Anju Goel, MD, MPH, is a board-certified physician who specializes in public health, communicable disease, diabetes, and health policy.
Certain sexually transmitted infection (STI) tests are conducted by swabbing the genital area, which can make some people hesitant to get them. While that is the only way some STIs can be diagnosed, there are several that can be accurately detected with a blood test instead.
This article details which STIs can be diagnosed with a blood test and what other tests may be used to accurately detect these and other STIs.
Options: Blood test, swab test
Herpes tests are used for people with symptoms of genital herpes, namely visible sores, and are not recommended for those without symptoms. The only exception is if you have a sex partner with genital herpes.
The herpes blood test looks at antibodies and proteins that are produced by the body in response to the virus. To ensure an accurate result, you need to allow time for these antibodies to build. As such, you need to wait at least 12 days from the time of exposure before getting tested.
Even if there are symptoms—which tend to start four to seven days after exposure—the level of antibodies in the blood may still not be enough to be detected.
If the initial result is positive, a second test will be used to confirm the results. This second test is not a repeat of the first test. Rather, it looks for other evidence of infection in the same sample of blood. Positive results from the two tests is considered definitive proof of infection.
If there are visible sores, the healthcare professional may want to take a swab instead. This is because any viruses on the swab can be directly detected using a genetic test called a nucleic acid amplification test (NAAT). The NAAT is considered the best test to determine if a person acquired the virus.
If a sexual partner is being tested and has no sores, a blood test will be used.
The herpes blood test can usually detect herpes antibodies 12 days after exposure to the virus. The blood test is very accurate, but swab tests are considered to be even better.
Options: Blood test, saliva test, urine test
HIV is commonly diagnosed through a blood test. The preferred tests not only detect antibodies but also proteins on the surface of the virus called antigens.
If an initial HIV test is positive, a second test will be performed on the same blood sample to confirm the result. When confirmed, HIV tests are extremely accurate and rarely return a false result.
There are also rapid HIV tests that can detect HIV antibodies in saliva. These include in-office and at-home tests that can return results in around 20 minutes. While useful, at-home saliva-based tests are less accurate, with one of 12 tests returning a false-negative result (meaning that you acquired HIV even if the test says you don't).
As with herpes tests, you may need to wait a while before getting tested for HIV. There is a window period after infection in which tests cannot reliably detect the virus. For combination antibody/antigen tests, the window period is 18 to 45 days. For traditional antibody tests, it is 23 and 90 days.
There are also urine-based HIV tests, but these are not often used.
HIV is commonly diagnosed with blood tests. Rapid saliva-based tests can give results in around 20 minutes, but are less accurate.
Options: Blood tests, swab test, spinal fluid test
There are several different blood tests used to detect syphilis, an STI caused by the bacteria Treponema pallidum . These tests are used in combination to determine if you acquired the condition. They can also tell if you have ever been infected in the past.
Treponemal tests are a group of tests that can detect syphilis antibodies in the blood. If positive, a second test to detect nontreponemal antibodies can confirm the results. These antibodies are related to the damage the bacteria cause to cells.
A nontreponemal test can also establish the stage of infection based on the level of antibodies in the blood, called the titer. Based on whether the titer is increasing or decreasing, the doctor can tell if treatment is working or if you have been reinfected.
Syphilis can also be detected from a swab of a sore or with a sample of spinal fluid.
The average window period for syphilis is around three to six weeks after infection.
Syphilis is diagnosed with blood tests that detect proteins produced by the body in response to the bacteria and the damage it causes. The tests can also tell if you are currently infected or have been infected in the past.
As with syphilis, there are multiple blood tests for hepatitis B. These can be used to determine your history of infection. They can also determine whether you're currently infected.
The diagnosis of hepatitis B involves different blood tests that detect an antigen called hepatitis B surface antigen (HBsAg), as well as two antibodies called hepatitis B surface antibody (HBsAb) and hepatitis B core antibody (HBcAb).
Each test can tell your healthcare provider different things:
The window period for hepatitis B can range from 60 to 150 days.
Hepatitis is diagnosed with three different blood tests that can tell if you have been infected, if you are contagious, or if you are immune to the virus through previous exposure or vaccination.
There are certain STIs that can only be diagnosed by testing swabs of genital tissues or sores.
In some cases, a swab may be used to culture (grow) the bacteria in a lab. In others, the cells on the swab may be genetically tested to confirm the presence of the STI.
STIs that are diagnosed by testing a swab sample include:
Testing for human papillomavirus (HPV), which requires a sample of cells from the opening of the cervix, can be done during a pelvic exam—either by itself or at the same time as a Pap smear.
Some people avoid STI screening due to the discomfort or embarrassment of having a genital swab. But, today many STIs can be diagnosed with blood tests, including genital herpes, HIV, syphilis, and hepatitis B.
Even so, STIs like chlamydia, gonorrhea, and trichomoniasis require a swab for diagnosis. Swabs are also an option for syphilis and herpes.
If you’re worried about a swab test and need one, talk to your healthcare provider. In some cases, you may be able to take the swab yourself.
Self-swabs can be a big help for people who have histories of sexual trauma or those who are simply reluctant to have a stranger touch their bodies.
Not all healthcare providers will allow this, but self-swabs have been shown to be effective for detecting many STIs. If nothing else, it’s better to have a self-swab test than no test at all.
STI blood tests are not part of a regular blood panel. You need a separate blood test for STIs. This is not routinely ordered by a healthcare provider unless requested.
Here's how long it takes for various STIs to show up on a blood test after exposure :
As with blood tests, a positive urine or swab test result after exposure will vary depending on the STI in question:
Centers for Disease Control and Prevention. Genital herpes screening FAQ (detailed).
Workowski KA, Bachmann LH, Chan PA, et al. Sexually transmitted infections treatment guidelines, 2021. MMWR Recomm Rep. 2021;70(4):1-187. doi:10.15585/mmwr.rr7004a1
National Institutes for Health. HIV testing overview.
Food and Drug Administration. Information regarding the OraQuick in-home HIV test.
Centers for Disease Control and Prevention. Types of HIV test.
Centers for Disease Control and Prevention. Syphilis - CDC fact sheet (detailed).
Wilkins T, Sams R, Carpenter M. Hepatitis B: screening, prevention, diagnosis, and treatment. Am Fam Physician. 2019;99(5):314-323.
Centers for Disease Control and Prevention. Hepatitis B questions and answers for health professionals.
Catarino R, Vassilakos P, Bilancioni A, et al. Randomized comparison of two vaginal self-sampling methods for human papillomavirus detection: dry swab versus FTA cartridge. PLoS One. 2015;10(12):e0143644. doi:10.1371/journal.pone.0143644
Centers for Disease Control and Prevention. Which STD tests should I get?
University of Oregon Health Center. STI screening timetable.
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