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False Positive vs False Negative Test

STD tests offer a high degree of accuracy if completed correctly, but there are a number of situations which might lead to an inaccurate result. This is known as either a false negative or a false positive result. In this blog, we take a look at these two outcomes, what might cause them and analyse the difference between the two.

What is a false negative?

A ‘false negative’ is when the test result comes back negative when an individual actually has an STD. For example if a person has been tested for chlamydia and the test result is negative, but it later turns out that the person has chlamydia, this would be a ‘false negative’.

What happens if you get a false negative STD result?

False Negatives can be extremely harmful as anyone with a false negative may infect future sexual partners unknowingly as they may believe they are STD-free. Identifying STDs and receiving treatment is an effective way of stopping the spread; false negatives prevent this from happening.

Not receiving treatment in a timely manner can also be harmful to your long term health, in particular your reproductive health. Many STDs cause more harm to your body the longer that they are present. STDs such as chlamydia can cause Pelvic Inflammatory Disease and can have an impact on fertility for both men and women.

What causes false negatives?

False negative results are actually more common than people think, but the majority of the time this is caused by patient error as opposed to a problem with the test. There are a number of potential causes of this.

Testing before the recommended incubation period

One reason that false negatives occur is due to testing prior to the recommended window period for testing. No STD test is accurate immediately after sex as it takes time for the bacteria or virus to reach a detectable state or for the body to present an immune response. 

The length of time that you are required to wait depends on the particular test that you take and can be anywhere from 7 – 28 days after exposure. You will see this on the STD tests on Your Sexual Health, displayed as the “Accuracy From” period.

Each test carries an Accuracy From date which is the time period when the results reach the required level of accuracy. You should always test after this date to avoid false negative results.

Incorrect Sample

Another cause of false negative STD test results can be patient error when collecting a sample. The quality of the sample plays a big part in the accuracy. Samples taken without following instructions properly can increase the chances of a false negative result.

One potential cause could be if you collect a urine sample, but don’t collect the first part of your urine flow. Another potential cause could be if you take a vaginal swab sample, but don’t swab the correct part of the vagina to collect enough mucous. 

If you take a clinic test, a nurse or doctor will usually be able to guide you through the process. For any home tests, ensure that you read the instructions correctly.

Not testing all areas that could be infected

Another cause of false negative results could be if you fail to collect a sample from an area of the body that is infected. STD results only offer accuracy if you test the area of the body that has become infected, for example an infection in the throat will only be picked up on a test if you take a throat swab.

An example could be if you have performed oral sex on someone, but failed to take a throat swab and instead, only collected a urine sample. In this instance, you may have been infected by an STD in the throat, but not infected in the genital area. 

You should always get tested in all areas of the body that have had potential exposure.

What is a false positive? 

A false positive is when a test result comes back positive, but the patient does not have an STD. This outcome is much rarer than a false negative, because it is a lot harder to detect something that is not there. It would usually be caused by an issue with the test itself. 

If you take a test that offers a positive result, but you are highly uncertain of the accuracy, you should take a retest. There are a number of different kinds of STD tests that work in slightly different ways, whether it’s a different sample type or a different manufacturer. Retesting with a different type of test should offer you increased confidence in the results.