Understanding False Positive or False Negative STI Test Results

Elizabeth Boskey, PhD, MPH, CHES, is a social worker, adjunct lecturer, and expert writer in the field of sexually transmitted diseases.

Updated on July 25, 2023

Monique Rainford, MD, is board-certified in obstetrics-gynecology, and currently serves as an Assistant Clinical Professor at Yale Medicine. She is the former chief of obstetrics-gynecology at Yale Health.

Table of Contents
Table of Contents

It can be confusing when two test results disagree. This can sometimes occur with tests used to diagnose sexually transmitted infections (STIs). For example, a person’s urine test for chlamydia may be positive but their genital culture may come back negative.

False-negative results are those in which you have a condition but the test says you don’t. False-positive results are those in which you don’t have a condition but the test says you do.

In the end, no diagnostic test is perfect. While most STI tests are highly accurate if used correctly, false results can and do happen.

This article explores some of the reasons why false results occur and what can be done if an STI test result is unclear or inconsistent.

Sensitivity and Specificity

Most modern STI tests are very good. However, no test is going to be 100% accurate 100% of the time.

A test’s accuracy is measured with two values:

• Sensitivity: Also known as the true positive rate, this is the percentage of time a test correctly picks up that an infected person has an STI.
• Specificity: Also known as the true negative rate, this is the percentage of time a test correctly eliminates the diagnosis of an STI in an uninfected person.

If a test has a sensitivity of 99%, this means that 99 out of 100 infected people will be correctly diagnosed and that one infected person will be given a false-negative result. False-negative means that a person has an STI even though the test says they do not. The lower the sensitivity, the higher the risk of false negatives.

If a test has a specificity of 99%, this means that 99 out of 100 uninfected people will be correctly diagnosed and that one uninfected person will be given a false-positive result. False-positive means that a person does not have an STI even though the test says they do. The lower the specificity, the higher the risk of false positives.

Recap

Sensitivity is the true positive rate of a test. Specificity is the true negative rate. Tests with lower sensitivity or specificity have a higher rate of false-negative or false-positive results, respectively.

False Positives vs. False Negatives

Because STI tests aren’t perfect, people who design them often might be able to choose whether it’s better to have more false-positive or more false-negative results. Which is better depends on the severity of the disease and the available treatments

If a condition has no serious long-term consequences but the treatment itself is grueling, a false positive may expose a person to needless treatments that cause more harm than good.

If an infection is serious and requires immediate treatment, a false-negative result could lead to a missed diagnosis and progression of a serious illness. One such example is HIV.

Factors that Affect Test Accuracy

Most STIs have a window period during which the body hasn’t produced enough antibodies to produce an accurate test result. If the tests are performed too soon after infection, it may return a false-negative result. In such cases, a person may be asked to come back for retesting if the exposure was recent and the risk of infection is high.

Disease Prevalence

Another factor that affects the rate of false results is disease prevalence, which is how common a disease is.

Imagine, for example, that there is a rare disease that only affects only one in a million people. If the test is very good at detecting the disease, it will usually find that one-in-a-million case. However, because there are so few people with the disease, the chances are high that there will be more false positives than true positives.

On the other hand, if a condition is common and a large part of the population has it, then the situation could be reversed. Even if the test has lower sensitivity, there may be more true positives than false positives simply because there are more people who have the infection.

Recap

The accuracy of a test is influenced by how common or uncommon a condition is. If an infection is rare, there will usually be more false positives than true positives. If a condition is common, there may be more true positives than false positives.

Coping With Inconsistent Results

If you get two different results from two different tests, what should you do? The answer depends largely on the type of STI involved.

If the condition is easily treated, and the treatment doesn’t have any serious side effects, then you may decide to take the treatment.

Two such examples are chlamydia and gonorrhea, which are sometimes treated presumptively. This means treatment is given if there’s a chance the person may be infected. For example, sexual partners of a person who tests positive may be treated without being tested themselves.

However, if treatment is costly or poses certain harms, then additional tests may be ordered to confirm the results. Such is the case with HIV tests, which require a confirmatory test to accurately diagnose the virus. When a confirmatory test is used, HIV tests are rarely ever wrong.

Summary

STI tests are very accurate, but they are not perfect. The accuracy of a test is based on its sensitivity (ability to pick up positives) and specificity (ability to eliminate negatives). Tests that have lower sensitivity or specificity have a higher risk of false results.

• False-negative results are those in which you have an STI but the test says you don’t.
• False-positive results are those in which you don’t have an infection but the test says you do.

Other factors can influence a test’s accuracy, including how common or uncommon a condition is. Certain STI tests also have window periods in which a test may return a false-negative result if it is performed too soon after STI exposure.

A Word From Verywell

If you end up with an inconsistent STI test result, stop, take a breath, and ask your doctor for advice on how to best proceed. If the treatment is simple, you may decide to take it even if you’re not sure you acquired an STI. Or, you may decide that it’s better to get retested or use a different kind of test.

Other issues can influence your decision. This includes knowing how to talk to a sex partner—not only about this incident but also about how to reduce your risk of STIs in the future.

Frequently Asked Questions

Are false-positive STI test results common?

False-positive STI test results are rare. Tests for common STIs, such as chlamydia and gonorrhea, have a specificity of more than 99%. This means that fewer than 1% of people who are not infected will test positive.

Are false-negative STI test results common?

False-negative results are rare. For most common STI tests, the sensitivity rate is over 90%. This means that the tests miss fewer than 10% of people who are infected.

How often should someone be tested for STIs?

It depends. Someone in a monogamous relationship won’t need to be tested as often as a person with multiple sex partners. For example, sexually active females age 24 and younger, females over age 24 who are at risk, and men who have sex with men may be advised to get tested for chlamydia at least once yearly. All adults and adolescents from ages 13 to 64 should be tested at least once for HIV,

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

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By Elizabeth Boskey, PhD
Elizabeth Boskey, PhD, MPH, CHES, is a social worker, adjunct lecturer, and expert writer in the field of sexually transmitted diseases.