The rapid plasma reagin (RPR) test looks for specific antibodies that will be present in the blood if a person's immune system is currently fighting off the syphilis infection.
If the test identifies an active syphilis infection, a second test will be required to confirm the diagnosis. Once confirmed, a doctor can then start treatment and help prevent further complications or the disease spreading to others.
When is the RPR test used?
The RPR test requires a blood sample.
A doctor will likely order an RPR test if a person presents signs and symptoms of syphilis, such as a rash or sores, especially if they have recently had a new sexual partner.
A doctor may use an RPR test:
- To check for an active syphilis infection.
- To check the progress of a person's condition after they have received treatment. If the treatment is effective, the RPR test will show a decreased number of antibodies in the blood.
- In routine checkups of sexual health in people who are sexually active.
- During pregnancy to make sure there are no active infections, as syphilis can affect a growing fetus.
The RPR test follows the same procedure as any other simple blood test.
The procedure may take place in a lab or a doctor's office. A person does not typically need to prepare in advance for an RPR test and will not need to fast beforehand. But, always follow the doctor's advice on what to do before, during, and after a medical test.
A doctor or technician will tie a rubber tube around the person's upper arm. This will help the blood pool in the vein, making it easier for the technician or doctor to find the vein.
Once they find a suitable vein, they will swab the area with antiseptic and use a small needle to collect a small sample of blood. The person may feel a slight pinch.
When the technician removes the needle, they will hold some pressure on the site for a few seconds then apply an adhesive bandage to the arm.
They then send the blood sample to a lab for testing. Ask the doctor about how long the results will take, as the time frames vary between labs.
Interpreting RPR results and accuracy
The presence of certain antibodies in the blood can indicate a syphilis infection.
A normal test result typically reveals that the blood does not contain any antibodies that are reactive to syphilis.
However, some factors can contribute to syphilis not appearing on the test, which can make the results less accurate.
In the early stages of a syphilis infection, the body may not have produced antibodies yet, or not enough antibodies to show up on the test. It can take several months before the antibodies build up.
It is also possible that the RPR test fails to detect syphilis after someone has recently received treatment because of the reduced number of antibodies in the blood.
It is best to do the RPR test in the middle stages of a syphilis infection — after the body has built up an antibody defense, but before any treatment takes place.
Even if the RPR test gives a negative result, a doctor may repeat the test after a few weeks.
Other infections can influence the results of an RPR test if they cause the body to produce similar antibodies to syphilis.
Some potential infections that could cause a false positive include:
When a person's test results come back as positive, meaning they may have syphilis, their doctor will likely run further tests. These tests, which may include a fluorescent treponemal antibody-absorption (FTA-ABS) test, look specifically for the presence of the antibodies used to fight syphilis.
RPR and pregnancy
Many states require people to take an RPR test during the early stages of pregnancy. Syphilis represents a significant risk to the fetus, so early detection and treatment can help prevent pregnancy loss due to syphilis infection.
In states that do not require the test, a doctor may still recommend that pregnant people take the test, particularly if the doctor suspects that the individual is at a higher risk for contracting syphilis.
If the test result is positive, the individual will receive treatment and periodic testing to ensure the infection is under control.
The RPR test is only an initial screening for syphilis. Because there are some problems with its interpretation, further testing is often needed to confirm a syphilis diagnosis.
If a person tests positive, they may require an additional test to confirm the presence of syphilis. If a person tests negative, their doctor may request that they repeat the test in a few weeks, especially if they suspect syphilis is present or if a person is considered high risk.
Syphilis is a treatable condition and, with treatment, a person can typically expect to make a full recovery. However, if syphilis is left untreated, people may experience severe complications. It is essential to talk to a doctor about the RPR test when experiencing symptoms of syphilis infection.