A rapid plasma reagin (RPR) test is a blood test used to screen you for syphilis. It works by detecting the nonspecific antibodies that your body produces while fighting the infection.

Syphilis is a sexually transmitted infection (STI) caused by the spirochete bacterium Treponema pallidum. It can be fatal if left untreated.

Combined with specific antibody testing, the RPR test allows your doctor to confirm the diagnosis of active infection and start your treatment. This reduces the chances of complications and the spread of the disease by an infected but unaware person.

Your doctor may order an RPR test for several reasons. It’s a quick way to screen those at high risk for syphilis. Your doctor may also order this test if you have syphilis-like sores or a rash. Doctors also routinely screen pregnant women for syphilis using an RPR test.

States used to require that people who are applying for a marriage certificate get a screening test for syphilis. The only state that still requires a blood test of any type is Montana, and a syphilis test is no longer included.

The RPR test measures antibodies that are present in the blood of someone who has syphilis, rather than the bacterium that causes the disease. It can also be used to check the progress of treatment for active syphilis.

After a course of effective antibiotic therapy, your doctor would expect to see the number of antibodies drop, and an RPR test could confirm this.

Doctors obtain blood for the RPR test with a simple procedure called a venipuncture. This can be done in your doctor’s office or a lab. You don’t need to fast or take any other special measures before this test. The test involves the following steps:

  1. A healthcare provider will ask you to sit in a comfortable chair or lie down on a cot or a gurney.
  2. They then tie rubber tubing around your upper arm to help make your veins stand out. When they find your vein, they will swab the spot with rubbing alcohol to cleanse it and insert a needle into the vein. The needle may produce a sudden, sharp pain, but it typically doesn’t last long.
  3. Once they have the blood sample, they’ll remove the needle from your vein, hold pressure on the puncture site for a few seconds, and offer you a bandage.

Venipuncture is minimally invasive and carries very few risks. Some people complain of soreness, bleeding, or bruising after the test. You can apply an ice pack to the puncture wound to help relieve these symptoms.

Some people may become light-headed or dizzy during the test. Tell the healthcare provider if your dizziness lasts longer than a few minutes.

A normal RPR blood sample shows no antibodies typically produced during an infection. However, your doctor cannot completely rule out syphilis if they see no antibodies.

Once you’ve been infected, it takes some time for your immune system to create antibodies. Shortly after infection, a test may not yet show any antibodies. This is known as a false negative.

False negatives tend to be more common in the initial and end stages of infection. Among people who are in the secondary (middle) stage of infection, the RPR test result is nearly always positive.

The RPR test also can produce false-positive results, suggesting you have syphilis when you actually don’t. One reason for a false positive is the presence of another disease that produces antibodies similar to the ones produced during a syphilis infection.

A few of the conditions that can cause a false positive include the following:

One small study found that Moderna COVID vaccine may cause some false positive RPR results.

If your result is negative, your doctor may ask you to wait a few weeks and then return for another test if you’re at a higher risk for syphilis. This is because of the RPR test’s potential for a false negative.

Due to the risk of false-positive results, your doctor will confirm the presence of syphilis with a second test, one that is specific for antibodies against the bacterium that causes syphilis, before starting your treatment. One such test is called the fluorescent treponemal antibody-absorption (FTA-ABS) test.

Your doctor will start you on antibiotic treatment, usually penicillin injected into the muscle, if your RPR and FTA-ABS test both show signs of syphilis. New infection usually responds to treatment quickly.

At the end of treatment, your doctor will most likely recommend that you get another RPR test to make sure your antibody levels are dropping.