The analysis, published online ahead of print by the Journal of Acquired Immune Deficiency Syndromes, builds upon CDC estimates released in October 2008 indicating that approximately 1.1 million people (1,106,400) were living with HIV in the United States at the end of 2006, and that about one in five of them (21 percent, or 232,700) were undiagnosed.

The problem with undiagnosed HIV among those infected with the virus is twofold. First, many who go undiagnosed often learn of their infection only after they have developed an AIDS-related complication, when the risk of death is substantially higher and HIV infection can be more difficult to treat. Second, research has shown that those who are infected but not diagnosed disproportionately contribute to the annual number of new infections, likely due to their high viral loads and lack of awareness that they are infectious.

According to Campsmith’s group, among those living with HIV, African Americans and Latinos were less likely to be diagnosed compared to their white counterparts (22.2 percent of blacks and 21.6 percent of Hispanics living with HIV were undiagnosed, vs. 18.8 percent of whites). Because of the substantially greater burden of HIV infection among African Americans, the rate of undiagnosed HIV infection among blacks was nine times that among whites and the rate for Hispanics was nearly three times that among whites.

Overall, blacks accounted for nearly half (48.6 percent) of HIV-positive persons unaware of their status—or an estimated 113,100 individuals living with undiagnosed infection. By comparison, whites accounted for 30.9 percent (72,000) and Hispanics accounted for 18.0 percent (41,900) of individuals living with undiagnosed infection. This analysis adds to an already substantial body of data showing the tremendous toll that HIV takes on African American and Latino communities.

Large proportions of HIV-infected American Indians/Alaska Natives (26.1 percent) and Asians/Pacific Islanders (29.8 percent) were also estimated to be undiagnosed, though these groups represent a relatively small proportion of the total population living with HIV in the United States.

By risk group, men infected through heterosexual contact were most likely to be undiagnosed (an estimated 26.7 percent), followed by men who have sex with men (23.5 percent), and women infected through heterosexual contact (21.1 percent). Although the reasons for the high percentage of heterosexual men who are undiagnosed are uncertain, this may be due in part to a low perception of HIV risk among these men or other barriers to accessing HIV testing services.

In terms of the total burden of undiagnosed infection, MSM still account for substantially greater numbers of undiagnosed individuals than any other transmission category, representing more than half (53.7 percent) of HIV-infected persons unaware of their status -- or an estimated 124,900 individuals living with undiagnosed infection. By comparison, heterosexual men accounted for 12.0 percent (27,900) and heterosexual women accounted for 18.3 percent (42,700) of individuals living with undiagnosed infection. These data suggest that expanded HIV testing among MSM remains critical to further reductions in HIV infection in the U.S.

The analysis also indicates that injection drug users (IDUs) were least likely to be undiagnosed (14.5 percent of male IDUs; 13.7 percent of female IDUs). While the reasons for this are unknown, increased knowledge of status among this group may be due in part to frequent interaction with needle exchange, drug treatment, and/or community outreach programs where HIV testing may be offered.

Campsmith’s group found that nearly half of young people (13-24) living with HIV were undiagnosed (47.8 percent) -- a much greater proportion than any other age group. By comparison, the proportion of HIV-infected individuals that were undiagnosed in the older age groups was in the range of 15 to 30 percent, depending on the specific age group. The high percentage of undiagnosed youth is likely due to lack of knowledge and misperceptions about risk in this group, as well as the lower likelihood that younger people would have been infected long enough to exhibit symptoms of the disease that may prompt an HIV test.

Because older individuals are much more likely to be living with HIV, the greatest burden of undiagnosed infection is among the older age groups. Individuals between 35 and 44 years of age account for the greatest number of undiagnosed individuals, representing an estimated 76,100 undiagnosed infections. By comparison, 54,300 individuals between the ages of 45 to 54, and 49,700 individuals between the ages of 25 to 34 were living with undiagnosed HIV infection. Young people between the ages of 13 to 24 accounted for an estimated 23,100 undiagnosed infections.

These findings, Campsmith and his fellow authors conclude, demonstrate the continued importance of HIV testing to ensure that all Americans know their HIV-infection status to protect themselves and their partners from infection. To increase knowledge of serostatus, CDC recommends that all persons likely to be at high risk for HIV be tested for HIV at least annually; further, CDC recommends that HIV screening become a routine part of health care for all individuals between the ages of 13 and 64, regardless of risk.