Can STI Test Results Be Wrong? Understanding False Positives & False Negatives

Can STI Test Results Be Wrong? Understanding False Positives & False Negatives

If you have recently received an STI test result and find yourself questioning whether it could be wrong, you are not alone. It is entirely natural to want reassurance that your result is accurate — whether it came back positive or negative. The good news is that modern laboratory STI tests are highly accurate when used appropriately, and the likelihood of an incorrect result is low. However, like all medical tests, they are not infallible. False positives and false negatives, while uncommon, can occur. Understanding why this happens, and what factors influence accuracy, can help you make informed decisions about your sexual health. In this article, we explain how STI test accuracy works, what can affect your results, and when repeat testing may be recommended.

How Accurate Are Modern STI Tests?

STI testing has advanced significantly over recent decades, and the tests used by accredited UK laboratories today are among the most reliable diagnostic tools available.

Nucleic Acid Amplification Tests (NAATs)

For infections such as chlamydia and gonorrhoea, nucleic acid amplification tests — commonly known as NAATs — are the standard diagnostic method. These tests detect the genetic material (DNA or RNA) of the infecting organism, making them highly sensitive and specific. NAATs can identify infections even when the bacterial load is very low, which significantly reduces the chance of a false negative result when testing is performed within the appropriate window period.

4th Generation HIV Tests

Modern HIV screening in the UK primarily uses 4th generation (antigen/antibody combination) tests. These detect both HIV antibodies produced by the immune system and the p24 antigen, a protein from the virus itself. This dual detection approach means that HIV can be reliably identified earlier than with older antibody-only tests. After the recommended window period, 4th generation HIV tests are considered highly reliable.

PCR Testing

Polymerase chain reaction (PCR) testing is another molecular technique used for certain infections. PCR tests can detect very small quantities of viral or bacterial genetic material, making them particularly useful for early detection — for example, in HIV PCR testing, which can identify infection sooner than standard antibody tests.

The key point is this: accuracy depends on correct timing relative to exposure and proper sample collection. A test performed at the right time, using the correct method, is highly likely to give you an accurate result.

What Is a False Positive STI Test?

A false positive means that a test indicates you have an infection when, in fact, you do not. While this sounds alarming, it is important to understand that false positive results are uncommon with modern laboratory testing, and confirmatory protocols exist specifically to catch them.

Why Do False Positives Occur?

There are several reasons a test might return a false positive result:

• Cross-reactivity — In rare cases, a screening test may react to substances in the blood or sample that are similar to, but not actually, the target pathogen. This is more relevant to certain blood-based screening tests, such as those for syphilis or HIV.
• Technical laboratory error — Although uncommon in accredited laboratories with robust quality controls, occasional processing errors can occur. This is one reason why confirmatory testing is standard practice for certain infections.
• High sensitivity by design — Some screening tests are intentionally designed to be highly sensitive, meaning they are calibrated to detect as many true positives as possible. The trade-off is a very small increase in false positive rates, which is why reactive results are confirmed using a different, more specific test.

What Happens After a Reactive Result?

For infections such as HIV and syphilis, a reactive (positive) screening result is always followed by confirmatory testing before a diagnosis is given. This two-step process is specifically designed to prevent false positive diagnoses. You would not receive a confirmed diagnosis based on a single screening test alone.

What Is a False Negative STI Test?

A false negative means that a test result comes back negative — suggesting you are not infected — when in fact an infection is present. False negatives are more common than false positives and are usually related to the timing of testing rather than a failure of the test itself.

Common Causes of False Negatives

• Testing during the window period — Every STI has a window period: the time between exposure and when the infection becomes detectable by a test. Testing before this window has passed is the single most common reason for a false negative result.
• Testing too soon after exposure — If you test within days of a potential exposure, the infection may not yet have produced enough antibodies, antigens, or genetic material for the test to detect.
• Incorrect sample collection — If a swab does not adequately sample the infected area, or if a urine sample is collected incorrectly, the test may miss the infection. This is particularly relevant for site-specific infections — for example, a genital swab will not detect a throat or rectal infection.
• Very early infection — In the earliest stages of some infections, the pathogen load may be below the detection threshold of even highly sensitive tests.

