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How Do RPR Blood Tests Work?
Updated on June 5, 2023
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At Home Health
How Do RPR Blood Tests Work?

The Rapid Plasma Reagin or RPR blood test is a screening test for a sexually transmitted infection (STI) called syphilis.1 It detects antibodies that react to syphilis bacteria called Treponema pallidum in the blood.

Syphilis is transmitted through contact with infected sores during unprotected sex. The bacteria enter the bloodstream through cuts or abrasions on your skin. Its symptoms depend on the stage of syphilis and can include:

  • Small, painless sores 
  • Skin rash on the palms of your hands or the bottoms of your feet
  • Fever
  • Swollen lymph glands
  • Hair loss
  • Headaches
  • Muscle aches
  • Weight loss
  • Tiredness or fatigue

Why is the RPR Test Important?

The RPR test is an important tool for identifying early-warning signs of syphilis. Early stages of syphilis can be asymptomatic. Early detection enables individuals to seek timely treatment, preventing the infection from progressing rapidly.

Additionally, the RPR test is important in determining how well the treatment for syphilis is progressing.2 

In the body, a syphilis infection can result in increased levels of antibodies. An RPR test can provide valuable information on the condition of the infection within the body.

When left untreated, syphilis can progress to irreversible neurological and cardiovascular complications, such as but not limited to:3,4 

  • Dementia
  • Heart disease
  • Seizures
  • Multi-organ failure

Thankfully, syphilis can easily be cured in the early stages, which is why it’s crucial to take an RPR test if you suspect you have the infection.1

When is an RPR Test Recommended?

An RPR test is recommended if you:4

  • Have unprotected sex, especially if you have several partners
  • Are a man who has sex with men (MSM)
  • Have HIV
  • Have had sex with someone who’s tested positive for syphilis
  • Tested positive for another STI, such as chlamydia, gonorrhea, or herpes
  • Are pregnant

The RPR test may be required at least once per year if you are an adult with HIV/AIDS who engages in sexual activity.1 

In addition, if you have engaged in unprotected sex, had multiple partners for sex, or engaged in sex while under the influence of illegal drugs, it is advised to get screened for syphilis and other STIs every 3 to 6 months. 

Window Of Infection

The window period is the period between exposure to an infection and the correct time a test can detect the infection. This indicates when the best time is to get tested for an infection.

Syphilis has an incubation period of 21 days, and symptoms appear between 10 and 90 days. The window of infection for syphilis is around 3-6 weeks or 90 days after exposure.10

RPR Test for Pregnant Women

Babies and young children can suffer from grave health consequences from syphilis, including severe internal organ damage or even death.5 About 40% of infants born to untreated syphilis carriers are either stillborn or die shortly after birth.9

Fetuses can contract syphilis transplacentally, or what is called congenital syphilis, which transfers the infection through the placenta during pregnancy.4,11 Worse, it can be transferred during the crucial point of development of the fetus, during the first trimester, or at birth. As a result, pregnant women are routinely tested for syphilis, regardless of whether they show symptoms.1,4

How is the RPR Test Performed?

The RPR test is performed by taking a simple blood sample from the patient. The procedure is as follows:5

  • The site of the blood draw is cleaned. Blood is typically drawn from a vein inside the elbow or on the back of the hand, as this is where veins are easily accessible. An elastic band will be tied around the arm to stimulate blood flow.
  • A needle punctures the vein and draws blood. The blood is stored in a vial or tube to be analyzed later for antibodies. The elastic band is removed, and the needle is withdrawn from the vein.
  • After the blood draw, your doctor may apply pressure to the injection site. They may also provide a bandage if additional bleeding occurs.

Once the test is finished, there are no restrictions on the regular activities you can do.5 

Interpreting RPR Test Results

The RPR test results can either be negative (also called nonreactive) or positive (also called reactive). 

A negative test result indicates that you probably do not have syphilis. However, antibodies can take several weeks to develop before being detected. False negatives can occur in both early- and late-stage syphilis.2 

If you suspect you have been exposed to syphilis but tested negative, you may need to undergo another screening test. Prioritize getting tested within the window of infection to decrease your chances of obtaining a false-negative result.10 

A positive test result means you likely have antibodies from a syphilis infection. If you test positive, you need to undergo confirmatory tests that detect antibodies specifically made to fight off syphilis.6 These confirmatory tests include:

  • Treponema pallidum particle agglutination assay (TP-PA)
  • Fluorescent treponemal antibody absorption (FTA-ABS) test
  • Microhemagglutination assay for antibodies to T. pallidum (MHA-TP)
  • T. pallidum hemagglutination assay (TPHA)
  • T. pallidum enzyme immunoassay (TP-EIA)
  • Chemiluminescence immunoassays (CLIA)

The best confirmatory tests are those that detect the actual bacterium instead of the antibodies. However, they are rarely used because they can only be conducted in specialized laboratories. 

