Your GP ran some bloods, the surgery rang to say everything was normal, and you took that as a clean bill of sexual health. It's one of the most common assumptions we hear, and in most cases it isn't what happened.
Routine GP blood tests (full blood count, liver function, kidney function, cholesterol) only check for sexually transmitted infections when STI tests are specifically requested. Even then, blood picks up some STIs and misses others. Chlamydia and gonorrhoea, the two most common, need a urine sample or a swab.
If you'd rather skip the reading, walk in to our Harley Street clinic any day of the week. Our Bronze Screen (£250) combines blood and urine testing for chlamydia, gonorrhoea, syphilis and HIV in one visit.
"A blood test is the only way to screen for HIV, syphilis and hepatitis. Swabs and urine can't detect these infections, so blood work should always be part of a sexual health check."
Dr Mohammad Bakhtiar, Clinical Lead, GMC 4694470
Which STDs can a blood test detect?
Blood tests can detect HIV, syphilis, hepatitis B, hepatitis C and herpes simplex virus (HSV-1 and HSV-2). Each of these triggers an immune response, producing antibodies or antigens that show up reliably in the blood once the window period has passed.
HIV
HIV testing on blood is the most reliable method. A 4th generation test looks for HIV antibodies and the p24 antigen, which appears earlier. It screens positive from 28 days and is conclusive from 45 days after exposure (BASHH, 2024).
Standard 4th generation HIV testing is £75, with results in 4 to 24 hours. If you need to test sooner, HIV RNA PCR can identify the virus from 10 days after exposure. It costs £238.75 at our clinic, with results within 24 hours. Our guide to HIV test accuracy and early detection explains which test suits which timing.
Syphilis
Syphilis is picked up by blood tests for treponemal antibodies (IgG/IgM), which become detectable 3 to 6 weeks after exposure (BASHH, 2024). The test costs £85, with results in 24 hours.
Because the first syphilis sore (chancre) is painless and heals by itself, a blood test is often how the infection is found. Rates in England have been rising, which makes routine screening more relevant every year.
Hepatitis B
Hepatitis B is detected by testing for the surface antigen (HBsAg), which appears in the blood 3 to 6 weeks after infection. The surface antigen test costs £68.13 plus a £50 blood draw (£118.13 total), with results within 3 working days. A full hepatitis B profile covers surface antigen, core antibody and immunity status, and is £180 plus the £50 blood draw (£230 total).
Hepatitis C
Hepatitis C can be found by antibody testing, from 4 to 10 weeks after exposure, or by antigen testing, from 2 to 3 weeks. The antibody test is £137.50 and the early-detection antigen test is £85. Both return results within 24 hours.
Herpes (HSV-1 and HSV-2)
Herpes blood testing looks for type-specific IgG antibodies. These take 12 to 16 weeks after exposure to show reliably. An IgG antibody profile costs £103.75, with a 24-hour turnaround.
If you have sores now, a swab PCR is more useful, because it doesn't depend on antibodies having developed. Swab testing costs £120.63, with results within 3 working days.
Which STDs do blood tests miss?
This is the gap that catches people out. The most common bacterial STIs in the UK need a urine sample or a swab.
Chlamydia
Chlamydia is the most diagnosed STI in England, and blood can't reliably detect it. It's tested by PCR on a urine sample or a swab (vaginal, throat or rectal). A chlamydia antibody blood test does exist, but it shows past or present exposure rather than a current infection, so it isn't used for diagnosis.
If you've only had blood tests, you haven't yet been screened for chlamydia. A urine PCR, combined with gonorrhoea, is £104, with results within 3 working days.
Gonorrhoea
Gonorrhoea also needs a urine sample or a swab. Throat and rectal gonorrhoea are particularly common, and each needs a swab from that site.
HPV
HPV has no blood test. It's detected by swab (PCR) from the cervix, anus, throat or genital skin, and HPV vaccination is the main way to prevent it.
Trichomoniasis
Trichomoniasis is detected by urine or swab PCR.
Mycoplasma and ureaplasma
Both are found by urine or swab PCR. Mycoplasma genitalium in particular is increasingly recognised as a cause of urethritis and cervicitis, and standard blood panels leave it out.
Blood test vs comprehensive STI screen
- Detects
- HIV, syphilis, hepatitis B, hepatitis C, herpes
- Misses
- Chlamydia, gonorrhoea, HPV, trichomoniasis, mycoplasma, ureaplasma, BV
- Detects
- Chlamydia, gonorrhoea (urethral/cervical)
- Misses
- Throat/rectal infections, blood-borne infections
- Detects
- All of the above
- Misses
- Nothing significant
| Test method | Detects | Misses |
|---|---|---|
| Blood only | HIV, syphilis, hepatitis B, hepatitis C, herpes | Chlamydia, gonorrhoea, HPV, trichomoniasis, mycoplasma, ureaplasma, BV |
| Urine only | Chlamydia, gonorrhoea (urethral/cervical) | Throat/rectal infections, blood-borne infections |
| Comprehensive screen (blood + urine + swabs) | All of the above | Nothing significant |
A blood-only test misses the most common STIs, and a urine-only test misses the blood-borne ones. For complete coverage, you need both.
Our Bronze Screen (£250) does exactly that for chlamydia, gonorrhoea, syphilis and HIV. The Gold Screen (£475 men, £490 women) adds hepatitis B and C, herpes, mycoplasma, trichomoniasis, ureaplasma and Gardnerella for near-complete coverage.
"My GP said my bloods were fine"
We hear this regularly. Routine GP bloods almost certainly covered your full blood count, liver function and kidney function, and possibly cholesterol and thyroid. None of these screen for STIs.
Even if your GP specifically added HIV and hepatitis, chlamydia and gonorrhoea still need a urine sample. To know your STI status, ask your GP specifically for full STI testing with a urine sample included, or come and see us at a sexual health clinic.
There's no judgement in getting tested. It's a routine part of looking after your health, no different from checking your blood pressure or cholesterol.
What happens at your visit
Walk-in STI testing is available seven days a week at our Harley Street clinic, with no appointment or GP referral needed. All samples are processed by UKAS-accredited laboratories. You don't need to use your real name, and we accept cash payment.
For our standard screens, results for blood tests come back in about 24 hours, and urine and swab results within 3 working days. If you need answers sooner, FAST same-day STI testing returns all results in 6 hours. FAST Screen Simple (£350) covers chlamydia, gonorrhoea, syphilis and HIV, and FAST Screen Complex (£450) adds hepatitis B and C.
References
- BASHH (2024). UK National Guidelines for HIV Testing.
- BASHH (2024). UK National Guidelines on the Management of Syphilis.
- NICE CKS (2024). HIV infection and AIDS: diagnosis.
- NHS (2024). Blood tests: overview.