tions often lead to a more disabling condition than would be predicted from the sum of their individual effects. The committee concludes that measures of functional capacity are critical in assessing whether patients living with HIV/AIDS can participate meaningfully in social and employment activities. However, upon reviewing the literature, the committee found no single test to measure the overall functional capacity or functional limitation of an individual with HIV. Functional limitations in adults are primarily assessed in three domains (i.e., physical, mental, neurocognitive), but are difficult to define or measure in a cost-effective and reproducible manner.
Without strong, valid, and easy-to-conduct functional measures, the committee sought to identify current equivalents to the earlier CDC definition of AIDS in the context of disability. Upon evaluating the medical literature, the committee identified four categories under which claimants should be considered disabled: those with CD4 ≤ 50 cells/mm3; those with imminently fatal or severely disabling HIV-associated conditions; those with HIV-associated conditions without listings elsewhere in the Listing of Impairments; and those with HIV-associated conditions with listings elsewhere in the Listing of Impairments.
The committee tried to identify a laboratory marker that could be used to make decisions about functional impairment and disability, but no direct associations were found. In the absence of such associations, the committee considered measures predictive of disease progression, morbidity, and mortality as surrogate markers of disability.
The CD4+ T-cell (also known as CD4 cells or T-cells) count is a common standard laboratory marker of disease stage for HIV/AIDS patients. The 1993 CDC AIDS definition was expanded to include a CD4 count below 200 cells/mm3 as indicative of an AIDS diagnosis. Varying CD4 levels indicate different levels of disease severity. A CD4 ≤ 50 cells/mm3 has been associated with poorer response to antiretroviral therapy, increased short-term all-cause mortality, and increased incidence of HIV-associated illnesses. Additionally, the majority of early mortalities from those with opportunistic infections occur at CD4 ≤ 50 cells/mm3. Although CD4 count is a continuous variable, CD4 ≤ 50 cells/mm3, as compared to other values, is most indicative of severe advanced immunodeficiency. It is comparable to the previous CDC AIDS definition based on opportunistic infections and cancers in its ability to indicate impairment. Although other clinical markers exist, such as HIV plasma viral load, none predict disease stage as well as CD4 count. The HIV viral load is clinically useful in monitoring the response to antiretroviral therapy and is a good predictor of the rate of CD4 decline, but it is not a direct measure of disease stage.