If you are concerned about your risk of exposure, understanding the window period for each STI is essential. You can read more about symptom timelines in our guide on how long after sex do STI symptoms appear.

Understanding the Window Period

The window period is the time it takes after exposure for an infection to become detectable by a specific test. Testing before this period has elapsed significantly increases the risk of a false negative result. The table below provides general guidance on typical reliable testing times:

These timeframes are general guidelines. Individual circumstances may vary, and your clinician can advise on the most appropriate time to test based on your specific situation.

Testing too early is the most common reason for false negative STI results. If you tested within the window period and received a negative result but remain concerned, repeat testing at the appropriate interval is usually recommended.

For more information about result timescales, read our guide on how long do STI test results take.

Are HIV False Positives Common?

Given the significance of an HIV diagnosis, it is understandable that people worry about the possibility of a false positive HIV test. The reassuring answer is that confirmed false positive HIV diagnoses are exceptionally rare in the UK.

How HIV Testing Works in Practice

Modern HIV screening uses 4th generation antigen/antibody combination tests, which are highly accurate after the window period has passed. However, no screening test is perfect, and a small number of initial reactive results may not represent true infection. This is precisely why the UK testing protocol includes confirmatory testing.

If an initial HIV screening test is reactive, a second, different test is performed on the same sample. In many cases, a further sample may be requested to confirm the result. Only after confirmatory testing has been completed would a positive HIV diagnosis be communicated.

The Key Reassurance

You would never receive a confirmed HIV diagnosis based on a single screening test. The confirmatory process is specifically designed to eliminate false positives, and this system is robust and well-established across the UK.

Can Chlamydia or Gonorrhoea Tests Be Wrong?

Chlamydia and gonorrhoea are among the most commonly tested STIs in the UK, and the NAAT tests used to detect them are highly reliable.

False Positives

False positive results for chlamydia and gonorrhoea using NAAT testing are uncommon. The specificity of these tests — meaning their ability to correctly identify people who do not have the infection — is very high. In the rare event of an unexpected positive result, your clinician may recommend a repeat test or confirmatory testing.

False Negatives

False negatives for chlamydia and gonorrhoea are more commonly related to:

• Timing — testing before the 7–14 day window period has elapsed
• Sample site — a genital test will not detect a rectal or pharyngeal (throat) infection. If you have had oral or anal sexual contact, site-specific testing is important
• Sample quality — incorrect self-collection technique can occasionally affect results

If you have symptoms consistent with chlamydia or gonorrhoea but have tested negative, discussing sample site and timing with your clinician is advisable.

What Should You Do If You Don't Trust Your Result?

It is perfectly reasonable to seek clarification if you are unsure about a test result. Here are some practical steps:

1. Review the timing of your test — Consider when the potential exposure occurred and whether the appropriate window period had passed before you were tested. Testing too early is the most common reason for inaccurate results.

1. Consider the window period carefully — Refer to the window period table above. If you tested within or very close to the minimum recommended timeframe, a repeat test may be appropriate.

1. Discuss your concerns with a clinician — A sexual health professional can review your test result in the context of your risk factors, symptoms, and timing. They may be able to provide reassurance or recommend further testing.

1. Repeat the test if clinically advised — If there is a legitimate clinical reason to question your result — such as testing during the window period, ongoing symptoms, or a high-risk exposure — your clinician may recommend retesting at an appropriate interval.

It is important to base any decision to retest on clinical reasoning rather than anxiety alone. Unnecessary repeat testing can sometimes increase worry rather than reduce it.

When Is Repeat Testing Recommended?

Repeat testing is a standard part of sexual health practice in certain circumstances. It is generally recommended when:

• You tested early — If your initial test was performed before the recommended window period had fully elapsed, a repeat test at the appropriate time is advisable to confirm the result.
• You have ongoing symptoms — If you are experiencing symptoms consistent with an STI but your test was negative, your clinician may recommend retesting or investigating alternative causes.
• You have had a high-risk exposure — Certain exposures carry a higher risk of transmission. In these cases, clinicians may recommend a baseline test followed by repeat testing at defined intervals to ensure nothing has been missed.
• Your result was inconclusive — Occasionally, a test result may be reported as equivocal or borderline. In these situations, repeat testing is standard practice.

