STI Testing & False Positive Results

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The accuracy and reliability of STI (sexually transmitted infection) testing is important because of the possibly serious consequences of an incorrect result. STI testing is the basis for the diagnosis of STIs, and therefore is the first step towards providing the treatment necessary to deal with these illnesses.

A number of STIs present with mild or no symptoms at all, and these include two of the most pervasive sexually transmitted diseases in the UK: Chlamydia and Gonorrhea. Over time however, these conditions can cause serious damage to a number of bodily systems, most notably the reproductive tract.

HIV, the virus which causes AIDS, is an STI with particularly nasty lifelong effects. Accurately diagnosing HIV is an important consideration as getting a sufferer on to treatment as soon as possible can have a strong effect on the progression and outcome of the disease.

What is a false positive?

False results are those which incorrectly indicate either the presence or absence of a particular infection. A false positive is where a test incorrectly reports that a particular condition is present, and so a person is incorrectly diagnosed with that disease and treatment begins.

What causes false positives?

False positive results are closely tied to a test’s ‘sensitivity’. Sensitivity is the term used to describe how capable a test is when it comes to detecting a bacteria or virus in small quantities, and while this doesn’t sound like it could be a bad thing, an overly sensitive test is what can contribute to the incidence of false positives.

Sensitivity can be best explained as a measure of how likely it is that a test can detect a particular illness, for example a specificity of about 98% suggests that in 98 cases out of every 100 the test will successfully detect a pathogen.

While it may not sound like a problem, and sensitivity certainly is something that clinicians and scientists look for in an STI test, over sensitivity is what causes false positives. Ultimately sensitivity needs to be balanced with specificity, which is a description of a test’s ability to specifically detect a particular pathogen accurately. If a test is too sensitive and not specific enough, it might incorrectly report the presence of a pathogen, otherwise known as a false positive.

The possibility of a false result is why many practices retest for particularly severe STIs like HIV. 

What are the negative consequences of a false positive result?

A false positive can be particularly embarrassing where STIs are concerned as once a positive result is reported, a patient will need to contact previous and current sexual partners and inform them of the condition. While necessary in the event of a true positive result to ensure that anyone who needs treatment and testing gets it, in the event of a false positive this process can be unnecessary unpleasantness.

A false positive can have a devastating effect on anyone testing for HIV, who will effectively be told that they carry an incurable condition that will affect them for the rest of their lives.

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