What STIs Can Be Detected by Blood Tests?

Some sexually transmitted infections (STIs) can be detected by a blood test, including genital herpes, HIV, syphilis, and hepatitis B. Not only do these tests give accurate results, but they may provide a more convenient way for some people to get tested.

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.

Genital Herpes

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.

HIV

Options: Blood test, saliva test, urine test

Human immunodeficiency virus (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 blood and saliva. These tests can return results in around 20 minutes. At-home saliva-based tests are also available. However, they 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).

Urine-based HIV tests are another option, but these are not often used.

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.

An HIV RNA test may be performed if someone has had a recent high-risk exposure or is experiencing acute symptoms of early infection. It's a blood-based test that can detect the HIV-virus as early 10 days after infection—sooner than other forms of testing.

Syphilis

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, a healthcare provider 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.

Hepatitis B

Option: Blood tests

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 hepatitis B surface antigen test can tell if you have syphilis and are currently contagious.
• The hepatitis B surface antibody test can tell if you are immune to hepatitis B, either because you have recovered from an infection or have been vaccinated.
• The hepatitis B core antibody test can tell if you currently have an acute or chronic hepatitis infection.

The window period for hepatitis B can range from 60 to 150 days.

STIs Detected by Swab Tests Only

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:

• Chlamydia
• Gonorrhea
• Trichomoniasis

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.

Summary

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.