Effective January 1993, the Centers for Disease Control and Prevention (CDC), in collaboration with state and territorial health departments, expanded the surveillance definition for the acquired immunodeficiency syndrome (AIDS) among adults and adolescents in the United States [1]. For persons with human immunodeficiency virus (HIV) infection, this expansion adds a measure of immunosuppression (a CD4⁺ T-lymphocyte count < 200 cells/µL or a CD4⁺ percentage < 14) and three clinical conditions: pulmonary tuberculosis, recurrent (two or more episodes within a 12-month period) pneumonia, and invasive cervical cancer. The expanded definition retains the 23 clinical conditions listed in the previous definition. This expansion will more accurately describe HIV-infected persons with advanced immunosuppression and other associated conditions of continuing or increasing public health significance.

Why was it necessary to expand AIDS surveillance criteria? AIDS surveillance is the principal source of information used to monitor and anticipate trends in the HIV epidemic, to describe the affected population, and to guide public health responses. To best serve these purposes, the criteria for AIDS reporting must reflect current knowledge of HIV-associated disease and clinical practice. The AIDS surveillance criteria were revised because the previous definition, in use since 1987, had become outdated.

In the past few years, the understanding of HIV immunopathogenesis has progressed, and the monitoring of CD4⁺ T-lymphocyte levels has become a standard of care for HIV-infected patients. Physicians use immunologic measures to assess the stage of infection and guide therapy. For example, antiretroviral therapy is recommended for those with CD4⁺ T-lymphocyte counts of less than 500 cells/µL [2], and prophylaxis against Pneumocystis carinii pneumonia, the most common life-threatening opportunistic infection in patients with AIDS, is recommended for those with CD4⁺ T-lymphocyte counts of less than 200 cells/µL [3]. Because of advances in care, development of previous AIDS-defining conditions has been prevented or postponed in many patients with severe immunosuppression.

Using a CD4⁺ T-lymphocyte measurement in AIDS surveillance provides a conceptually and operationally simple way of broadening the definition to include more persons with advanced HIV-related immunosuppression as well as many with severe diseases not included in the previous definition. Thus, AIDS surveillance will be more representative of persons at high risk for a broad spectrum of severe morbidity and those who most need regular medical follow-up.

The three added diseases each have substantial current or potential importance in the HIV epidemic. Pulmonary tuberculosis was included because HIV-induced immunosuppression increases the likelihood that latent tuberculosis infection will become active, the HIV epidemic has contributed to the re-emergence of tuberculosis, and tuberculosis is preventable [4]. Recurrent pneumonia was included because, among conditions not included in the 1987 definition, pulmonary infections are the leading cause of serious HIV-related morbidity and death [5]. Laboratory-based confirmation of the infectious cause of pneumonia was not required because in practice such confirmation is often not obtained. Requiring pneumonia to be recurrent enhances the specificity of this indicator. Cervical cancer was added for the following reasons: cervical dysplasia, a precursor lesion that may progress to invasive cancer, is common in HIV-infected women and associated with HIV-induced immunosuppression [6]; cervical cancer is preventable; and its inclusion emphasizes the importance of gynecologic care for HIV-infected women. Although cervical cancer is an uncommon cause of death among HIV-infected women [7], it could emerge as a more frequent problem if HIV-infected women increasingly survive earlier infectious complications yet fail to receive adequate gynecologic care [8]. These three diseases are particularly common among populations showing the largest relative increases in cases of AIDS in recent years: women, persons of minority race or ethnicity, and intravenous drug users and their sex partners [9].

The expansion of the definition will have a substantial effect on the number of reported cases of AIDS, especially in 1993, because many persons will be counted earlier in their course of HIV infection. The largest increase will result from the CD4⁺ T-lymphocyte criteria. The CDC estimates that there are 120 000 to 190 000 HIV-infected persons with such immunosuppression who have not yet developed AIDS-indicator diseases. However, many of these persons may have undiagnosed HIV infection or immunosuppression; thus, the actual effect will be considerably less. Currently, approximately 50 000 cases of AIDS are reported annually, a number that had been expected to increase gradually during the next 2 to 3 years based on previous reporting criteria. Assuming that health departments successfully implement the new criteria, case reports will increase by approximately 75% over previously projected numbers in 1993 and by 10% to 20% in 1994 [10].

What does expanded AIDS surveillance mean for physicians? Physicians play an important role in AIDS case reporting. This role is increasing as more HIV-infected individuals are diagnosed with AIDS-indicator conditions in outpatient settings rather than hospitals, where infection control specialists are often responsible for disease reporting. Including a single measure of immunosuppression will, in many instances, enable physicians to more easily note that a patient meets AIDS reporting criteria. For example, in one study, 85% of HIV-infected persons who developed conditions covered in the 1987 surveillance definition had CD4⁺ T-lymphocyte counts of less than 200 cells/µL [5]. In those areas where health departments receive reports from laboratories of low CD4⁺ T-lymphocyte counts, staff will contact physicians to document whether patients have HIV infection, to exclude the possibility that a CD4⁺ T-lymphocyte count was spurious, and to assist in completing case reports.

What does the 1993 surveillance definition mean for individuals infected with HIV? A change in the surveillance definition for AIDS does not alter the health of HIV-infected persons, nor does it alter their health care needs. Care is optimally initiated as soon as HIV infection is diagnosed to provide recommended immune-system monitoring, screening, treatments, and preventive services. Because HIV infection is characterized by incremental deterioration in immune status, punctuated by a highly variable succession of clinical events, defining AIDS is inherently somewhat arbitrary. For example, there is no clinical or laboratory difference between CD4⁺ T-lymphocyte counts of 200 and 199/µL. Therapeutic decisions should not be based on AIDS reporting criteria; they should be based on the individual patient's clinical and immunologic status and should include an understanding of the variability of CD4⁺ T-lymphocyte determinations. For persons who are concerned because they now "have AIDS," counseling should focus on their clinical and immunologic status rather than on the label of "AIDS". Because some service agencies have included the AIDS definition in their eligibility criteria, access to care may be expanded if those agencies adopt the new definition. Increased access to care is clearly desirable, even though the surveillance definition was not developed to define program eligibility.

AIDS surveillance will continue to serve as an essential means to track the epidemic, to project trends, and to assist in targeting health care and prevention resources. The expanded surveillance definition will more accurately represent persons with severe HIV-related immunosuppression and morbidity and is intended to facilitate reporting by physicians. Physicians have a critical and legally mandated role in AIDS reporting. The effectiveness of AIDS surveillance depends on their diligence in assuming this public health responsibility.