i5-1-laboratory-diagnosis-of-hiv-may-2025-pdf.html
3. UK Standards for Microbiology Investigations, SMI V 44: Laboratory diagnosis of syphilis, 23 May 2025. https://www.rcpath.org/resourceLibrary/uk-smi-v-44i3-1-laboratory-diagnosis-of-syphilis-may-2025-pdf.html
4. UK Standards for Microbiology Investigations, SMI V 4: Investigation of hepatitis B infection, 20 July 2021. https://www.rcpath.org/resourceLibrary/uk-smi-v-4i6-1-investigation-of-hepatitis-b-infection-july-2021-pdf.html
5. British HIV Association, British Association for Sexual Health and HIV and British Infection Association, Adult HIV testing guidelines, 2020. https://bhiva.org/wp-content/uploads/2024/10/HIV-testing-guidelines-2020.pdf
6. British Association for Sexual Health and HIV, UK guidelines for the management of syphilis, 2024. https://bashh.org/_userfiles/pages/files/syphilis_2024.pdf
7. British Association for Sexual Health and HIV, UK national guideline for the management of infection with Chlamydia trachomatis, 2026. https://www.bashh.org/_userfiles/pages/files/national_guideline_for_the_management_of_infection_chlamydia_trachomatis_2026.pdf
8. British Association for Sexual Health and HIV, UK national guideline for the management of infection with Neisseria gonorrhoeae, 2025. https://www.bashh.org/_userfiles/pages/files/fiferetal2025britishassociationofsexualhealthandhivuknationalguidelineforthemanagementofinfection_1.pdf
9. British Association for Sexual Health and HIV, UK national guideline for the management of anogenital herpes, 2024. https://www.bashh.org/_userfiles/pages/files/pateletal2024britishassociationofsexualhealthandhivuknationalguidelineforthemanagementofanogenital.pdf
10. British Association for Sexual Health and HIV, Guidelines on the management of viral hepatitis, 2026. https://www.bashh.org/_userfiles/pages/files/guidelines_on_the_management_of_viral_hepatitis_2026.pdf
11. NHS, HIV and AIDS: diagnosis, reviewed 9 July 2025. https://www.nhs.uk/conditions/hiv-and-aids/diagnosis/
12. NHS, Hepatitis C: diagnosis, reviewed 19 March 2026. https://www.nhs.uk/conditions/hepatitis-c/diagnosis/