Can an STI test result be wrong?

How accurate lab testing is

For chlamydia and gonorrhoea, the lab uses a nucleic acid amplification test (NAAT), usually PCR, which looks for the bacteria’s genetic material. The Australian STI Management Guidelines (opens in new tab) describe NAATs as highly sensitive, and they work on samples you collect yourself: the guidelines endorse self-collected vaginal, throat and anal swabs, which is how many clinics collect them too.

No test is perfect, though, and a result can be wrong in either direction. A false negative says “not detected” when an infection is there. A false positive says “detected” when it isn’t.

Why an infection can be missed

• Testing inside the window period. Chlamydia and gonorrhoea take around 2 weeks after an exposure to show up reliably, and HIV takes 6 weeks on a lab blood test. Test sooner and a negative result isn’t the final word. See how soon after sex you can test.
• Not testing every site that could be infected. A urine sample only tells you about the urethra, not the throat or anus. Infections at those sites usually cause no symptoms, so they’re only found when those sites are swabbed. See urine or swab.
• A sample without enough in it. First-pass urine needs to be the first part of the stream, at least an hour after you last peed. For a vagina, the guidelines say a self-collected swab picks up more infections than urine does. Each swab needs to touch the right place, as the collection guide shows.
• A sample that can’t be tested. A sample that’s too small, leaks or is held up on its way to the lab can come back as inconclusive or failed rather than negative, and needs collecting again. That’s why lab samples are posted back the same day they’re collected.

Why a result can say positive when there’s no infection

• A rare false positive. The guidelines (opens in new tab) note that gonorrhoea NAATs aren’t validated for sites other than the genitals, and that false positives can rarely occur. It’s most often discussed for gonorrhoea in the throat. If a result doesn’t fit your circumstances, a doctor can talk it through with you and arrange testing again where that helps.
• Testing too soon after treatment. PCR finds genetic material, and it can still be there after the infection has been treated. That’s why, when a chlamydia test of cure is needed, the guidelines say it’s done no earlier than 4 weeks after treatment, so leftover DNA doesn’t give a false positive. For gonorrhoea, a test of cure, when one is needed, is done 2 weeks after treatment.

Don’t decide a positive result is wrong on your own. An untreated infection can cause problems later and can pass to partners, so talk to a doctor before you set a result aside.

A positive months later is often a new infection

Getting chlamydia or gonorrhoea again is common, which is why the guidelines recommend testing again 3 months after a positive result. A positive then is often a new infection, for example from a partner who hadn’t been tested and treated, rather than a wrong result. Telling recent partners, and holding off sex with them until they’ve been tested and treated if necessary, is how the cycle stops.

HIV self-test results

A fingerprick HIV self-test is a screening test you read yourself, so its accuracy depends partly on how it’s done. In laboratory testing, the Atomo HIV Self Test correctly identified 99.6% of HIV-positive samples (its sensitivity) and 99.6% of HIV-negative samples (its specificity). In field evaluations with first-time self-testers, those figures were 94.3% and 95.2%, so real-world results depend on the test being performed correctly.

Two things to know about any HIV self-test: the test may not detect HIV acquired in the last 3 months, so consult a medical practitioner if exposure was possible during this time; and the test is a screening test and a positive test result requires confirmatory laboratory testing on the request of a medical practitioner. If it doesn’t work, or reads positive, see a sexual health clinic (opens in new tab), GP clinic (opens in new tab) or urgent care clinic (opens in new tab) for a lab blood test. More in HIV testing at home.

If you think your result is wrong

• You tested soon after an exposure. Test again once you’re past the window period.
• A partner has told you they’ve tested positive. Tell a doctor, even if your own result was negative. You may need testing again, at the right sites, or treatment.
• You have symptoms. See someone in person, whatever your result said. Testing at home is for people without symptoms.
• A positive doesn’t fit. Raise it at your follow-up appointment. Your doctor can explain the result and whether testing again would help.

For what each result means and what happens next, see understanding your STI test results.

How at-home testing works with Prickly

Prickly is an Australian at-home STI testing service for people without symptoms. Your samples are tested by PCR at a NATA-accredited partner lab, the same method sexual health clinics and GP clinics use, and a doctor reviews every lab test result before it reaches you.

• Request online. Complete a five-minute online request, or book a short call with a doctor – bulk billed with Medicare. Either way, a doctor confirms that at-home STI testing is suitable for you.
• Receive your equipment. Collection equipment tailored to your exposures ships the same or next business day in an unmarked mailer box with Australia Post tracking.
• Collect at home. Follow the included STI testing self-collection guide – swabs and urine for the lab, and, if you’ve added HIV testing, a fingerprick HIV self-test you read at home in 15 minutes.
• Post to the lab. Post your lab samples back the same day using the included prepaid return label – at any Australia Post Office, red street posting box or Parcel Locker.
• Get your results. A doctor reviews every lab test result and emails you privately. Telehealth follow-up included – with help accessing treatment and telling partners, if needed.

The same steps are on the home page under How it works, and fees are under Pricing.

When at-home testing isn’t right

See someone in person at a sexual health clinic (opens in new tab), GP clinic (opens in new tab) or urgent care clinic (opens in new tab) instead if you have symptoms or if you’re under 16. And if you may have been exposed to HIV in the last 72 hours, don’t wait for a test – visit www.getpep.info (opens in new tab) or go to a sexual health clinic (opens in new tab) or emergency department (opens in new tab) as soon as you can. If something happened without your consent, 1800RESPECT (opens in new tab) is also available 24/7 for free, confidential support.