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.
Blood tests used for STIs are generally very accurate, especially modern 4th-generation tests for HIV and treponemal tests for syphilis. Accuracy depends on timing (testing outside the window period) and the specific test used. Understanding what blood tests can and can’t detect helps you interpret your results correctly.
Here’s a practical guide.
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STIs Detected by Blood Tests
| STI | What the Blood Test Detects | Accuracy |
|---|---|---|
| HIV | Antibodies + p24 antigen (4th-gen) | Over 99% past window period |
| Syphilis | Antibodies to Treponema pallidum | Over 95% |
| Hepatitis B | HBsAg (current infection), antibodies (past or vaccination) | Very high |
| Hepatitis C | Antibodies and HCV RNA | Very high |
| Herpes (HSV) | Type-specific antibodies | Variable — more false positives than some other tests |
STIs Not Detected by Blood Tests
Some STIs are diagnosed differently:
- Chlamydia and gonorrhoea — urine or swab NAAT (not blood)
- Trichomoniasis — swab test
- Mycoplasma genitalium — swab or urine NAAT
- HPV — cervical screening sample
- Herpes (active lesion) — swab PCR more accurate than blood
Window Periods for Blood Tests
- HIV (4th-gen) — reliable from 4 weeks, definitive by 6 weeks
- Syphilis — usually reliable from 3-4 weeks; definitive by 12 weeks
- Hepatitis B — reliable from 6 weeks to 3 months
- Hepatitis C — reliable from 6 weeks to 3 months
Testing too early can cause false negatives.
How Blood Tests Work
- Most STI blood tests detect antibodies — the body’s immune response to infection
- 4th-generation HIV tests also detect p24 antigen (viral protein)
- Some tests detect viral genetic material (HIV RNA, HCV RNA)
- A small blood sample is taken from a vein or finger-prick
Accuracy Factors
- Timing — most important factor
- Test type — lab-based tests are most sensitive
- Cross-reactivity — some conditions can cause rare false positives
- Laboratory quality
- Proper sample handling
Confirmatory Testing
Any reactive or positive blood test result is confirmed with a second test before treatment is started. This prevents false positives from causing unnecessary worry or treatment.
Home Blood Tests
Home finger-prick blood tests are available for HIV, syphilis, hepatitis B and C. They are:
- Convenient and discreet
- Accurate when processed at reputable labs
- Often bundled with urine or swab tests for a comprehensive screen
- Free through many NHS services
When to Get a Blood Test
- Routine screening for HIV, syphilis and hepatitis
- After a new or high-risk sexual partner
- If symptoms suggest infection
- During pregnancy
- Before or during PrEP use
Read more: How accurate are STI tests? | Home HIV testing | Rapid test accuracy
Frequently Asked Questions
Are blood tests better than urine tests?
Neither is inherently better — they detect different infections. HIV and syphilis need blood; chlamydia and gonorrhoea need urine or swabs.
Can a blood test diagnose all STIs?
No. Chlamydia, gonorrhoea, trichomoniasis and several others are detected by urine or swab tests.
How long does it take to get blood test results?
Usually 1-5 days for lab processing. Rapid tests can give immediate results.
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?
- Before You Get Intimate: Why You Should Ask About STI Testing
- Blood Test for Syphilis, Hepatitis B and Hepatitis C
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.
