For adults and adolescents (i.e., persons aged >13 years), the human immunodeficiency virus (HIV) infection classification system and the surveillance case definitions for HIV infection and acquired immunodeficiency syndrome (AIDS) have been revised and combined into a single case definition for HIV infection (1-3). In addition, the HIV infection case definition for children aged 13 years), the case definitions for HIV infection and AIDS have been revised into a single case definition for HIV infection that includes AIDS and incorporates the HIV infection classification system. Laboratory-confirmed evidence of HIV infection is now required to meet the surveillance case definition for HIV infection, including stage 3 HIV infection (AIDS). Diagnostic confirmation of an AIDS-defining condition alone (Appendix A), without laboratory-confirmed evidence of HIV infection, is no longer sufficient to classify an adult or adolescent as HIV# Introduction
Since the beginning of the human immunodeficiency virus (HIV) epidemic, case definitions for HIV infection and acquired immunodeficiency syndrome (AIDS) have undergone several revisions to respond to diagnostic and therapeutic advances and to improve standardization and comparability of surveillance data regarding persons at all stages of HIV disease. HIV testing is now widely available, and diagnostic testing has continued to improve; these changes are reflected in the 2008 revised case definition for HIV infection, which now requires laboratory-confirmed evidence of HIV infection to meet the case definition among adults, adolescents, and children aged 18 months to <13 years. Historically, the case definition for AIDS included adults and adolescents without laboratory-confirmed evidence of HIV infection if other clinical criteria were met. In 1993, the existing case definition for AIDS (1) was expanded to include 1) all HIV-infected persons with a CD4+ T-lymphocyte count of <200 cells/L or a CD4+ T-lymphocyte percentage of total lymphocytes of <14 and 2) three additional clinical conditions (pulmonary tuberculosis, recurrent pneumonia, and invasive cervical cancer), in addition to retaining the 23 clinical conditions in the previous AIDS case definition (2). Despite these changes, the case definition for AIDS continued to include a subset of adults and adolescents without laboratory-confirmed evidence of HIV infection whose illness still met the surveillance case definition for AIDS. Illness in a person who did not have any other known cause of immunodeficiency met the surveillance case definition for AIDS if the illness met any of the following three criteria: 1) no laboratory testing performed or inconclusive laboratory evidence of HIV infection but a definitive diagnosis of a condition included in a subset of AIDS-defining conditions, 2) negative laboratory results for HIV infection but a definitive diagnosis of Pneumocystis jirovecii pneumonia, or 3) negative laboratory results for HIV infection but a definitive diagnosis of a condition included in a subset of AIDS-defining conditions and a CD4+ T-lymphocyte count of <400 cells/L. Because of improvements in diagnostic capabilities and treatment, including increased use of new HIV-testing technologies, CDC collaborated with CSTE to recommend in 2005 an interim change in the AIDS case definition, which required laboratory confirmation of HIV infection. This recommended change required laboratory-confirmed evidence of HIV infection in addition to a CD4+ T-lymphocyte count of <200 cells/L, a CD4+ T-lymphocyte percentage of total lymphocytes of <14, or diagnosis of an AIDS-defining condition (5). This CDC/CSTE interim recommendation has been incorporated into the 2008 HIV infection case definition, which includes AIDS (stage 3).
In 1993, the revised classification system for HIV infection and the expanded AIDS surveillance case definition for adults and adolescents were based on three clinical categories (i.e., A, B, and C) and three ranges of CD4+ T-lymphocyte counts (i.e., >500 cells/L, 200-499 cells/L, and <200 cells/L) or the concordant CD4+ T-lymphocyte percentages (2). Clinical category A comprised asymptomatic acute or primary HIV infection or persistent generalized lymphadenopathy. Clinical category B comprised symptomatic conditions in an HIVinfected adult or adolescent that were not included in clinical categories A or C but were attributed to a cell-mediated immunity defect or for which the clinical course or management was complicated by HIV infection. Clinical category C comprised the 26 AIDS-defining conditions. In the context of treatment and diagnostic improvements since 1993, clinical categories A and B pose particular difficulties because they include many conditions that are not discrete diseases, are not necessarily indicators of immunodeficiency, poorly match current treatment guidelines, and are not integrated into routine surveillance practices. The classification system of the 2008 case definition for HIV infection, which includes AIDS, has been simplified, with less emphasis on clinical conditions by elimination of clinical categories A and B while retaining the 26 AIDS-defining conditions in clinical category C (1,2).
The role of CD4+ T-lymphocyte counts and percentages also has been clarified. The 2008 case definition highlights the central role of the CD4+ T-lymphocyte counts and percentages, which are objective measures of immunosuppression that are routinely used in the care of HIV-infected persons and are available to surveillance programs. The three CD4+ T-lymphocyte count categories have been renamed for HIV infection, increasing in severity from stage 1 through stage 3 (AIDS); an unknown stage also is included. For surveillance purposes, HIV disease progression is classified from less to more severe; once cases are classified into a surveillance severity stage, they cannot be reclassified into a less severe stage.

