False Positive STD test is like scoring a self goal

People search for the STD testing nearby when they have symptoms of STDs or for various reasons like if they had unprotected sex (stranger) or with a sex worker or came in contact with the infected area of someone they suspect already have STDs etc. In many cases the test result turns out to be a False positive one.

false positive STD result is when a test says that a person has been infected when they are not. Conversely, a false negative result is when a test incorrectly says a person does not have a disease.

False Positive or negative can both be bad considering the severness of the disease and its ability to be treated. For example : if the treatment of the disease is more grueling than the disease itself or where treatment delay doesn’t have any long-term consequences. Thus in both cases a false positive result, is more troublesome.

If a person tests False positive for the Herpes and joins a Herpes dating community. That could be a disaster, a biggest catastrophe of his life to pull in STD which he never had. !!

On the contrary, if early treatment is necessary for the disease to curb, false negative will cause problems for it will be the reason for delayed in treatment.

The measurement of how good a test is depends on two factors i.e.  sensitivity and specificity. Although they are just indications, the false positive or negative not entirely depends on these parameters, but it depends mainly on positive predictive value. Positive predictive value (PPV) is basically an indication of how likely the test results would be positive for that particular STD at its current stage. It varies with the above mentioned factors such as test sensitivity, test specificity, and disease prevalence.

Let us now take a look at some false positive STD results and their repercussions.

False positive Herpes

Depending on types of test the probability of being it ‘false positive’ varies. For example IgM test, practiced by some or very few clinics in USA (this type of test has been rejected by Health Dept. of USA and CDC) for herpes testing has a bigger chances of turning out to be false positive.

Genital Herpes is caused by virus named  ‘HSV type 2’, it is a sexually transmitted disease very common in America, with the rate of 1 in 6 people.Most of the cases are asymptotic, i.e. they don’t show any signs or symptoms and patient wouldn’t know they were carriers without blood tests.

Now lets see how a common test like ‘The HerpeSelect‘ may cause a test result to be false positive. A cutoff for HSV2 virus to be positive is 1.1. This means for a Herpes test to qualify as positive the score should be minimum 1.1. A 2005 study published in the journal BioMed Central Infectious Disease found that index values above 3.5 yielded over 90 percent accuracy — but scores between 1.1 and 3.5 had around a 50 percent chance of being false positive &  Scoring just above 1.1 has 90% of the chance to be wrong .

Thus more screening is necessary when any one scores between 1.1 and 3.5 in their Herpes testing.

Also, the patients scoring ‘positive low’ are at higher risk of being tested as false positive. No good data exist on how often patients with questionable positive results are actually re-tested.

So what is the way out ? Well, certain conformity tests are available like facility at Western blot is considered highly accurate, it is expensive and cumbersome to perform. Each test costs over $200 and the University of Washington is the only lab that provides it. Biokit’s HSV-2 Rapid Test and Quest’s own HSV-2 IgG Inhibition assay.

False positives test result can occur for any test that diagnoses viral infection based on antibodies reaction, rather than direct detection of the virus, so it is not limited to Herpes alone. HIV and Hepatitis C testing are also prone to False Positive result.

False positive Hepatitis C

To understand the reason for the false positive result of Hepatitis C, first we need to understand the types and nature of the test for Hepatitis C. There are two types of test use to determine  the presence of Hepatitis C in the body 1) enzyme-linked immunosorbent assay, or ELISA, screen 2) RNA Test

  1. enzyme-linked immunosorbent assay (ELISA): Here in this test, the reaction of the antibodies are observed that the immune system develops in response to an infection (to hepatitis C). So it is a kind of an indirect test, where the response of antibodies determines the presence of virus.
  2. RNA Test : It looks directly for the virus in the bloodstream, and it is an expensive test compared to the general screening(ELISA) purposes. It is typically only used as a confirmatory test if ELISA is positive.

A False positive Hepatitis C occurs when the ELISA test comes up positive for hepatitis C antibodies.But the person taking the test was never exposed to hepatitis C virus, which leads the RNA test to read as negative.  This may happen because the ELISA test may have picked up the antibodies produced by immune system other than the reason of Hepatitis C, this is what is known as “Cross-reactive”. The ELISA ends up picking up on these antibodies’ presence and incorrectly coming up positive. There are many reasons why your immunity system can produce the antibodies like lack of sanitation and clean water etc. Thus, leading to false positive result.

Related article : What happens when STDs are kept Untreated for long? Shocking health risks !

False positive Gonorrhea

Not many times you get False positive result for Gonorrhea because it is mainly tested via urine. But in some cases or laboratory tested for antibodies via blood to Neisseria gonorrhoeae. There is a deep possibility that the body creates antibodies due to various reasons and can be mistaken for gonorrhea. Also the virus gonorrhea rarely passes through bloodstream increases the possibility of being a False Negative result. Thus it is inconclusive to tell for certain that an individual has gonorrhea at the time of the test, but can tell whether or not the individual has had it in the past. This type of test should not be used to diagnose gonorrhea.

Thus NAA test is carried where urine sample is taken from the patient.  Nucleic Acid Amplification (NAA) test finds the DNA of the gonorrhea as it searches for bacteria’s  DNA (genetic material).

False positive Chlamydia

Very rarely, the Chlamydia test can turned out to be a false positive one. Nucleic acid amplification tests (NAA) like the one for Gonorrhea is used for chlamydia. This test finds genetic material of Chlamydia. A polymerase chain reaction (PCR) test is an example of a nucleic acid amplification test. This test can also be done on a urine sample. this test is generally recommended for testing of Chlamydia can be 90-95% accurate, thus they are reliable as there are no test which are 100% accurate.

False positive Syphilis

Again, very rare that a a Syphilis test turning out to be false positive. A rapid plasma reagin (RPR) test is a blood test used to screen you for syphilis. It works by detecting the antibodies that your body produces to fight the infection. The most common reason for a false-positive, is the generation of antibodies by immune system due to reason other than Syphilis (other illness).

Thus if RPR is indicating a positive test, it is always recommended to get tested for Syphilis using FTA ABS test. It is a conformity test for the detection of Syphilis. FTA ABS test using antibodies specific for the Treponema pallidum species, such tests would be assumed to be more specific than RPR/VDLR.

 Related article : Latest CDC report Infographic on STD 

False positive HIV 

HIV is very sensitive and there are changes of turning out to be a False positive. Like in case of Hepatitis C doctor usean enzyme-linked immunosorbent assay, or ELISA test, to test for HIV. The test result detects the measure of std-testing-near-meantibodies in the blood produced by immunity. If you have the virus the body takes minimum of 4-6 weeks to produce these antibodies. Thus antibody test may not detect anything during this window period.

Like in case of Hepatitis C receiving positive ELISA doesn’t confirm you HIV infection, a small percentage of people may receive a false-positive result.This can happen if the test picks up on other antibodies in your system.

Thus a follow up test truly determines if you really have a HIV inside you. If that test too go positive then  you’re considered to be HIV-positive.