Can Blood Tests Detect STIs? What Standard Testing Does — and Doesn't — Reveal

One of the most common questions people ask before booking a sexual health check is whether a routine blood test will pick up sexually transmitted infections. It is a reasonable assumption — blood tests are used for many health checks — but the reality is more nuanced than most people expect.

A standard blood panel — the kind typically ordered to check cholesterol, liver function or blood cell counts — does not automatically screen for STIs. These are general health markers. STI testing must be specifically requested, and different infections require different types of samples.

Understanding which infections can and cannot be detected through blood testing is important. Relying on a routine blood test result to rule out STIs can create false reassurance, potentially delaying diagnosis of infections that are easily identifiable through appropriate screening.

What Does a Standard Blood Test Actually Check?

When most people refer to a "blood test," they are usually describing a general health panel. This typically includes a full blood count (FBC), which measures red blood cells, white blood cells and platelets. It may also include liver function tests, kidney function markers, cholesterol levels and blood glucose.

These tests provide valuable information about overall health and organ function, but they are not designed to detect sexually transmitted infections. A full blood count might show signs of infection in a general sense — such as elevated white blood cells — but it cannot identify specific STIs or confirm which pathogen is responsible.

STI-specific blood tests look for entirely different markers — typically antibodies produced in response to a particular virus or bacterium, or the presence of viral genetic material. These are separate laboratory analyses that must be requested specifically and are not part of routine health screening unless expressly included.

This distinction is important because many people assume their "blood work came back normal" means they have been tested for everything, including STIs. In most cases, that is not the case.

Which STIs Can Be Detected Through Blood Tests?

Several important sexually transmitted infections are detected through blood testing. These are typically viral infections that enter the bloodstream and trigger an immune response that can be measured through antibody or antigen testing.

HIV

HIV is detected through blood testing that identifies antibodies and antigens produced in response to the virus. Modern fourth-generation tests can detect both the p24 antigen (produced by the virus itself) and HIV antibodies, allowing for earlier detection than older testing methods. An HIV test is the only reliable way to determine HIV status — the infection cannot be diagnosed through symptoms alone.

Syphilis

Syphilis is caused by the bacterium Treponema pallidum and is detected through blood tests that look for specific antibodies. Screening typically involves an initial test followed by a confirmatory test if reactive. Syphilis progresses through stages — primary, secondary, latent and tertiary — and blood testing can identify infection even during stages where no visible symptoms are present.

Hepatitis B

Hepatitis B is a viral infection affecting the liver that can be transmitted sexually. Blood testing detects surface antigens (HBsAg) and antibodies, helping to determine whether someone has an active infection, has recovered from a past infection, or has immunity through vaccination.

Hepatitis C

While hepatitis C is more commonly associated with blood-to-blood transmission, sexual transmission is possible in certain circumstances. Blood testing detects hepatitis C antibodies, and if reactive, a follow-up PCR test can confirm whether the virus is currently active.

We provide confidential laboratory-based STI screening. All blood samples are processed by accredited laboratories.

Which STIs Do NOT Show Up in Blood Tests?

Several of the most common sexually transmitted infections cannot be reliably detected through blood testing. These infections are localised to specific body sites and require direct sampling — either through a urine sample or a swab — for accurate detection.

Chlamydia

Chlamydia is caused by the bacterium Chlamydia trachomatis and is one of the most frequently diagnosed STIs. It does not circulate in the bloodstream in a way that allows reliable blood-based detection. Instead, it infects specific mucosal surfaces — genital, rectal or throat — and is detected through nucleic acid amplification testing (NAAT) performed on a urine sample or swab from the affected site.

Gonorrhoea

Gonorrhoea, caused by the bacterium Neisseria gonorrhoeae, behaves similarly to chlamydia in terms of detection. It infects mucosal tissues and is identified through NAAT testing on urine or swab samples. A chlamydia and gonorrhoea test can screen for both infections simultaneously using a single sample.

Trichomoniasis

Trichomoniasis, caused by the parasite Trichomonas vaginalis, is another infection that does not appear in blood tests. It requires swab-based testing for detection. Trichomoniasis is frequently asymptomatic, particularly in men, which means relying on symptoms or blood results alone may miss the infection entirely.

