Confirmatory Test - Double Checking Test Results

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

Why Do Doctors Use Confirmatory Tests?

Diagnostic testing isn't perfect. It is basically. impossible to design a diagnostic test that perfectly identifies both those people who have a disease and those people who don't. This is why diagnostic testing regimens often involve multiple tests. In these circumstances, an initial test result is not considered accurate until it has been double-checked by a confirmatory test.

How Do Doctors Choose a Confirmatory Test?

Testing order depends on several factors. The most important tend to be cost and whether false positives or false negatives are more dangerous. In general, when it is important to not cause false positive test results, initial testing is designed to definitively identify people who do not have a disease. The second round of confirmatory testing is then performed to check whether the people who initially tested positive actually are. Avoiding false positives are important for highly stigmatized conditions such as HIV. Many STDs have a lot of associated stigma. Therefore, doctors don't want to say someone has them when they don't. 

Designing a test regimen in this way has several benefits.It allows the confirmatory test. to be run only on a high prevalence population. That improves the positive predictive value of the test. It also potentially allows doctors and scientists to save money without putting anyone at risk.

(Confirmatory tests are often more expensive than the initial test.)

Also Known As: second round test, follow-up test

Syphilis Testing As An Example of Confirmatory STD Testing. 

Testing for syphilis is one area where using a confirmatory test is important. Doctors are supposed to initially test using a VDRL or other non-treponemal test.

They then use treponemal tests for follow-up. The first test identifies all people who may have syphilis. The second test then weeds out people who test positive for other reasons.

When financial considerations lead to this testing being done in the reverse order, it's a big problem. Doing that can lead to significant problems with false positives. People can end up diagnosed with and treated for conditions they don't have. That's both expensive and unpleasant. No one likes to experience the side effects of taking antibiotics if they don't have to!

In fact, the issue of reverse order syphilis testing was problematic enough that the Centers for Disease Control released a report about it in 2011. New ELISA tests had made the second round test easier and cheaper than the usual first round tests. That caused people to end up with positive results that they shouldn't have. 

Source:

sub>CDC (2011, February 11) Discordant Results from Reverse Sequence Syphilis Screening --- Five Laboratories, United States, 2006--2010.

MMWR. February 11, 2011 / 60(05);133-137

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