RPR (rapid plasma reagin) is a screening test for syphilis. It looks for antibodies that are present in the blood of people who have the disease.

The test is similar to the venereal disease research laboratory (VDRL) test.

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell with blood.

A needle is inserted into the vein, and the blood is collected in an airtight vial or a syringe. During the procedure, the band is removed to restore normal blood flow. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a small glass tube (pipette), on a slide, onto a test strip, or into a small container. A bandage may be placed on the puncture site if bleeding continues.

No special preparation is usually needed.

When the needle is inserted to draw blood, you may feel moderate pain, or only a prick or stinging sensation. Afterward, you may feel some throbbing.

The RPR test can be used to diagnose syphilis. It is used to screen people who have symptoms of sexually transmitted diseases and is routinely used to screen pregnant women for the disease.

Several states also require that couples be screened for syphilis before getting a marriage license.

The test is also used to see how treatment for syphilis is working. After treatment with antibiotics, the levels of syphilis antibodies should fall. These levels can be monitored with another RPR test. Unchanged or rising levels can mean a persistent infection.

A negative test result is considered normal. However, the body does not always produce antibodies specifically in response to the syphilis bacteria, so the test is not always accurate. False-negatives may occur in persons with early- and late-stage syphilis. More testing may be needed before ruling out syphilis.

A positive test result may mean that you have syphilis. If the screening test is positive, the next step is to confirm the diagnosis with a more specific test for syphilis, such as FTA-ABS. The FTA-ABS test will help distinguish between syphilis and other infections.

How well the RPR test can detect syphilis depends on the stage of the disease. The test is most sensitive -- almost 100% -- during the middle stages of syphilis. It is less sensitive during the earlier and later stages of the disease.

The following conditions may cause a false positive test:

• HIV
• Lyme disease
• Certain types of pneumonia
• Malaria
• Systemic lupus erythematosus

• Excessive bleeding
• Fainting or feeling light-headed
• Blood accumulating under the skin (hematoma)
• Infection
• Many punctures to find veins

U.S. Preventive Services Task Force. Screening for Syphilis Infection: Recommendation Statement. Ann Fam Med 2004; 2: 362-365.