Can You Have a Positive RPR Test and Not Have Syphilis?

The RPR test can only detect antibodies associated with syphilis infection, not antibodies specific to syphilis. Occasionally, this results in a false-positive result. Factors that can contribute to a false-positive result include:2

  • Lyme disease
  • Certain types of pneumonia
  • Malaria
  • Pregnancy
  • Systemic lupus erythematosus and some other autoimmune disorders
  • Tuberculosis

What Happens if the RPR Test is Reactive?

A reactive RPR test indicates that antibodies were detected. 

This means you may have a syphilis infection or a previous syphilis infection that was successfully treated.

If your RPR test is reactive, the next step is to take a confirmatory test. This is to determine the concentration of antibodies in your blood sample and whether or not you definitively have syphilis.6

If the confirmatory test confirms you are infected with syphilis, your healthcare provider will discuss treatment options with you. Following treatment, you may need repeat tests to determine if the treatment is working or not.

The only way to prevent syphilis and other STIs is to abstain from sex. However, you can reduce your chance of infection by using a condom or dental dam if you are sexually active.4 Be sure to inquire about your sexual partner’s history to ensure they have been properly screened for STIs. 

Can Syphilis Be Cured?

Syphilis can be cured through a type of antibiotic called penicillin.6 Antibiotic treatment can completely cure most early-stage syphilis infections.

While antibiotic treatment can also treat late-stage syphilis, it cannot undo severe damage, such as internal organ damage that can arise at this stage. This emphasizes the importance of early detection to avoid irreversible complications.

RPR Test vs. VDRL Test

A Venereal Disease Research Laboratory (VDRL) test is another screening and confirmatory test for syphilis.7 The RPR test was developed as a refined version of the original VDRL test. 

Both the RPR test and VDRL test are nontreponemal tests. This means they detect non-specific antibodies of the syphilis bacteria called Treponema pallidum. They are both used as initial screening tests for syphilis.

The RPR test detects the reagin antibody produced in response to syphilis. On the other hand, the VDRL test measures antibodies produced one to two weeks after the initial sore has formed.5

Both the RPR and VDRL tests are valid screening tools for syphilis.9 Both tests can yield false-positive results associated with other medical conditions unrelated to syphilis.7 Regardless of which test you take, both must be followed up by a treponemal test to confirm an active syphilis infection.9

Updated on June 5, 2023
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11 sources cited
Updated on June 5, 2023
  1. Rapid Plasma Reagin”. University of Rochester Medical Center Rochester.
  2. RPR test”. Icahn School of Medicine at Mount Sinai.
  3. Syphilis”. National Library of Medicine. 
  4. Syphilis”. Cleveland Clinic. 
  5. Syphilis Tests”.
  6. Syphilis Tests”. Medline Plus.
  7. VDRL Test and its Interpretation”. National Library of Medicine. 
  8. Laboratory Evaluation”. ScienceDirect.
  9. Syphilis”. Centers for Disease Control and Prevention.
  10. How Soon After Unprotected Sex Should I Get Tested for STDs?”
  11. Congenital syphilis”. Icahn School of Medicine at Mount Sinai.
Dr. Rizza Mira
Dr. Rizza Mira
Medical Reviewer
Dr. Rizza Mira is a medical doctor and a general practitioner who specializes in pediatrics, nutrition, dietetics, and public health.

As a pediatrician, she is dedicated to the general health and well-being of children and expecting parents. She believes that good nutrition, a healthy lifestyle, and prevention of illness are key to ensuring the health of children and their families.

When she’s not in the hospital, Rizza advocates and mobilizes causes like breastfeeding, vaccination drives, and initiatives to prevent illness in the community.
Will Hunter
Will Hunter
Content Contributor
Will is a content writer for KnowYourDNA. He received his B.A. in Psychology from the University of California, Los Angeles. Will has 7 years of experience writing health-related content, with an emphasis on nutrition, alternative medicine, and longevity.
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