False Postive and Negatives 

The Number One Reason STI Tests Go Wrong

Here is something very few people understand.

Most false negatives in sexual health happen not because the lab made a mistake and not because the test itself malfunctioned.

They happen because people are being tested in the wrong anatomical location.

Bacterial STIs do not magically migrate through the body. They infect the place they are exposed to.

• If the exposure happened through oral sex, the infection will be in the throat.
• If the exposure happened through receptive anal sex, the infection will be in the rectum.
• If the exposure happened through genital sex, the infection will be in the genitals.

Three different locations.

Three different ecosystems.

Three different results.

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Sensitivity and Specificity

These are the two concepts that determine how good any medical test is.

Sensitivity answers the question:

If the person actually has the infection, how often will the test detect it?

Specificity answers:

If the person does not have the infection, how often will the test correctly be negative?

A test with lower sensitivity will miss infections.

A test with lower specificity will falsely label people as positive.

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Shameless Care’s Lab Partner’s Sensitivity and Specificity

Chlamydia, Gonorrhea, and Trichomonas (CT/NG/TV) PCR

Aptima CT/NG/TV PCR (Nexus Validation Data)

• Sensitivity (PPA): 100%
• Specificity (NPA): 100%

Multiplex CT/NG/TV PCR – Aptima

• Sensitivity (PPA): 98.9%
• Specificity (NPA): 98.3%

Cobas CT/NG/TV PCR (Bridge Study)

• Sensitivity (PPA): 100%
• Specificity (NPA): 100%

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Mycoplasma genitalium PCR

• Sensitivity (PPA): 98.3 percent
• Specificity (NPA): 96.7 percent

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HIV (4th-Generation Antigen/Antibody, Dried Blood Spot EIA)

Nexus HIV Combo EIA Validation Data

• Sensitivity (PPA): 97 percent
• Specificity (NPA): 98 percent

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Hepatitis C (HCV Antibody, Dried Blood Spot EIA)

• Sensitivity (PPA): 97 percent
• Specificity (NPA): 100 percent

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Syphilis (Treponemal IgG, Dried Blood Spot EIA

• Sensitivity (PPA): 96 percent
• Specificity (NPA): 100 percent

A positive treponemal test should be followed with a non-treponemal assay (RPR) to determine active infection.

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Hepatitis B Surface Antigen (HBsAg, Dried Blood Spot EIA)

• Sensitivity (PPA): 96.7 percent
• Specificity (NPA): 100 percent

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HSV-2 Antibody (Dried Blood Spot EIA)

• Sensitivity (PPA): 100 percent
• Specificity (NPA): 100 percent

Positive results indicate past exposure and may require confirmatory testing in low-prevalence settings.

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How People and Providers Accidentally Create False Negatives

Even with excellent laboratory technology, human behavior can reduce accuracy.

Here are the most common real-world causes.

Urinating too soon before a urine test

Almost every guideline recommends waiting at least one hour after your last urination before collecting a sample. That’s because you need enough time for organisms to accumulate in the urethra.

Urinating right before a test can flush out or dilute the bacteria and produce a false negative.

Poor sample collection

This matters a lot, especially for throat swabs.

Some clinicians barely touch the swab to the throat for half a second. Others avoid the tonsils because they don’t want to trigger a gag reflex. That swab is unlikely to pick up enough DNA for a high-sensitivity test to work properly.

Proper technique touches both tonsillar pillars and the posterior throat for several seconds.

The difference in accuracy between a good swab and a superficial swab is substantial.

At-home collection works extremely well when done correctly. But rushing the process or barely touching the swab to the site can cause a false negative. The PCR machine does not know whether the patient followed instructions. It only knows whether the organism’s DNA is present.

Testing the wrong site

A urine test cannot detect throat gonorrhea.

A genital swab cannot detect rectal infection.

A blood test cannot detect oral chlamydia.

This is the most preventable cause of false negatives.

Recent antibiotic use

Even a single dose of an antibiotic taken for an unrelated issue can temporarily suppress bacteria enough to produce a negative result, even if the infection is not fully cleared.

Very early infection

If someone tests very soon after an exposure, the organism may not have multiplied to detectable levels. Gonorrhea and chlamydia NAATs generally detect most infections by about a week after exposure and almost all by two weeks. Testing earlier can produce a false negative.

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The Truth

False positives and false negatives exist in every corner of medicine.

The goal is not perfection. The goal is accuracy with the lowest possible risk of error.

Modern PCR based STI testing in a high complexity CLIA certified lab is the best technology we have today. It is extremely accurate.

But the most important ways to reduce the risk of false results are simple.

• Test every site where an infection could be.
• Take your time with sample collection.
• Avoid urinating right before a urine test.
• Retest if your symptoms do not match your results.
• Use a panel that actually includes the infections you care about.

This is the entire philosophy behind Shameless Care.

We want your negative result to mean something.

We want your positive result to be real.

And we want you to understand how these tests work, so you never again mistake a partial test for a complete one.