# Children Aged <18 Months
The 1999 surveillance guidelines recommended four categories of HIV infection for children aged <18 months: definitively HIV infected, presumptively HIV infected, definitively uninfected with HIV, and presumptively uninfected with HIV (3). Because of improved accuracy and the widespread availability of viral detection and antibody tests to diagnose HIV infection, changes have been made in the surveillance case definition of presumptively uninfected with HIV for children aged <18 months at the time of diagnosis (1,3,4). Thus, compared with infants categorized using the previous surveillance case definition, fewer HIV-exposed infants who have a very low probability of infection will be categorized as having indeterminate infections (3). No major revisions have been made to the remaining three categories for children aged <18 months, and no changes have been made to the AIDS surveillance case definition for children in this age group (1,3,4). Because of the greater uncertainty associated with diagnostic testing for HIV in this population (i.e., because maternal antibodies from the HIV-infected mother might exist in the infant after birth, possibly affecting HIV diagnostic testing of the infant that occurs soon after birth), children in this age group whose illness meets clinical criteria for the AIDS case definition but does not meet laboratory criteria for definitive or presumptive HIV infection are still categorized as HIV infected when the mother has laboratory-confirmed HIV infection.

# Aged 18 Months to <13 Years
For children aged 18 months to <13 years, laboratoryconfirmed evidence of HIV infection is now required to meet the surveillance case definition for HIV infection and AIDS. Diagnostic confirmation of an AIDS-defining condition alone, without laboratory-confirmed evidence of HIV infection, is no longer sufficient to classify a child as HIV infected for surveillance purposes (1,3,4). No changes have been made to the 24 AIDS-defining conditions (1,4) or the HIV infection classification system for children aged <13 years (4).

# Surveillance Case Definition for HIV Infection Among Adults and Adolescents
The 2008 HIV infection case definition for adults and adolescents (aged >13 years) replaces the HIV infection and AIDS case definitions and the HIV infection classification system (1)(2)(3)5). The case definition is intended for public health surveillance only and not as a guide for clinical diagnosis. The definition applies to all HIV variants (e.g., HIV-1 or HIV-2) and excludes confirmation of HIV infection through diagnosis of AIDS-defining conditions alone. For surveillance purposes, a reportable case of HIV infection among adults and adolescents aged >13 years is categorized by increasing severity as stage 1, stage 2, or stage 3 (AIDS) or as stage unknown (Table ).

# Criteria for HIV Infection
Laboratory

# Case Classification
A confirmed case meets the laboratory criteria for diagnosis of HIV infection and one of the four HIV infection stages (stage 1, stage 2, stage 3, or stage unknown) (Table ). Although cases with no information on CD4+ T-lymphocyte count or percentage and no information on AIDS-defining conditions can be classified as stage unknown, every effort should be made to report CD4+ T-lymphocyte counts or percentages and the presence of AIDS-defining conditions at the time of diagnosis. Additional CD4+ T-lymphocyte counts or percentages and any identified AIDS-defining conditions can be reported as recommended (6).

# HIV Infection, Stage 1
No AIDS-defining condition and either CD4+ T-lymphocyte - count of >500 cells/L or CD4+ T-lymphocyte percentage of total lymphocytes of >29.

# HIV Infection, Stage 2
No AIDS-defining condition and either CD4+ T-lymphocyte - count of 200-499 cells/L or CD4+ T-lymphocyte percentage of total lymphocytes of 14-28.
- Rapid tests are EIAs that do not have to be repeated but require a confirmatory test if reactive. Most conventional EIAs require a repeatedly reactive EIA that is confirmed by a positive result with a supplemental test for HIV antibody. Standard laboratory testing procedures should always be followed.  For HIV screening, HIV virologic (non-antibody) tests should not be used in lieu of approved HIV antibody screening tests. A negative result (i.e., undetectable or nonreactive) from an HIV virologic test (e.g., viral RNA nucleic acid test) does not rule out the diagnosis of HIV infection.  Qualified medical-care providers might differ by jurisdiction and might include physicians, nurse practitioners, physician assistants, or nurse midwives.  An original or copy of the laboratory report is preferred; however, in the rare instance the laboratory report is not available, a description of the laboratory report results by a physician or qualified medical-care provider documented in the medical record is acceptable for surveillance purposes. Every effort should be made to obtain a copy of the laboratory report for documentation in the medical record.

