Blood drop test kit for HIV, hepatitis B, hepatitis C and syphilis, on a white background.

Some home syphilis tests also detect several other infections, such as HIV and hepatitis C.Credit: Pixel-shot/Alamy

Cases of sexually transmitted infections (STIs) are increasing in some countries around the world. Even the COVID-19 pandemic – which has brought life to a standstill in many ways – hasn’t halted the trend. In April, the US Centers for Disease Control and Prevention reported that there were 133,945 cases of syphilis in the first year of the pandemic, a 52% increase since 2016 (see “Resurgence”).

And that’s likely an underestimate, the CDC says, given that early in the pandemic, health clinics had to limit in-person visits and STI surveillance programs found their resources shifted elsewhere. The situation sparked a surge in home testing for syphilis and other sexually transmitted diseases.

Encouraged by the popularity of at-home testing for COVID-19, self-testing has been embraced by many, including policymakers and people in marginalized communities. In January, California passed a law requiring private health insurers to pay for STI testing at home. Proponents say self-testing has the benefit of allowing individuals to collect samples in the privacy of their homes, free from any stigma that might come with going to clinics, and it allows people of different backgrounds and income levels to test frequently.

Resurgence: Bar chart showing the total number of cases of syphilis in the United States from 1980 to 2020.

Source: US Centers for Disease Control and Prevention.

But the postponement could also have trade-offs — some of them are similar to those experienced with COVID-19, warns Shweta Patel, a gynecologist at the University of Alabama at Birmingham. With at-home testing, people don’t get the advice that comes with in-person testing, and public health departments may lose out on valuable statistics. Users need to report their own findings and do their own contact tracing to let others know they may need testing, Patel says, and that doesn’t always happen.

Still, self-testing for COVID-19 has proven useful during the pandemic, says Natalie Cramer, associate executive director for programs at the National Alliance of State and Territorial AIDS Directors in Washington DC. She adds that it’s high time for a similar approach to STIs like syphilis and HIV. “I think we’re all more comfortable self-testing with COVID,” says Cramer. “It opened up both a need and a level of comfort that people have around them.”

Anatomy of a pathogen

For about half a millennium, syphilis has been one of the most well-known — and feared — STIs. It fueled the development of the first drug to be called a miracle cure (Salvarsan, an arsenic-based compound, in 1910) and has largely been considered curable since the advent of penicillin three decades later. Proper use of condoms can reduce disease transmission to almost zero. However, if left untreated, the disease can prove fatal and cause serious neurological problems, especially if transmitted at or before birth.

Color scanning electron micrograph (SEM) showing yellow spiral-shaped bacteria on a red background.

Many syphilis tests detect antibodies against the family of bacteria that includes the corkscrew form Treponema pallidum.Photo credit: Dr. Kari Lounatmaa/Science Photo Library

Evidence of the disease caused by the corkscrew-shaped bacterium Treponema pallidum, is simple, says Deborah Williamson, a microbiologist at the University of Melbourne in Australia who has spent several decades studying syphilis. In a clinic, a healthcare worker will typically draw a sample of blood and run an assay to look for antibodies to the family of bacteria that it contains T.pallidum. If this assay is positive, the clinician will run additional tests on the original sample to make a diagnosis.

Only a small subset of tests that require swabs from open wounds or other areas of the body actually look for the bacterium itself, whether it’s looking for it under a microscope or identifying parts of its DNA or protein. Regardless of the method, however, the treatment requires a doctor’s prescription if a person tests positive, Patel says.

Testing goes DIY

When the COVID-19 pandemic hit, clinical testing around the world took a hit. For example, when SARS-CoV-2 infection rates in New York City skyrocketed in the second quarter of 2020, STI testing fell sharply. Clinics limited visits and people were reluctant to see doctors in rooms where they could contract COVID-19. STI transmission started to rise. While necessary, these changes “unleashed a perfect storm,” says Jennifer Mahn, director of clinical and sexual health at the National Coalition of STD Directors in Washington DC.

But the pandemic has also sparked a boom in home testing. Governments began encouraging at-home COVID-19 testing so people can check if they are contagious before traveling or socializing. A variety of studies showed that frequent self-testing, even with rapid antigen tests that could only detect infections when viral loads were high, could support the public health response1,2. The move also made doctors, researchers, and the public more confident about the accuracy and usefulness of home test kits in general.

Advocates hope the popularity of at-home testing for COVID-19 will spur use and demand for at-home STI testing. Research shows these may be popular. For example, a study on men having sex with men in Zimbabwe, first published in April 20213showed that participants were eager to self-test for syphilis.

Self-test kits would go a long way in helping these men access testing services, says study co-author Definate Nhamo, senior program manager at the Pangea Zimbabwe AIDS Trust in Harare. “You can get a result in 20 to 30 minutes and you don’t have to endure some of the negative attitudes of healthcare providers.”

test accuracy

In 2012, the US Food and Drug Administration approved its first at-home HIV test, which detects antibodies to HIV-1 and HIV-2 in saliva. Users could get results in minutes without having to send samples to a lab. The test was not as popular as first expected, and its price tag was high at $40-$50. However, it inspired similar initiatives for other STIs: Syphilis self-testing appeared in the late 2010s.

Like their counterparts in clinics, home syphilis tests detect antibodies against the family of bacteria to which they belong T.pallidum in a blood sample. But they only need a drop of blood and not a tube full. They work like a cross between a blood glucose meter and a home pregnancy test.

Other self-administered syphilis tests ask users to take separate swabs from their mouth, anus, and genitals and send the samples to a clinical laboratory for analysis. These kits test directly for pieces of bacterial protein T.pallidum, or for substances secreted by cells during syphilis infection. When used properly, some tests can have sensitivities (risk-positive rates) and specificities (risk-negative rates) that are comparable to clinical tests—over 98%. However, the results can take several days to come in, making them less useful for testing before each sexual encounter, says Weiming Tang, an infectious disease epidemiologist at the University of North Carolina at Chapel Hill.

One downside is that kits are often expensive, starting at $50, with some options costing over $100 — although some include testing for multiple STIs. Another disadvantage of home tests, which provide results within minutes, is that clinical studies have found differences between different batches and users.

Self-test challenges remain

The push for syphilis self-testing also creates public health challenges parallel to those of the COVID-19 pandemic. The accuracy of a home test depends on how well the user can collect the sample, Cramer says, whether it’s a nasopharyngeal swab for SARS-CoV-2 or a genital swab for syphilis. Perhaps most concerning for some, home testing lacks the counseling and contact tracing that comes with clinical testing, especially for STIs like syphilis4.

Cramer says that while self-test results aren’t automatically uploaded to public health databases, that may not result in as large a drop in reporting as expected. Many people she’s seen tested at home wouldn’t go to a clinic to get tested anyway, so there wouldn’t be a net data loss for them. Additionally, people still need to contact the healthcare system to receive treatment, providing another way to track infections and contacts. Some self-testing programs mail antibiotic prescriptions to people who test positive without requiring a formal clinic visit, but this method offers other ways of collecting data. Overall, Hecht says, society is better off with home testing than without.

Initiatives to encourage the use of home syphilis testing kits have gained momentum during the pandemic. In the United States, Hecht is testing a program to send free STI kits to people who sign up. Importantly, the program also offers telephone advice and connects users to local health departments for follow-up and contact tracing.

“We’re seeing quite a number of positive cases, which shows us that we’re reaching a group of people who really need this type of service,” says Hecht. “We want to make access as easy as possible for people.”

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