Site-Specific Infections

Infections affecting the throat or rectum — including pharyngeal chlamydia, pharyngeal gonorrhoea and rectal infections — cannot be detected through blood or urine testing. These require site-specific swabs from the affected area. This is particularly relevant for individuals who have had oral or anal sexual contact, as genital-only screening would miss these infections entirely.

NAAT testing — nucleic acid amplification testing — is the gold-standard laboratory method for detecting chlamydia, gonorrhoea and similar bacterial infections. It amplifies and identifies the genetic material of the pathogen, providing highly accurate results from urine or swab samples.

Why Testing the Correct Body Site Matters

STIs do not always infect the same body site. Depending on sexual activity, an infection may be present in the genital area, the throat, the rectum, or more than one site simultaneously. Standard urine-based testing screens the genital tract, but it will not detect infections elsewhere.

Pharyngeal (throat) infections with chlamydia or gonorrhoea are commonly asymptomatic. An individual may have a throat infection with no sore throat, no discomfort and no visible signs — yet still be capable of transmitting the infection. Similarly, rectal infections may produce no symptoms but require specific swab testing for detection.

This is why a thorough sexual history — even briefly discussed — helps determine which tests are appropriate. A comprehensive screening approach, covering all relevant body sites, provides the most reliable picture of infection status.

Confidential screening covering all relevant sites is available at our private sexual health clinic London location.

Can You Have an STI and a Normal Blood Test?

This is one of the most common sources of false reassurance. Receiving "normal bloods" can create confidence that everything has been checked, when in reality, STI screening was never part of the panel. If sexual health is a concern, specific STI testing should be requested rather than relying on general health blood work.

Comprehensive STI screening — combining blood tests for HIV, syphilis and hepatitis with urine or swab tests for chlamydia, gonorrhoea and other infections — is the most thorough approach to establishing infection status.

Understanding Window Periods

The window period is the time between potential exposure to an infection and the point at which a test can reliably detect it. Testing too early — within the window period — may produce a negative result even if infection is present.

HIV

Fourth-generation HIV tests, which detect both antibodies and the p24 antigen, can typically provide reliable results from around four weeks after potential exposure. However, some guidelines recommend confirmatory testing at a later point for complete confidence. The window period varies depending on the type of test used.

Syphilis

A syphilis test may detect antibodies within a few weeks of infection, but earlier testing may miss it. If symptoms such as painless sores appear, testing should be arranged promptly regardless of timing since exposure.

Chlamydia and Gonorrhoea

NAAT-based testing for chlamydia and gonorrhoea can typically detect infection from around two weeks after exposure. Testing earlier than this may produce a false negative. If initial results are negative but concern remains, repeat testing may be appropriate.

Understanding window periods helps ensure testing is performed at the right time for the most reliable results. If you are unsure about timing, the clinical team can advise on when to schedule testing based on your circumstances.

When Should You Request Full STI Screening?

STI screening is appropriate in a range of situations. It does not have to be prompted by symptoms — in fact, because many STIs are asymptomatic, routine screening is an important part of sexual health management.

Regular screening — even in the absence of symptoms — provides the most reliable way to identify infections early. Many STIs are straightforward to manage when identified promptly, making early detection a practical and sensible approach.

What Happens If a Test Is Positive?

A positive STI result can feel concerning, but it is important to understand that the vast majority of sexually transmitted infections are manageable with appropriate medical care. Early identification allows for timely intervention and helps prevent potential complications or onward transmission.

At our clinic, we provide screening and laboratory testing. If a test returns a positive result, we will explain the findings clearly and confidentially. Treatment is arranged separately — appropriate medical care can be coordinated to ensure the infection is addressed promptly.

Partner notification may also be discussed where relevant, as some infections can be transmitted to others without symptoms being present. This is handled discreetly and with sensitivity.

Receiving a positive result is not uncommon. STIs are among the most frequently diagnosed infections worldwide, and effective management options exist for all commonly tested infections. The most important step is obtaining an accurate result through laboratory testing.