A rapid plasma reagin (RPR) test is one of the various tests that screen for a sexually transmitted infection (STI) called syphilis. It is a simple blood test that checks for unique syphilis antibodies. The RPR test can be inconclusive on its own. It’s usually followed by other screenings.
Syphilis is an STI that is typically transmitted through contact with someone’s infected sores during unprotected sex. The syphilis bacterium is called treponema pallidum. It enters your body through your mucous membranes or cuts or abrasions on your skin.
Syphilis has four stages. Each has its own set of symptoms. If untreated, syphilis can become deadly. Early treatment can cure syphilis completely before symptoms worsen.
Symptoms. Your doctor may recommend an RPR test if you begin showing syphilis symptoms after having sex with someone with syphilis. These symptoms include:
- Sores, called chancres, at the infection site
- Swollen lymph nodes
- Muscle aches
Symptoms in the final stages of syphilis include loss of feeling, paralysis, blindness, dementia, and death.
Treatment. Syphilis can be cured in the primary stage with a single dose of long-acting penicillin or other antibiotics if you’re allergic to penicillin. Later stages of syphilis require long-term antibiotic treatment.
An RPR test simply requires a blood sample. Little to no preparation is needed.
If you might have syphilis, don’t have sex with anyone until you receive confirmation you don’t have it. You can unknowingly put others at risk for infection if your results are positive.
Preparation for a blood test. Certain blood tests may require you to fast beforehand or stop taking certain medications. Your doctor will provide instructions on how to prepare for the RPR test. Most tests don’t require any preparation.
What to Expect
Procedure. Blood will be drawn from the back of your hand or inside of the elbow. The site will be cleaned with antiseptic to prevent dirt and bacteria from getting into the puncture.
Your doctor may wrap an elastic band on your upper arm to cause your veins to swell with blood. Then they will insert a needle to draw out the necessary amount of blood. A bandage will be placed on the injection site to stop any bleeding.
Your test results are typically available within 7 to 10 days after the blood is taken. You’ll need to be cautious during that time to minimize potential infections.
Risks. There are few risks when having blood drawn. These include:
- Excessive bleeding at the injection site
- Lightheadedness or fainting
- Hematoma (build-up of blood under the skin)
- Multiple punctures to locate the vein
Negative results. A negative result may mean you don’t have syphilis or have recovered if you’ve had it previously. Depending on the stage of syphilis, the RPR test may produce false-negative results.
Positive results. You may have syphilis if the RPR test results are positive. Further testing is often required to confirm a positive diagnosis and that the test wasn’t a false-positive.
Abnormal results, false-negative. Certain factors may cause you to receive false-negative results on the RPR test:
- You are in the early or late stages of syphilis.
- It has been less than 14 days since the infection.
- It has been longer than 21 days since the infection.
- You drank alcohol within 24 hours of the RPR test.
Additional testing will be required to rule out syphilis as the cause if your results are questionable or the symptoms persist.
Abnormal results, false-positive. The following conditions can make the RPR test appear positive for syphilis:
- Intravenous drug use
- Chronic liver disease
- Recent vaccinations
- Inflammation of the heart lining or valves
- Rickettsial infections (typhus, Rocky Mountain spotted fever)
There are other tests used to screen for syphilis. Several others are used to confirm a diagnosis.
Venereal disease research laboratory (VDRL) test. This screening test checks your blood or spinal fluid for syphilis antibodies. Similar to RPR tests, the VDRL test can be inaccurate.
Rapid immunochromatographic test. This screening test also checks for syphilis antibodies. The blood sample can be taken and analyzed during a regular doctor visit.
Tests to confirm a syphilis infection include:
- Enzyme immunoassay (EIA) test: This test is often paired with an RPR or VDRL test.
- Fluorescent treponemal antibody absorption (FTA-ABS) test: This test checks for antibodies after the first 3 to 4 weeks of infection.
- Treponema pallidum particle agglutination assay (TPPA): This test checks for antibodies and is often paired with an initial screening.
- Darkfield microscopy: This test looks at a syphilis germ in a sample of fluid or tissue under a microscope. It is typically used in the early stages.
- Microhemagglutination assay (MHA-TP): This test is often done after another test gives a positive result.