Emergency rooms, where many people without health insurance go when they're sick, offer a prime opportunity to identify undiagnosed cases of HIV. But a study by the University of Cincinnati found that even ERs with HIV testing programs were missing the chance to test as many as 28 percent of their patients. This hurts the national effort to find the one in five people with HIV who are undiagnosed so they can be connected with care and treatment.

Why the wildly fluctuating stats? For one thing, many ERs are overwhelmed with patients most hours of the day, and frantic staff are often hard-pressed to add another routine task to their regimen. Plus, some ERs don't have a staffer poised at all times to conduct HIV testing.

But ERs still outperform other testing spots. In a New Jersey survey of 70 voluntary rapid HIV test sites, the 18 that were ERs turned up the highest percentage of the HIV cases diagnosed in the state between 2005 and 2008.