20.4 Eias and elisas  (Page 5/10)

Part 3

Although the indirect ELISA for HIV is a sensitive assay, there are several complicating considerations. First, if an infected person is tested too soon after becoming infected, the test can yield false-negative results. The seroconversion window is generally about three weeks, but in some cases, it can be more than two months.

In addition to false negatives, false positives can also occur, usually due to previous infections with other viruses that induce cross-reacting antibodies. The false-positive rate depends on the particular brand of test used, but 0.5% is not unusual. Thomas, Justin G., Victor Jaffe, Judith Shaffer, and Jose Abreu, “HIV Testing: US Recommendations 2014,” Osteopathic Family Physician 6, no. 6 (2014). Because of the possibility of a false positive, all positive tests are followed up with a confirmatory test. This confirmatory test is often an immunoblot (western blot) in which HIV peptides from the patient’s blood are identified using an HIV-specific mAb-enzyme conjugate. A positive western blot would confirm an HIV infection and a negative blot would confirm the absence of HIV despite the positive ELISA.

Unfortunately, western blots for HIV antigens often yield indeterminant results, in which case, they neither confirm nor invalidate the results of the indirect ELISA. In fact, the rate of indeterminants can be 10–49% (which is why, combined with their cost, western blots are not used for screening). Similar to the indirect ELISA, an indeterminant western blot can occur because of cross-reactivity or previous viral infections, vaccinations, or autoimmune diseases.

• Of the 1300 patients being tested, how many false-positive ELISA tests would be expected?
• Of the false positives, how many indeterminant western blots could be expected?
• How would the hospital address any cases in which a patient’s western blot was indeterminant?

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