Confusion can arise when two test results are inconsistent. This sometimes happens with tests used to diagnose sexually transmitted infections (STIs). For example, a person may have a positive urine test for chlamydia, but their genital culture may be negative.
False negative results are those for which you have the condition but the test says you don’t. False positive results are those that you don’t have the condition but the test shows you have.
In the end, no diagnostic test is perfect. While most STI tests are very accurate when used correctly, false results can and do occur.
This article explores some of the reasons for erroneous results and what to do if STI test results are unclear or inconsistent.
Sensitivity and specificity
Most modern STI tests are very good. However, no test will be 100% accurate 100% of the time.
The accuracy of the test is measured by two values:
- Sensitivity: Also known as the true positive rate, this is the percentage of time the test correctly finds an infected person with an STI.
- Specificity: Also known as the true negative rate, this is the percentage of time that the test correctly eliminates an STI diagnosis in an uninfected person.
If a test has a sensitivity of 99%, that means 99 out of 100 infected people will be correctly diagnosed, while one infected person will get a false negative result. A false negative means a person has an STI, even if the test shows they don’t. The lower the sensitivity, the higher the risk of false negatives.
If a test has a specificity of 99%, that means 99 out of 100 uninfected people will be correctly diagnosed, while one uninfected person will get a false positive result.A false positive means that a person does not have have sexually transmitted infections, even if tests show they have. The lower the specificity, the higher the risk of false positives.
Sensitivity is the true positive rate of a test. Specificity is the true negative rate. Tests with lower sensitivity or specificity have higher rates of false negative or false positive results, respectively.
Sensitivity and specificity of medical tests
false positives and false negatives
Because STI tests aren’t perfect, the person who designs them can often choose whether it’s better to get more false positives or more false negatives.Which is better depends on the severity of the disease and available treatments
If a disorder has no serious long-term consequences, but the treatment itself is painful, a false positive could put a person on unnecessary treatments that do more harm than good.
If the infection is severe and requires immediate treatment, false-negative results can lead to missed diagnosis and progression of serious disease. One such example is HIV.
Which HIV test is the most accurate?
Factors Affecting Test Accuracy
Most STIs have a window period during which the body does not produce enough antibodies to produce an accurate test result. If tested too soon after infection, it may return false negative results. In this case, a person may be asked to come back and retest if the exposure is recent and the risk of infection is high.
Another factor that affects the rate of false results is disease prevalence, which is how common the disease is.
For example, imagine a rare disease that affects only one in a million people. If a test is very good at detecting a disease, it usually finds one in a million cases. However, since there are so few people with the disease, there is a high chance of more false positives than true positives.
On the other hand, if a condition is widespread, and a large portion of the population suffers from it, the situation can be reversed. Even if the test is less sensitive, there may be more true positives than false positives, simply because more people are infected.
The accuracy of the test is affected by how common or uncommon the condition is. If infection is rare, there are usually more false positives than true positives. If a condition is common, there may be more true positives than false positives.
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Coping with inconsistent results
What should you do if you get two different results from two different tests? The answer largely depends on the type of STI involved.
If the condition is easily treatable and the treatment does not have any serious side effects, then you may decide to go for it.
Two such examples are chlamydia and gonorrhea, sometimes presumed to be treated. This means that if the person is at risk of being infected, treatment will be given. For example, the sexual partner of someone who tests positive may be treated without being tested.
However, if the treatment is expensive or causes certain harms, additional tests may be required to confirm the results. This is the case with HIV testing, where confirmatory testing is needed to accurately diagnose the virus. HIV tests rarely go wrong when a confirmatory test is used.
What is the window period for different STDs?
STI tests are very accurate, but not perfect. The accuracy of a test is based on its sensitivity (ability to detect positives) and specificity (ability to eliminate negatives). Tests with 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 shows you don’t.
- False positive results are those in which you do not have the infection but the test shows that you have it.
Other factors can affect the accuracy of the test, including how common or uncommon conditions are. Some STI tests also have a window period, and if tested too soon after STI exposure, the test may return false-negative results.
If you end up with inconsistent STI test results, stop, take a deep breath, and ask your doctor how best to proceed. If treatment is simple, you may decide to get treatment even if you are not sure if you have an STI. Alternatively, you may decide that it is better to retest or use a different type of test.
Other issues may affect your decision. This includes knowing how to talk to a sexual partner—not just about it, but about how to reduce your risk of developing an STI in the future.
Frequently Asked Questions
Are false positive STI test results common?
False positive STI test results are rare. Tests for common sexually transmitted infections such as chlamydia and gonorrhea are more than 99% specific. That means less than 1% of uninfected people have tested positive.
Are false negative STI test results common?
False negative results are rare. For most common STI tests, the sensitivity rate is over 90%. That means these tests miss less than 10 percent of infected people.
How often should someone be tested for STIs?
it depends on. People in monogamous relationships don’t need to be tested as often as people with multiple sexual partners. For example, sexually active women 24 and younger, at-risk women over 24, and men who have sex with men may be advised to get tested for chlamydia at least annually. All adults and adolescents between the ages of 13 and 64 should be tested for HIV at least once,