Syphilis testing relies on a blood draw, and in most cases you’ll need two different types of blood tests to get a definitive answer. A single test isn’t enough because syphilis diagnostics work in a two-step process: one test screens for signs of infection, and a second, different test confirms whether the syphilis bacterium is actually responsible. The whole process typically starts with a standard blood sample at a clinic, lab, or sexual health center.
The Two Types of Blood Tests
Syphilis blood tests fall into two categories, and understanding the difference matters because you’ll likely encounter both on your lab results.
Nontreponemal tests (RPR and VDRL) detect your body’s immune response to cell damage caused by the infection, not the bacterium itself. These are the traditional screening tests. They’re inexpensive and fast, but they can be triggered by other conditions, which is why a positive result always needs confirmation. During secondary syphilis (when a rash or other symptoms appear), RPR and VDRL catch nearly 100% of cases. But in primary syphilis, when you might only have a painless sore, their sensitivity drops to roughly 50% to 76%.
Treponemal tests (TPPA, FTA-ABS, and automated immunoassays) look for antibodies your immune system makes specifically against the syphilis bacterium. These are more accurate across all stages. TPPA detects about 95% of primary syphilis cases and reaches 100% sensitivity for secondary syphilis in most studies. The trade-off: once you’ve had syphilis, treponemal tests usually stay positive for life, even after successful treatment. So they can’t tell you whether a current infection is active or old.
How the Testing Sequence Works
There are two testing pathways labs use, and which one you encounter depends on the lab your provider works with.
In the traditional algorithm, the lab runs a nontreponemal screening test (RPR or VDRL) first. If it’s reactive, a treponemal test confirms the result. This approach has been standard for decades. It’s good at identifying active infections because the nontreponemal test produces a titer, a number that rises with active disease and falls after treatment.
In the reverse sequence algorithm, the lab starts with an automated treponemal immunoassay. If that’s reactive, a nontreponemal test follows. Many large labs now use this method because the first step can be fully automated, making it faster to process high volumes. The downside is that the treponemal test may flag people who were treated for syphilis years ago and no longer have an active infection. When the treponemal test is reactive but the nontreponemal test is not, a second, different treponemal test is run to sort out whether it’s a true positive or a lab artifact.
Either way, a confirmed diagnosis requires at least two tests agreeing. If your results are discordant (one positive, one negative), your provider will typically order a third test or recommend retesting in a few weeks.
When Testing Is Most Accurate
Timing matters more for syphilis testing than many people realize. After exposure, the bacterium has an incubation period of about 10 to 90 days before a sore (chancre) appears. Blood tests may not turn positive until your immune system has had time to build detectable antibodies, which can take several weeks after the sore develops.
If you test too early, during the first few weeks after exposure, a blood test can come back negative even if you’re infected. This is especially true for nontreponemal tests, which miss 25% to 50% of primary syphilis cases. Treponemal tests perform better in early infection but still aren’t perfect. If you have a suspicious sore but your blood test is negative, retesting in two to four weeks is standard practice.
By the time syphilis reaches the secondary stage, usually six weeks to six months after the initial sore, blood tests become highly reliable. Both nontreponemal and treponemal tests reach near-100% sensitivity at this point. In later latent stages (when there are no symptoms but the infection is still present), nontreponemal tests become less reliable again, dropping to 63% to 66% sensitivity for late latent syphilis. Treponemal tests hold up better, staying above 84% to 100% even in late stages.
Testing a Sore Directly
If you have a visible sore or lesion, a provider can test it directly rather than relying only on blood work. Darkfield microscopy, which involves examining fluid from the sore under a specialized microscope, was the original gold-standard method. It can identify the corkscrew-shaped syphilis bacterium in real time, but it requires specialized equipment and an experienced technician, so it’s not available everywhere.
PCR testing of lesion swabs is increasingly used as an alternative. It detects the bacterium’s genetic material and works well on primary chancres and the rashes or mucous patches of secondary syphilis. Direct testing is especially useful in early primary syphilis, when blood tests are most likely to give a false negative.
False Positives and What Causes Them
Nontreponemal tests are known for producing false positives. Because they detect general immune activity rather than syphilis-specific antibodies, a range of other conditions can trigger a reactive result. Common causes include lupus and other autoimmune diseases, recent viral infections (including hepatitis and HIV), pregnancy, IV drug use, and certain vaccinations. These are called biological false positives, and they’re the main reason a reactive screening test always requires confirmation with a treponemal test.
Treponemal tests are much more specific, but they aren’t immune to error either. False positives can occur with other spirochetal infections like Lyme disease, though this is uncommon. The bigger practical issue with treponemal tests is that they remain reactive for life in most people who’ve ever had syphilis, so a positive treponemal test in someone with a history of treated syphilis doesn’t necessarily mean reinfection.
Who Should Get Tested and How Often
The CDC recommends routine syphilis screening for several groups. All pregnant women should be tested at the first prenatal visit, with retesting at 28 weeks and at delivery if they have risk factors such as substance use, multiple partners, a new partner, or a partner with an STI.
Men who have sex with men should be tested at least once a year, and every three to six months if they have multiple partners or other risk factors. People living with HIV should be screened at their first evaluation and at least annually after that. The CDC also recommends screening for adults with a history of incarceration, people involved in transactional sex work, and men under 29 in areas with high syphilis rates. Transgender and gender-diverse individuals should be screened at least annually based on sexual behaviors and exposure.
Testing for Neurosyphilis
If syphilis goes untreated for years, it can affect the brain and spinal cord. Testing for neurosyphilis requires a spinal tap (lumbar puncture) to collect cerebrospinal fluid. A VDRL test run on that fluid is highly specific: if it’s reactive, neurosyphilis is confirmed. The diagnosis can also be made based on an elevated white blood cell count of 20 cells per microliter or greater in the spinal fluid. Protein levels in the fluid are also measured.
Spinal taps aren’t routine. They’re reserved for people with neurological symptoms (vision changes, hearing loss, headaches, confusion, difficulty walking) or for those with late-stage untreated syphilis where nervous system involvement is suspected.
Testing Newborns for Congenital Syphilis
Babies born to mothers who tested positive for syphilis during pregnancy need their own evaluation. The infant’s blood is tested with the same type of nontreponemal test used on the mother so the results can be directly compared. A titer in the infant that’s four times higher than the mother’s suggests active infection rather than passively transferred antibodies.
The newborn also receives a physical exam looking for signs like an enlarged liver or spleen, skin rash, or limited limb movement. Depending on the situation, additional evaluation may include spinal fluid analysis, a complete blood count, and long-bone X-rays to check for bone abnormalities. Importantly, testing umbilical cord blood is not recommended because it produces unreliable results. The placenta may also be examined using special staining or PCR to look for the bacterium directly.
Where to Get Tested
You can get syphilis testing at your primary care provider’s office, sexual health clinics, community health centers, and many urgent care facilities. Public health departments often offer free or low-cost testing. Some at-home test kits are also available, where you collect a blood sample via finger prick and mail it to a lab, though these vary in which test types they include. If you have an active sore, an in-person visit is more useful because the provider can swab the lesion directly for faster, more accurate results in early infection.
Results from a standard blood draw typically come back within one to three business days, though rapid point-of-care treponemal tests can produce results in under 20 minutes at clinics that offer them.