A rapid plasma regain (RPR) test is a blood test used to screen patients for syphilis. It works by detecting the antibodies that the body produces to fight the infection.
Syphilis is a sexually transmitted infection that can be fatal if left untreated. The RPR test allows a doctor to confirm diagnosis and begin treatment, reducing the chances of complications and the spread of the disease by an infected but unaware person.
Your doctor may order an RPR for several reasons. It is a quick way to screen those at high risk for sexually transmitted infections (STIs) for syphilis. The doctor may also prescribe this test if a patient has symptoms of syphilis-like sores or a rash.
Doctors also routinely screen pregnant women for syphilis using an RDR test.
A few states still require couples that want to get married to get a syphilis-screening test. These states include Mississippi, Montana, and the District of Columbia (NOLO) .
Finally, since the RPR measures antibodies to the disease rather than the disease itself, it can be used to check the progress of treatment for syphilis. For instance, after a course of antibiotics, the doctor would expect to see the number of antibodies drop and an RPR test could confirm this.
Doctors obtain blood for the RPR with a simple blood test (also called a “venipuncture”). The procedure can be done in a doctor’s office or a lab. You do not need to fast or take any other special measures before this test.
A laboratory technician will have you sit in a comfortable chair or lie down on a cot or a gurney. If you are wearing long sleeves, the technician will ask you to roll one sleeve up or to remove your arm from the sleeve.
The technician then ties a length of rubber tubing around your upper arm to help make the veins stand out. The tubing feels tight and can pinch a little, but it is not really painful. When the technician locates a vein, he or she swabs the spot with rubbing alcohol to cleanse it and inserts a needle into the vein. The needle may produce a sudden, sharp pain, but it typically doesn’t last long.
Once the technician has withdrawn the blood sample, he or she will remove the needle from your vein, hold pressure on the puncture for a few seconds, and offer you a bandage.
Unless your doctor has told you otherwise, you should be able to resume normal activities as soon as the test is over.
Venipuncture is a minimally invasive procedure and carries very few risks. Some people complain of soreness, bleeding, or bruising after the test. An icepack applied to the puncture wound may help relieve these symptoms.
Some people may become light-headed or dizzy during the test. If the dizziness persists, tell the lab technician.
A normal blood sample carries no antibodies to syphilis. A doctor, however, cannot completely rule out a syphilis diagnosis, even if he or she sees no antibodies. Once you have been infected, it takes some time for the body’s immune system to create antibodies to fight the bacteria. Shortly after infection, a test may not show any antibodies. This is known as a “false negative.” False negatives tend to be more common in the initial and in the end stages of infection. Among people who are in the second or middle stage of infection, the RPR’s results are nearly always accurate (NIH, 2011).
The RPR also can produce false positive results—suggesting you have syphilis when you don’t. One reason for a false positive is the presence of another disease that produces antibodies similar to the ones that fight syphilis. A few of the conditions that can account for a false positive include HIV, Lyme disease, malaria, lupus, and certain types of pneumonia, especially those associated with a compromised immune system.
Because of the potential for false negatives, doctors may recommend that a person who shows symptoms of syphilis, but has a negative RPR, wait a few weeks and then return for another test. Due to the risk of false-positive results, doctors confirm the presence of syphilis with a second test, the FTA-ABS, before initiating treatment.
If your RPR and your FTA-ABS both show signs for syphilis, the doctor will start you on a course of antibiotics, usually penicillin. New infections usually respond quickly to treatment. If the syphilis bacteria have been in your body for longer than a year, you may need to take antibiotics for a longer period of time before you show improvement.
At the end of treatment, the doctor will probably administer another RPR to make sure your antibody levels are dropping.