What Is a VDRL test?

Syphilis
Scanning Electron Micrograph of Syphilis Bacteria. CDC/SCIENCE PHOTO LIBRARY/GETTY IMAGES

VDRL is a blood test that is used to determine whether or not you have an active syphilis infection. The test is run on a standard blood sample. 

A positive VDRL test usually means that you have syphilis. However, that's not always the case. VDRL looks for antibodies to a non-syphilis specific protein called cardiolipin. Unfortunately, that means there can be false positive test results. False positives occur in 1 to 2 percent of the population.

They can be caused by many conditions including pregnancy, HIV infection, tuberculosis and certain other bacterial infections.

A VDRL test can be used to detect new syphilis infections. It can also be used to follow the course of syphilis treatment. How does that work? The levels of anti-cardiolipin antibodies in the blood will change depending on how active a syphilis infection is in the body. Therefore, if treatment is working, VDRL test numbers can go down. Unfortunately, false positives aren't the only potential problem with the VDRL test. Over the long-term, latent syphilis infections can lead to false negative tests. That means that people who have had syphilis for a long time without symptoms may test negative, even though they are infected. 

Did You Know? VDRL stands for  "Venereal Disease Research Laboratory."

What are Other Syphilis Tests?

RPR is another non-specific test for syphilis that is used in similar ways to VDRL.

It is also known as the rapid plasma reagin test. Both RPR and VDRL are known as nontreponemal tests. This is because they don't test directly for the syphilis pathogen. 

There are also more specific tests for syphilis. These tests look specifically for antibodies to the organism that causes the disease - Treponema pallidum.

Treponemal tests are somewhat less likely to cause a false positive diagnosis. However, they can remain positive even after an infection has been cured. A positive treponemal test just shows someone has been infected with syphilis at some point in the past. It doesn't show whether or not they are currently infected.

Do I Need Other Tests or Just VDRL?

It is important to know that VDRL is only designed to be used as a test for syphilis. It is most accurate in the middle stages of infection, rather than very early or late. If you are concerned you may have HIV or another STD, you will need to be tested for those diseases separately. The VDRL test can not be relied on to detect any other STD. Different STDs require different tests. 

You may also need follow-up testing after a positive VDRL test. This type of testing is done if there is a reason to believe the VDRL test may be incorrect. For example:

  • if a person's history suggests it is unlikely they have been exposed to syphilis
  • if the person has signs and symptoms more likely to be caused by another STD
  • if there are other reasons to believe a false positive is likely, such as recent immunization 

What is Reverse Screening for Syphilis?

In general, the CDC recommends testing for syphilis using a non-treponemal test first.

This can then be followed up with a treponemal test, if necessary. However, some laboratories like to do reverse screening for syphilis. This involves using the treponemal test first. If that test is reactive, or positive,the result can be checked with an RPR. 

Reverse screening has both advantages and disadvantages. On the positive front, it may be more likely to detect very early and very late syphilis infections. These are the infections that are most often missed by VDRL or RPR testing. On the negative side, reverse screening can lead to many more positive tests. That can be expensive, potentially leading to unnecessary follow-up and treatment.

The risk of unnecessary follow-up is particularly high when there are discordant test results -- i.e. where the treponemal test is positive and the non-treponemal test is negative. There are many reasons why such discordant results can occur, and the appropriate response isn't always clear. With reverse testing, someone who has been successfully treated for syphilis can look the same as someone with an early infection. However, doctors want to handle those two situations in very different ways. 

As with many types of STD testing, syphilis testing is a balancing act. Diagnostic tests aren't perfect. That's why scientists work so hard to figure out the most effective ways to use them. In general, doctors want to detect and treat as many cases as they can,. However, they need to weigh that against the risks of exposing too many people to unnecessary stress or medication. Currently, the evidence suggests that standard screening is the best way to accomplish those goals in most settings. That could change as new tests are developed, and as population risks ebb and flow with time. 

Sources:

Binnicker MJ. Which algorithm should be used to screen for syphilis? Curr Opin Infect Dis. 2012 Feb;25(1):79-85. doi: 10.1097/QCO.0b013e32834e9a3c

Centers for Disease Control and Prevention (CDC).. Discordant results from reverse sequence syphilis screening--five laboratories, United States, 2006-2010. MMWR Morb Mortal Wkly Rep. 2011 Feb 11;60(5):133-7.

Centers for Disease Control and Prevention (CDC).. Syphilis testing algorithms using treponemal tests for initial screening--four laboratories, New York City, 2005-2006. MMWR Morb Mortal Wkly Rep. 2008 Aug 15;57(32):872-5. 

Nayak S, Acharjya B. VDRL test and its interpretation. Indian J Dermatol. 2012 Jan;57(1):3-8. doi: 10.4103/0019-5154.92666.

 

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