Reliability of Blood Tests for Sexually Transmitted Diseases

Reliability is an important aspect of any kind of test which aims to diagnose an illness or disease, but it is particularly important when it comes to infections of a sensitive nature like STIs (sexually transmitted infections). Moreover a number of highly prevalent STIs like Chlamydia and Gonorrhea have proven difficult to diagnose because of the lack of symptoms which makes it difficult to control the spread of these illnesses. As such a reliable method of testing and diagnosing STIs is important to controlling their spread and preserving public health and safety. Blood tests are a commonly used method of determining whether or not a person has been infected by an STI causing pathogen (bacteria or virus usually), and in this article we look at how reliable this type of testing can be.

What affects the reliability of blood tests?

While blood tests are still broadly used, their reliability depends on a number of different factors including what type of blood screen is being applied and when. ‘Blood test’ is a generic term for any investigation which involves taking a blood sample and examining it for the presence of any disease causing pathogens. A blood test can, for example, aim to detect signs of the pathogen directly, or alternatively look for evidence of its presence like your body’s immune response against disease. This response is highly specific to particular organisms, and involves the production of small molecules called antibodies which can be studied as evidence of the presence of particular diseases.

When you take a blood test also has an important bearing on how reliable said test can be, and this is because of an important consideration during testing called the window period. This is a length of time after the point of infection during which a test is not particularly reliable or effective, and can report a false negative (incorrectly indicate that there is no disease when in fact there is). Every type of test and every illness has its own window period, and the general aim of improving these tests is to reduce the window period to allow for safer and more accurate testing, particularly in cases where the point of infection is not entirely clear.

How reliable are STI blood tests?

Different blood tests have different reliabilities as mentioned in the previous section, and a good way of illustrating these is in the context of Herpes testing. Herpes is caused by the Herpes Simplex virus, which is usually tested for using a number of different methods, the foremost of which are the ELISA (Enzyme Linked Immunosorbent Assay) and the Immunoblot.

ELISA gives a reliable positive result for HSV-1 (a strain of the Herpes virus) 92% of the time, while the Immunoblot is accurate in about 95% of positive tests for the same type of virus. The figures are fairly similar for the other strain of the virus, HSV-2, for which ELISA gives reliable positive results 92% of the time and the Immunoblot does so 94% of the time.

About 99% of negative results reported by Immunoblot for both strains of the virus are accurate, while about 98% of negative ELISA results are accurate.

These figures give a rough idea of how reliable blood tests can be, particularly when looking for a condition like Herpes Simplex. However an important consideration is of course the window period, during which these tests will not be so reliable.

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