Skip to content
Blood Test Tracker

RPR (Rapid Plasma Reagin)

A blood test used to help find syphilis and to follow its treatment. A reactive result needs a second, more specific test to confirm it.

AI-verified against cited sources. Awaiting expert reviewAlso called RPRLOINC 20507-0

What it is

RPR looks for antibodies your body makes when syphilis damages cells. These are called nontreponemal antibodies, and they are not antibodies to the bacterium itself. The lab mixes your serum with a test reagent and looks for clumping. The result is printed as nonreactive or reactive. If it is reactive, the lab often runs a titer. It dilutes the sample step by step (1:2, 1:4, 1:8 and so on) and reports the last dilution that still clumps. A higher second number means more antibody.

Why it matters

RPR is used in two ways. It helps find syphilis, but it is always paired with a second, treponemal test to confirm, because a reactive RPR can be a false alarm. It also tracks how well treatment is working: after treatment the titer usually falls at least fourfold within a year. Syphilis can be cured, and untreated infection can cause serious harm later, so finding it matters. Syphilis testing is a standard part of prenatal care in the US.

If it’s high

A reactive result means antibodies of the type RPR detects were found. This can be a sign of syphilis, but a reactive RPR can also be a biologic false positive, which is why the lab or your clinician confirms it with a treponemal test. If a titer is reported, it shows how much antibody was found and is used to follow treatment over time. A titer can stay low and reactive for a long time after successful treatment (the serofast state).

Common causes

  • Syphilis infection, current or recent
  • Past syphilis that was treated, with the titer not yet back to nonreactive (including a long-lasting low titer called the serofast state)
  • A biologic false positive from other infections (such as HIV, malaria or leprosy), recent vaccination, autoimmune conditions or injection drug use
  • Pregnancy or older age, which CDC lists among factors linked to false-positive nontreponemal tests

What can help

  • Ask your clinician about a confirming treponemal test and what your combined results mean
  • Ask about testing for other infections, such as HIV, chlamydia and gonorrhea, and whether partners should be tested
  • If you have been treated, ask when to repeat the RPR, and have it done at the same lab each time

If this applies to you

RPR means a little more for some people. Open the one that fits you.

Taking HIV PrEP

Syphilis is checked with a blood test as part of routine PrEP care. For people on pills, the CDC schedule is every 3 months for men who have sex with men and transgender women who have sex with men, and every 6 months for other sexually active people. Cabotegravir and lenacapavir shots have their own schedules.

Blood tests on HIV PrEP

What can affect your result

  • Stage of infection: RPR can miss very early syphilis, and the RPR-first testing path might be less sensitive in latent syphilis (CDC 2024)
  • Conditions unrelated to syphilis can cause a reactive result (a biologic false positive)
  • Treatment: titers usually fall after treatment, and may become nonreactive
  • Prozone effect: very high antibody levels can occasionally cause a falsely nonreactive result unless the sample is diluted
  • Lab and method: RPR and VDRL titers cannot be compared with each other, so follow-up should use the same method and preferably the same lab (CDC 2021)
  • Conditions unrelated to syphilis can make the RPR reactive (a biologic false positive), including other infections such as HIV, malaria and leprosy, recent vaccinations, autoimmune conditions and injection drug use (CDC 2024). Pregnancy and older age are also listed (CDC 2021).
  • Very high antibody levels can occasionally cause a falsely nonreactive result, called the prozone effect. The lab can find it by testing a diluted sample, which a clinician can ask for (CDC 2024).
  • Plasma samples and grossly hemolyzed, lipemic or contaminated samples are rejected (Labcorp). RPR on spinal fluid is not done (Mayo).

After treatment, the titer usually falls at least fourfold within 12 months, especially when syphilis was treated early, and it might become nonreactive over time (CDC 2024).

