Syphilis Testing: Window Periods, Two-Step Blood Tests, and What National Data Show

Syphilis is a sexually transmitted infection caused by the bacterium Treponema pallidum. It is spread through direct contact with a syphilis sore during vaginal, anal, or oral sex, and it can also pass from a pregnant person to a fetus. The infection develops in stages — primary, secondary, latent, and tertiary — and each stage looks different.

Syphilis is different from chlamydia and gonorrhea in one important way for testing: it is identified with blood tests (serology), not urine or swabs. That means one blood draw answers the syphilis question regardless of which sites were involved in a sexual encounter.

Primary syphilis classically presents as a painless sore (chancre) at the site of infection — on the genitals, in the mouth, or elsewhere — that resolves on its own in about 3 to 6 weeks whether or not anyone notices it. Secondary syphilis can follow with a rash (sometimes on the palms and soles), mucous patches, swollen lymph nodes, fever, or fatigue; those signs also resolve without intervention.

After secondary syphilis comes the latent stage: no visible signs or symptoms at all, even though the infection is still in the body. CDC notes that latent syphilis is detected by serologic testing — which is exactly why blood testing exists for people who feel well. In its 2023 data, nearly half of reported syphilis cases were staged as unknown duration or late — the kind of cases that routine screening is designed to catch.

Syphilis testing uses two families of blood tests: treponemal tests, which detect antibodies against T. pallidum, and non-treponemal tests, which detect a different antibody response used to help confirm infection and follow disease activity. CDC's 2024 laboratory recommendations describe a two-step approach — a screening test of one type, followed by a test of the other type to confirm — so a single reactive result is not treated as the whole story.

As with any serology, timing governs accuracy: antibodies take time to develop, so a blood test drawn too soon after exposure can miss an early infection. There is no single published cutoff; antibodies are generally detectable within the first several weeks. CDC post-exposure guidance supports repeating serology at about 4 to 6 weeks and again around 3 months when an initial result is negative and a recent infection cannot be ruled out. A test taken too early can miss a recent infection — planning the draw (and any repeat) around that window is what makes the result trustworthy.

Because the primary and secondary sores and rashes go away on their own, people can live for years with syphilis without knowing it. CDC documents that without the right treatment, the infection moves into the latent stage and, in some people, into tertiary syphilis years or decades later, when it can affect the heart, blood vessels, brain, and nervous system. Not everyone with untreated syphilis develops tertiary disease — but staging data show how many infections are caught late when screening is infrequent.

Syphilis at any stage also increases the risk of acquiring HIV, and it can pass to a baby during pregnancy, which is why CDC screening recommendations are broad. A blood test is a quiet, private way to close the loop on a new partner, a missed period of screening, or a routine checkup.

CDC's 2023 surveillance report counted 53,007 primary and secondary (P&S) syphilis cases — the most infectious stages — a national rate of 15.8 per 100,000 and a 10.7 percent decrease from 2022, which CDC describes as the first substantial drop in P&S syphilis since 2001. Total syphilis across all stages (excluding congenital cases) was 205,371, roughly flat from 2022, with increases in cases staged as unknown duration or late offsetting the P&S decline.

Across the 100 largest US metros, 2023 metro-level surveillance data show a mean P&S syphilis rate of 23.05 per 100,000 with a standard deviation of 11.59 — metro rates vary several-fold from one another. The honest read of the national picture: recent declines in the most infectious stages are real progress, while the continued rise in infections identified late is an argument for routine screening rather than symptom-driven testing.

Lab-based syphilis testing through an online order is processed at a Labcorp or Quest patient service center, with thousands of locations nationwide. Syphilis testing is a blood draw, and it can be ordered alone or as part of a broader STD panel — a common pairing with chlamydia and gonorrhea testing when more than one infection is a realistic possibility, since CDC recommends testing for syphilis alongside other STIs in many situations.

Insurance is optional. When a claim is processed through insurance it can show up on an explanation of benefits; paying out of pocket keeps the visit off insurance paperwork, a frequent reason people choose self-pay for STI testing. Current single-test and panel pricing is listed on the STDcheck pricing page, and a $10-off coupon flow is available at checkout.