# HIV Infection, Stage Unknown

# No information available on CD4+ T-lymphocyte count or 
percentage and no information available on AIDS-defining conditions. (Every effort should be made to report CD4+ T-lymphocyte counts or percentages and the presence of AIDS-defining conditions at the time of diagnosis.)

# Discussion
To meet the surveillance case definition for HIV infection among adults and adolescents, laboratory-confirmed evidence of HIV infection is required. The lowest CD4+ T-lymphocyte count (or concordant CD4+ T-lymphocyte percentage of total lymphocytes) or the presence of AIDS-defining conditions is used to determine the stage of infection. If the CD4+ T-lymphocyte count and the CD4+ T-lymphocyte percentage are both available but do not correspond to the same severity stage, select the more severe stage. For surveillance purposes, disease progression is from less to more severe; once cases are classified in a more severe surveillance stage, they cannot be reclassified into a less severe surveillance stage.
A diagnosis of acute HIV infection indicates documented evidence of detectable HIV RNA or DNA or of p24 antigen in plasma or serum in the presence of a documented negative or indeterminate result from an HIV antibody test. These laboratory tests should be conducted on the same specimen or on specimens obtained on the same day. Acute HIV infection occurs approximately during the time from viral acquisition until seroconversion (i.e., the development of measurable levels of HIV-specific antibodies). During this period, early immune responses to the virus produce distinctive characteristics; 40% to 80% of patients develop clinical symptoms of a nonspecific viral illness (e.g., fever, fatigue, or rash) typically lasting 1-2 weeks (7-12). Acute HIV infection often is not detected because the date of HIV acquisition is unknown, no specific clinical signs are present, no single laboratory marker is present, and the diagnostic window is small. High viral loads typically are associated with acute HIV infection, potentially increasing the risk for transmission. CD4+ T-lymphocyte counts have decreased in certain patients with acute HIV infection, especially during the months immediately following viral acquisition (7,11,12). However, the viral load and CD4+ T-lymphocyte count usually stabilize once equilibrium is reached between HIV and the immune response (i.e., the viral set point). The changing CD4+ T-lymphocyte counts associated with acute HIV infection might have implications when using these counts to stage HIV infection for surveillance purposes; for example, persons might experience a particularly low, but temporary, CD4+ T-lymphocyte count and be categorized as having a more severe stage of HIV infection than they actually have after reaching the viral set point.

# Surveillance Case Definition for HIV Infection Among Children Aged <18 Months
The 2008 case definition of HIV infection among children aged <18 months replaces the definition published in 1999 (3) and applies to all variants of HIV (e.g., HIV-1 or HIV-2). The 2008 definition is intended for public health surveillance only and not as a guide for clinical diagnosis.
The 2008 definition takes into account new available testing technologies. Laboratory criteria for children aged <18 months at the time of diagnosis include revisions to one category: presumptively uninfected with HIV. No substantial changes have been made to the remaining three categories (definitively HIV infected, presumptively HIV infected, and definitively uninfected with HIV), and no changes have been made to the conditions listed under the AIDS criteria in the 1987 pediatric surveillance case definition for AIDS for children aged <18 months (1,3,13). Because diagnostic laboratory testing for HIV infection among children aged <18 months might be unreliable, children in this age group with perinatal HIV exposure whose illness meets the AIDS case definition on the basis of clinical criteria are considered presumptively HIV infected when the mother has laboratory-confirmed HIV infection. The definitive or presumptive exclusion of HIV infection for surveillance purposes does not mean that clinical HIV infection can be ruled out. For the purposes of calculating the exact timing of tests (e.g., when a specimen was obtained for laboratory testing) based on the surveillance case definition, 1 month corresponds to 30 days.

# Criteria for Definitive or Presumptive HIV Infection
A child aged <18 months is categorized for surveillance purposes as definitively or presumptively HIV infected if born to an HIV-infected mother and if the laboratory criterion or at least one of the other criteria is met.

# Laboratory Criterion for Definitive HIV Infection
A child aged <18 months is categorized for surveillance purposes as definitively HIV infected if born to an HIV-infected mother and the following laboratory criterion is met.
Positive results on two separate specimens (not includ-- ing cord blood) from one or more of the following HIV virologic (non-antibody) tests: HIV nucleic acid (DNA or RNA) detection -HIV p24 antigen test, including neutralization assay, for a child aged >1 month HIV isolation (viral culture) -

# Laboratory Criterion for Presumptive HIV Infection
A child aged <18 months is categorized for surveillance purposes as presumptively HIV infected if 1) born to an HIVinfected mother, 2) the criterion for definitively HIV infected is not met, and 3) the following laboratory criterion is met.
Positive

# MMWR December 5, 2008
When test results regarding HIV infection status are not - available, documentation of a condition that meets the criteria in the 1987 pediatric surveillance case definition for AIDS (1) (Appendix A).

