Background:  At the end of 2006, >1 million persons age ≥13 years were estimated to be living with HIV infection—both diagnosed and undiagnosed—in the US. Undiagnosed persons cannot benefit from clinical treatments to prevent immune system damage and opportunistic infections; they may also disproportionately transmit HIV to others as a result of their unrecognized HIV status. This analysis presents estimates of the prevalence of undiagnosed HIV infection in the US at the end of 2006.

Methods:  An extended back-calculation method was used to estimate HIV prevalence, including the percentage undiagnosed, based on reported HIV diagnoses and disease severity (whether the individual received an AIDS diagnosis within the same year as the HIV diagnosis). AIDS data from all states and the District of Columbia and HIV data from 40 states with confidential name-based HIV surveillance were used; data were adjusted for reporting delays, under-reporting, and redistribution of unknown HIV transmission categories.

Results:  Overall 21.0% (232,700 of 1,106,400) of persons estimated to be living with HIV were undiagnosed, 21.6% for males and 19.1% for females. Whites had the lowest percentage undiagnosed (18.8%) compared to Hispanics (21.6%), African Americans (22.2%), American Indians and Alaska Natives (26.0%), and Asians and Pacific Islanders (29.8%) (all comparisons to whites p <0.001). The majority of undiagnosed cases were among men who have sex with men (53.7% overall). African Americans made up 48.6% of undiagnosed persons with HIV, despite comprising only 12% of the US population.

Conclusions:  The number of persons in the United States living with HIV infection presents challenges for planners and providers of medical, preventive, and social services. Differences in the percentage of undiagnosed HIV were evident across race/ethnicity and behavior groups. Effective testing programs, along with early access to treatment and prevention services, are integral for reducing undiagnosed HIV infections. Recent national public health activities have focused on expanding HIV testing in routine clinical settings, and addressing racial disparities in the impact of HIV. Future analyses will help determine the effect of prevention efforts on HIV prevalence in the United States.