When to talk to a clinician

A reactive RPR is not an emergency, but contact the clinician who ordered it soon, because syphilis is curable and untreated infection can cause serious problems later. See a clinician promptly for a new sore, a rash (including on the palms or soles), or changes in vision, hearing, balance, memory or strength, which can signal syphilis involving the eyes, ears or nervous system. If you are pregnant, tell your pregnancy care team right away.

Questions you could ask

  • How often should I be screened, given my age and situation?
  • If I am pregnant or planning a pregnancy, when should syphilis testing be done?
  • Which confirming test do I need, and what do my results mean together?
  • Should my partners be tested, and should I be tested for other infections?
  • When should my RPR be repeated, and should it be done at the same lab?

Questions people ask

What does a titer like 1:16 mean?

The lab diluted your sample step by step and 1:16 is the last dilution that still showed clumping. A bigger second number means more antibody. What matters most is how your titer changes over time on the same test. CDC treats a fourfold change, which is two dilutions such as 1:16 to 1:4, as meaningful.

Can an RPR be nonreactive if I have syphilis?

Yes. The RPR can miss very early infection, can be less sensitive in latent infection, and, rarely, can read falsely nonreactive on samples with very high antibody levels (the prozone effect). Your clinician may use a treponemal test or repeat the RPR, depending on your situation.

Why is my RPR still reactive after treatment?

Titers usually fall at least fourfold within a year, but some people stay reactive at a low level for a long time even after successful treatment. This is called the serofast state. Your clinician follows the titer over time to check that it is not rising.

Show the science and sources

For clinicians and researchers

RPR is a nontreponemal (lipoidal antigen) flocculation test that detects IgG and IgM antibodies to cardiolipin-type antigens released from damaged host cells and from Treponema pallidum. Labcorp's RPR uses charcoal flocculation; Mayo runs a multiplex flow immunoassay for the screen and flocculation for the titer. Labcorp and Mayo list a reference value of Nonreactive (Mayo also prints Negative for the reflex titer). CDC (2024) puts the sensitivity of RPR in primary syphilis at 48.7-76.1% against darkfield microscopy in most studies (one study reported 92.7%), notes that the RPR-first algorithm might be less sensitive in early or late latent syphilis, and estimates biologic false positives at 0.2-0.8% of the population. A fourfold change in titer (two dilutions, for example 1:16 to 1:4) between results from the same test is considered clinically significant, and RPR and VDRL quantitative results cannot be compared. After treatment, titers usually fall at least fourfold within 12 months; in one trial of early syphilis, 14% had a smaller decline at 12 months, and some people stay serofast. Prozone-related false negatives were under 0.85% in two large studies, and diluting the serum removes the prozone. In the traditional algorithm a reactive RPR is followed by a treponemal test; in the reverse algorithm a positive treponemal screen is followed by a quantitative RPR. Treponemal antibodies usually persist for life, so RPR is the test used for follow-up. Titers are reported in doubling dilutions, commonly from 1:1 up to 1:256 or higher.

LOINC 20507-0 (primary), 31147-2. Specimen: serum.

Sources

  1. CDC Laboratory Recommendations for Syphilis Testing, United States, 2024, MMWR Recommendations and Reports (CDC) (2024)
  2. Sexually Transmitted Infections Treatment Guidelines, 2021: Syphilis, Centers for Disease Control and Prevention (2021)
  3. Rapid Plasma Reagin Screen with Reflex, Serum (test RPRS), Mayo Clinic Laboratories (Extended Catalog)
  4. Rapid Plasma Reagin (RPR) Screen with Reflex to Titer, Serum (test RPRT1), Mayo Clinic Laboratories (Extended Catalog)
  5. Rapid Plasma Reagin Titer, Serum (test RPRT2), Mayo Clinic Laboratories (Extended Catalog)
  6. Syphilis: RPR With Reflex to RPR Titer (test 006072), Labcorp
  7. LOINC codes verified with the NLM Clinical Tables LOINC search, U.S. National Library of Medicine

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

Related markers