Appendix B

Comparison of the Revised World Health Organization and CDC Surveillance Case Definitions and Staging Systems for HIV Infection

In 2007, the World Health Organization (WHO) revised the standard human immunodeficiency virus (HIV) infection and acquired immunodeficiency syndrome (AIDS) clinical staging system and the clinical and surveillance case definitions (1). The definitions were revised to 1) provide standardized HIV infection and AIDS surveillance case definitions, 2) simplify clinical staging, 3) coordinate the WHO 2002 three-stage pediatric staging system with the WHO 1990 four-stage adult system, 4) include immunologic criteria and clinical staging in case definitions, and 5) coordinate the clinical staging and surveillance case definitions. This appendix summarizes the revised criteria for WHO case surveillance and compares the 2007 revised WHO definitions with the 2008 revised CDC definitions. Despite differences in WHO and CDC disease classification and staging and because CDC recommends reporting all CD4+ T-lymphocyte counts for persons, the revised WHO case definition still allows comparison of CDC surveillance data from the United States with WHO data from other countries.

Revised WHO Definitions

Surveillance Case Definitions

WHO recommends reporting cases of HIV infection as HIV infection or advanced HIV disease (AHD), including AIDS. All cases of HIV infection, AHD, and AIDS require a confirmed diagnosis of HIV infection based on laboratory testing, using the appropriate national testing algorithm (1). The revised WHO surveillance case definitions include the following: HIV infection (stages 1 and 2), AHD (stage 3), and AIDS (stage 4) (1).

Clinical Staging and Immunologic Criteria

Four clinical stages have been established for persons with confirmed HIV infection. These stages include the full spectrum of HIV infection and coincide with WHO clinical treatment recommendations: 1) no symptoms, 2) mild symptoms, 3) advanced symptoms, and 4) severe symptoms (1). The revised staging systems include presumptive clinical diagnoses that can be made in the absence of laboratory tests and definitive clinical criteria that require confirmatory laboratory tests. The clinical stage provides useful information when HIV infection is first diagnosed, when a person begins receiving care for HIV infection, for tracking patients in treatment programs, and to guide decisions on when to initiate cotrimoxazole prophylaxis and antiretroviral therapy (ART).

Age-specific immunologic criteria for the disease classification are presented. For children aged <5 years, the CD4+ T-lymphocyte percentage of total lymphocytes rather than the absolute CD4+ T-lymphocyte count should be used because the absolute count tends to vary, more than the percentage, per individual child in this age group. Immunologic and clinical criteria should be documented (when available) to describe a case of HIV infection.

Comparison of the WHO and CDC Definitions

Both the WHO and CDC surveillance case definitions for HIV infection now require laboratory confirmation of HIV infection. Differences between the WHO and CDC definitions and staging systems include the following (Table):

1. WHO recommends reporting cases of HIV infection as HIV infection or AHD (including AIDS), whereas CDC recommends reporting cases of HIV infection by stage (i.e., stage 1, stage 2, stage 3, or stage unknown).
2. WHO presents four clinical stages for disease classification to reflect the WHO ART treatment guidelines, whereas CDC presents three, combining WHO stages 2 and 3 into CDC stage 2.
3. Because of increased, although not universal, availability of CD4+ T-lymphocyte testing, WHO recommends using clinical and immunologic criteria for clinical staging. CDC recommends using only immunologic criteria for staging, with the exception of stage 3, for which cases must have a CD4+ T-lymphocyte count of <200 cells/µL or a CD4+ T-lymphocyte percentage of <14 or one of 26 AIDS-defining conditions.

   Despite these differences in disease classification and clinical staging and because CDC recommends reporting all CD4+ T-lymphocyte counts, CDC and WHO stages can still be compared.

   Reference

   1. World Health Organization (WHO). WHO case definitions of HIV for surveillance and revised clinical staging and immunological classification of HIV-related disease in adults and children. Geneva, Switzerland: WHO Press; 2007. Available at http://www.who.int/hiv/pub/guidelines/hivstaging/en/index.html.
   
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   Date last reviewed: 11/20/2008