# Criteria for Uninfected with HIV, Definitive or Presumptive
A child aged <18 months born to an HIV-infected mother is categorized for surveillance purposes as either definitively or presumptively uninfected with HIV if 1) the criteria for definitive or presumptive HIV infection are not met and 2) at least one of the laboratory criteria or other criteria are met.

# Laboratory Criteria for Uninfected with HIV, Definitive
A child aged <18 months born to an HIV-infected mother is categorized for surveillance purposes as definitively uninfected with HIV if 1) the criteria for definitive or presumptive HIV infection are not met and 2) at least one of the laboratory criteria or other criteria are met.

# Laboratory Criteria for Uninfected with HIV, Presumptive
A child aged <18 months born to an HIV-infected mother is categorized for surveillance purposes as presumptively uninfected with HIV if 1) the criteria for definitively uninfected with HIV are not met and 2) at least one of the laboratory criteria are met.
Two

# Criteria for Indeterminate HIV Infection
A child aged <18 months born to an HIV-infected mother is categorized as having perinatal exposure with an indeterminate HIV infection status if the criteria for infected with HIV and uninfected with HIV are not met.

# Discussion
The exclusion of HIV infection (definitive or presumptive) for surveillance purposes does not mean that clinical HIV infection can be ruled out. These categories are used for surveillance classification purposes and should not be used to guide clinical practice. A child with perinatal HIV exposure should continue to be monitored clinically according to nationally accepted treatment and care guidelines (17)(18)(19) to 1) monitor for potential complications of exposure to antiretroviral medications during   Suspected cases of HIV infection among children aged 4 weeks, specimens should be obtained on separate days.
the perinatal period and 2) confirm the absence of HIV infection with repeat clinical and laboratory evaluations.
No changes have been made to the existing classification system for HIV infection among children aged <18 months (4). To classify HIV-infected children in this age group, use the 1994 revised classification system for HIV infection among children aged <13 years (4).

# Surveillance Case Definitions for HIV Infection and AIDS Among Children Aged 18 Months to <13 Years
These 2008 surveillance case definitions of HIV infection and AIDS supersede those published in 1987 (1) and 1999 (3) and apply to all variants of HIV (e.g., HIV-1 or HIV-2). They are intended for public health surveillance only and are not a guide for clinical diagnosis.
The 2008 laboratory criteria for reportable HIV infection among persons aged 18 months to <13 years exclude confirmation of HIV infection through the diagnosis of AIDS-defining conditions alone. Laboratory-confirmed evidence of HIV infection is now required for all reported cases of HIV infection among children aged 18 months to <13 years (20).

# Criteria for HIV Infection
Children aged 18 months to <13 years are categorized as HIV infected for surveillance purposes if at least one of laboratory criteria or the other criterion is met.

# Criteria for AIDS
Children aged 18 months to <13 years are categorized for surveillance purposes as having AIDS if the criteria for HIV infection are met and at least one of the AIDS-defining conditions has been documented (Appendix A).
The 2008 surveillance case definition for AIDS retains the 24 clinical conditions in the AIDS surveillance case definition published in 1987 (1) and revised in 1994 (4) for children aged <13 years (Appendix A). Because the 2008 definition requires that all AIDS diagnoses have laboratory-confirmed evidence of HIV infection, the presence of any AIDS-defining condition listed in Appendix A indicates a surveillance diagnosis of AIDS. Guidance on the diagnosis of these diseases in the context of all nationally notifiable diseases is available at . gov/epo/dphsi/casedef/case_definitions.htm.

# Discussion
To meet the surveillance case definition for HIV infection, laboratory confirmation of HIV infection is now required for children aged 18 months to <13 years. To meet the surveillance case definition for AIDS, in addition to the presence of one or more AIDS-defining conditions, laboratory-confirmed evidence of HIV infection is now required for children aged 18 months to <13 years. These revisions will increase the specificity of the HIV infection and AIDS surveillance case definitions by excluding patients without laboratory-confirmed evidence of HIV infection, reinforcing the public health message that HIV infection is the cause of AIDS. Improved specificity will provide more accurate data regarding number of HIV infection cases, which can be used to refine public health policies and determine appropriate use of HIV resources.
No changes have been made to the existing classification system for HIV infection among children aged 18 months to <13 years (4). To classify HIV-infected children in this age group, refer to the 1994 revised classification system for HIV infection among children aged <13 years (4

# 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 (

# Appendix A AIDS-Defining Conditions

# Members of the CDC Pediatric Surveillance Case Definition for HIV Infection and AIDS Working Group