STD testing is a scary experience for many people. That’s why you might feel relieved when the test comes back negative.

But before you go out and celebrate or have any more sex, consider the possibility of a false negative.

This is when an STD test says you are free from a sexually transmitted infection but in reality, you are infected.

False Negatives and False Positives

STD tests

Modern STD tests have become much more accurate thanks to better technology and new testing methods.

Chlamydia and Gonorrhea tests, the most common STD tests in the United States, are especially accurate.

Labs use a nucleic acid amplification test to look for the bacteria’s RNA or DNA. This test can accurately detect over 95% of infection cases.

Tests for syphilis, herpes, and HIV are pretty accurate as well.

However, there is still a possibility that the test results could be wrong.

If the test shows you are positive when you don’t have that STD, it’s called a false positive.

If the test shows you are negative when you actually have that STD, it’s called a false negative.

In this post, we’ll focus on false negatives, why they happen and how to avoid the potentially dangerous effects of getting a false negative on your STD test.

Possible Causes of a False Negative STD Test

negative test

Here are some reasons why you may still have a sexually transmitted infection even if your test came back negative.

1. You Took a Test during the Window Period

If you had sex last night and there are strong chances you might have been exposed to an STD, any test you take today or even tomorrow will most likely indicate you are negative.

That’s because the STD takes time before it shows up in your blood, urine or swab.

Don’t confuse the window period with the incubation period, which is the period between getting infected and displaying symptoms.

Window period varies with each STD.

The window period is longer for tests like syphilis and HIV which detect the body’s immune response (antibodies) rather than the STD bacteria or virus.

Modern testing techniques have drastically shortened the window period. But there is still a risk that you get a test too early.

For HIV and syphilis, the window period is around 6 weeks, meaning there is a higher risk of a false negative during the first one and a half months.

For chlamydia and gonorrhea, the window period is 2-7 days.

Note: Inaccuracy of a test during the window period only applies to false negatives. If you get a positive test result two days after a sexual encounter, you are most likely infected (though it is a good idea to test again).

2. You Used a Home Test

Lab STD tests are the most accurate for obvious reasons.

The test is done by pros in a highly regulated environment and using reliable equipment.

Lab tests where you take the sample yourself at home and send it to a lab are slightly less accurate because you might take the sample wrongly or its quality could be compromised during transit.

That’s why we recommend online tests like STD Check. You order online but go to a lab (anonymously) to get your sample taken and tested.

Essentially, you enjoy the confidentiality of a home test with the reliability of a lab test.

The least accurate tests are home instant tests where you take a sample and get results right there and then.

Fortunately, most home tests skewer more towards false positives (if positive, get a second test at the lab). False negatives are less common.

3. The Test Was Simply Wrong

Even the most accurate tests can be wrong now and then.

Three things determine the accuracy of an STD test.

Sensitivity: How good the test is at finding people with a specific infection. High sensitivity means fewer chances of a false negative.

Specificity: How good the test is at finding people without a specific infection. High specificity means fewer chances of a false positive.

Prevalence: How common disease is. The number of people with an infection can determine how accurate a test for that infection is.

I won’t go deeper into these factors because they involve some technical stuff and even some calculations.

To learn more about them, Very Well Health has an article on how to calculate a test’s accuracy based on the three factors.

All you need to know is that while STD tests will be accurate most of the time, they can also be wrong.

The most important thing is to ensure a false negative does not worsen your health because of delayed diagnoses and treatment.

How to Deal with a Potentially False Negative

std testing window

If you take an STD test and get negative test results, the best thing is to get another test after some time.

Don’t get a repeat test immediately in case it’s still during the window period.

Wait for a few weeks; around 3-6 weeks.

Make sure the repeat test is done at a lab. Either go to a lab or clinic yourself or order an online STD lab test from STDCheck.

During the waiting period, do not have sex with anyone just in case it was a false negative and you really do have an STD.

If the second test is positive, remember to inform your sexual partner(s).

I Tested Negative, But Could I Still Have an STI?

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