Use of P24 Antigen Screening For HIV May Be Desirable in Primary Care

WESTPORT, CT (Reuters Health) Jan 02 - For diagnosis of primary HIV infection, the p24 antigen assay deserves consideration for incorporation into primary care practice, although it is not without limitations, according to a report and an editorial in the January 2nd issue of Annals of Internal Medicine.

Dr. Eric S. Daar, of Cedars-Sinai Medical Center in Los Angeles, and a multicenter team prospectively studied 436 patients who had symptoms suggestive of primary HIV infection. Of these, 12.4% had a positive result on a virologic test even though they had undetectable HIV antibodies or an indeterminate Western blot test.

The researchers found that no sign or symptom accurately predicted acute infection. When fever, myalgia and rash were combined, the predictive value increased but to no more than 75%.

Dr. Daar and associates note that while the HIV RNA assay was more sensitive than the p24 antigen assay, at 100% and 88.7%, respectively, it was more expensive, at $100 versus $20 per test. Moreover, the HIV RNA test also had lower specificity, 97.4% versus 100%. A higher false-positive rate implies the need for more follow-up testing and post-test counseling, they point out.

"It could be argued that if resources for assays and pre- and post-test counseling are unlimited, then tests for HIV RNA may be the best screening tool," the authors state. "Alternatively, when testing patients for suspected primary symptomatic HIV infection, clinicians can be confident that more than 90% of those with negative results on assays for HIV antibody or indeterminate Western blots will be identified by the assay for p24 antigen alone."

In an editorial, Dr. Timothy Flanigan and Dr. Karen T. Tashima, of Brown University School of Medicine in Providence, Rhode Island, call the p24 antigen assay preferable to the HIV RNA assay, even though it misses a small percentage of HIV-positive patients.

"We now need to take the next step [beyond routine HIV testing of pregnant women] and incorporate screening for primary HIV infection among all patients who present for evaluation with compatible symptoms," the editorialists write.

Physicians in primary-care settings, including emergency departments, "should consider evaluating primary HIV infection by using the standard serologic tests — enzyme immunoassay and Western blot — and adding a p24 antigen assay," Drs. Flanigan and Tashima conclude.

Ann Intern Med 2001;134:25-29,75-77.