Repeat testing should always be guided by clinical advice rather than performed routinely without reason.

How Private Clinics Ensure Accurate Results

Reputable clinics use accredited UK laboratories that operate under strict quality control standards. Here is how reliable providers ensure accurate results:

• Accredited laboratories — Testing is performed by laboratories accredited to ISO 15189 or equivalent standards, ensuring that testing processes meet rigorous quality benchmarks. These are the same standards applied across NHS and private sector testing.
• Confirmatory testing protocols — For infections such as HIV and syphilis, confirmatory testing is built into the diagnostic pathway. A single reactive screening result does not constitute a diagnosis — it is always verified through additional testing.
• Quality-controlled systems — Accredited laboratories participate in external quality assurance schemes, where their testing performance is regularly assessed and benchmarked against other laboratories.
• Appropriate test selection — Reputable clinics use tests that are appropriate for the infection being screened, the sample type, and the time since exposure. This ensures that the right test is used at the right time.

These measures collectively mean that when you receive a result from a reputable, accredited provider, you can have a high degree of confidence in its accuracy.

Frequently Asked Questions

How common are false positive STI tests?

False positive STI test results are uncommon with modern laboratory testing. For most infections, the specificity of current tests is very high, meaning that the chance of a test incorrectly identifying an infection that is not present is low. Where false positives are more likely to occur — such as with initial HIV or syphilis screening — confirmatory testing protocols are in place to prevent misdiagnosis.

Can you get a false negative HIV test?

Yes, a false negative HIV test is possible, most commonly when testing is performed during the window period — before the body has produced enough antibodies or antigen for the test to detect. With a 4th generation HIV test, the recommended window period is 18–45 days. Testing after this period is considered highly reliable. If you have tested during the window period, repeat testing at the appropriate time is recommended.

Should I retest if I have symptoms?

If you have symptoms consistent with an STI but have tested negative, it is worth discussing this with a clinician. Possible explanations include testing during the window period, testing at the wrong sample site, or the symptoms having a non-STI cause. Your clinician can advise whether repeat testing is appropriate or whether further investigation is needed.

Are home STI tests accurate?

Home STI testing kits that use accredited laboratory analysis can be accurate when used correctly. The reliability of a home test depends on the quality of the kit, the laboratory processing the sample, and whether the user follows collection instructions carefully. Home tests that provide instant or point-of-care results may be less reliable than laboratory-processed tests. If you have concerns about a home test result, confirmatory testing through a clinic or laboratory is advisable.

Can anxiety affect STI test results?

No — anxiety does not directly affect the accuracy of STI test results. STI tests detect the presence of specific pathogens, antibodies, or genetic material, and emotional state does not influence these biological markers. However, anxiety can sometimes lead to testing too early (before the window period has passed) or to difficulty interpreting results without clinical context. If anxiety about STI testing is affecting your wellbeing, discussing your concerns with a healthcare professional can help provide clarity and reassurance.

When Should You Seek Medical Advice?

While most questions about STI test accuracy can be addressed through routine clinical consultations, you should seek prompt medical advice if you experience:

• Persistent symptoms — such as unusual discharge, pain during urination, or genital sores that do not resolve, regardless of a negative test result
• Worsening symptoms — any symptoms that are becoming more severe or spreading should be assessed without delay
• Fever accompanied by a rash — this combination may indicate a systemic infection that requires clinical assessment
• A new exposure — if you have had a new sexual exposure since your last test, particularly if it involved a higher-risk situation, testing and clinical advice are recommended

These symptoms do not necessarily indicate a serious problem, but prompt assessment ensures appropriate care is provided.

Confidential STI Testing & Follow-Up in the UK

If you have questions about your STI test result or would like repeat testing based on exposure timing, confidential appointments are available at our UK clinic. Testing recommendations depend on your individual circumstances — including the type of exposure, the time since exposure, and any symptoms you may be experiencing.

Whether you are seeking reassurance about a previous result, follow-up testing after a known exposure, or guidance on interpreting your results, our nurse-led team can provide evidence-based testing support tailored to your situation. If treatment is required, we will advise you on the appropriate next steps with your GP or sexual health service. Book a confidential appointment at a time that suits you.