Neal Rzepkowski, MD, brings a special expertise to his practice treating HIV-positive people: He’s a longtimer himself, diagnosed in 1985. Thanks to a combo including the injectable drug Fuzeon (enfuvirtide), then a move to a simpler regimen of Truvada (tenofovir plus emtricitabine) and Reyataz (atazanavir), his CD4s have climbed to the 500s—and his virus is undetectable for the first time.

But in late 2005, the Buffalo doctor had a CD4 count of 280, along with some ill-health warning signs: serious skin rashes, diarrhea and fevers. CD4s above 250 might not sound bad, but his lab report showed a CD4 percentage hovering at 11 percent—“way in the ‘AIDS range,’” Rzepkowski, now 55, recalls.

Your CD4 percentage is the proportion of total lymphocytes (white blood cells powering the immune system) that are CD4s (others include CD8s and B cells). A percentage of 21 or more means your immune system is functioning properly, no matter how many CD4s you have in total. A percentage at or below 13 heralds immune system damage, leaving you vulnerable to infections like PCP pneumonia and triggering an AIDS diagnosis—even if your CD4 count exceeds 200. If your percentage is lower than 13, docs recommend taking preventive meds to ward off infections.

CD4 counts can fluctuate meaninglessly from one test to the next—even in the course of a day—so doctors regard CD4 percentage as a more reliable long-term barometer of immune fitness. A study in the February 1 issue of Journal of Infectious Diseases found that it’s especially important to consider CD4 percentage when deciding whether to start HIV meds with CD4s in the “gray area” between 250 and 400 (CDC guidelines currently suggest starting treatment between 200 and 350 CD4s).

The study found that even someone with 350 CD4s but a CD4 percentage of 14 would likely progress to AIDS more quickly than someone with a CD4 count of 200 but a percentage of 28. Says study researcher Todd Hulgan, MD, “two individuals with the same CD4 count should be considered differently if their percentages are different.” He adds, however, that a consistent CD4 count of 200 should be treated as an AIDS diagnosis, regardless of the CD4 percentage.

Hulgan’s study was based on nearly 2,000 folks who’d never before taken a med cocktail that fully suppressed HIV. Therefore, he says, the study’s findings about the practical value of CD4 percentage can’t be applied directly to people who are on meds and undetectable. But five top HIV docs told POZ that CD4 percentage should be considered whenever you’re considering tweaking or dropping a regimen. “If the CD4 percentage is lower than what you’d expect based on the actual CD4 count,” says New York City’s Gal Mayer, MD, “I definitely push a little harder” to change treatment course.

If Rzepkowski had been watching his CD4 percentage, he says, he would have switched his combo sooner: Even when his CD4s were at 371, his percentage had hit 12 (it’s double that now). He says he’s learned that CD4 counts, viewed in isolation, can “give a false sense of security.” Rzepkowski mentions one of his patients with a CD4 count rising to 359 but a percentage stuck at 9. Despite the robust count, Rzepkowski recommended continuing Bactrim (to prevent PCP) until the percentage improves. He’ll consider CD4 percentage in treatment decisions more often, both for himself and his patients, he says, “because I always try to treat them the way I treat myself.” And that’s better than ever.