Background This fact sheet contains terms, definitions, and methods of calculation that are commonly applied to HIV surveillance data.
The data include persons with diagnosed HIV infection and those whose infection has been classified as having progressed to stage 3 AIDS , and have been reported to CDC by state and local health departments through a given point in time.
Data for the most current year are considered preliminary as they are based on 6 months reporting delay. Due to delays in reporting, CDC recommends allowing for a month reporting delay before including data in trend analyses. The HIV Surveillance Report marked the transition to presenting diagnosis, death, and prevalence data without statistical adjustments for delays in reporting of cases to CDC. In determining that adjustments for reporting delays were no longer necessary, CDC considered improvements in data quality as a result of the following: CDC continues to statistically adjust transmission category data by using multiple imputation techniques to account for missing transmission category information in cases reported to CDC.
Diagnoses of HIV infection and deaths of persons with diagnosed HIV infection are the number of persons diagnosed with HIV infection and the number of persons with a diagnosed HIV infection who have died in a given time period, respectively. Note that diagnoses of HIV infection are regardless of stage of disease at diagnosis that is, persons diagnosed with HIV infection who have not progressed to stage 3 AIDS ; persons who were diagnosed with HIV infection and classified as stage 3 AIDS at the same time; and persons who were diagnosed with HIV infection that later received a stage 3 classification.
Also note that deaths of persons with a diagnosis of HIV infection may be due to any cause i. To provide the reader with a more accurate understanding of the number of persons diagnosed with HIV infection who have died, CDC includes in its surveillance report data on persons diagnosed with HIV infection regardless of the stage of disease at death, which includes persons with infection that may have been classified as stage 3 at the time of death. Note that deaths of persons with infection ever classified as stage 3 can be due to any cause i.
Uses of these data: Diagnoses of HIV infection including stage 3 classifications , and death data provide trends of the burden of disease and are useful for tracking the time from a diagnosis of HIV infection to a stage 3 classification or death. Disparities between populations in the time from HIV infection diagnoses to stage 3 classifications or time to death underscore inequities in access to testing and care; this knowledge can help direct resource allocation. HIV incidence refers to persons newly infected with HIV, whereas individuals newly diagnosed with HIV may have been infected years before being diagnosed.
Incidence estimates are useful for planning and for allocating of funds, as well as evaluating the impact of prevention programs. Persons living with diagnosed HIV infection or infection ever classified as stage 3 AIDS These terms denote the number of persons in the 50 states and 6 US dependent areas who have received a diagnosis of HIV infection and are still alive, or the number of persons with infection that has been classified as stage 3, and are still alive. The data in the HIV Surveillance Report represent the number of persons living with HIV infection who have been diagnosed, have been reported to the HIV surveillance system, and have not been reported as deceased.
HIV prevalence The number of persons living with HIV disease at a given time regardless of the time of infection, whether the person has received a diagnosis aware of infection , or the stage of HIV disease.
Although prevalence does not indicate how long a person has had a disease, it can be used to estimate the probability that a person selected at random from a population will have the disease. CDC reports prevalence as the number of persons living with HIV infection in a given population at a given time and also reports prevalence rates, calculated per , population.
Prevalence is useful for planning and resource allocation, as it reflects the number of people currently needing care and treatment services for HIV infection. Prevalence rates are useful for comparing HIV disease between populations and for monitoring trends over time. Rate A measure of the frequency of an event compared with the number of persons at risk for the event.
Rates are calculated by dividing the number of events numerator by the size of the population denominator and including a measure of time. When comparing rates between populations, it is typical to standardize the denominator in order to make direct comparisons. This standardization will depend on the magnitude of the local surveillance data—for national data, the population size is most often standardized to , Incidence rate is calculated as the number of new cases during a specified period divided either by the average population usually mid-period or by the cumulative person-time the population was at risk.
Percentage A proportion of the whole, in which the whole is Proportion A portion of a population or a data set, usually expressed as a decimal fraction e. This surveillance case definition revises and combines the surveillance case definitions for human immunodeficiency virus HIV infection into a single case definition for persons of all ages i.
The revisions were made to address multiple issues, the most important of which was the need to adapt to recent changes in diagnostic criteria. Laboratory criteria for defining a confirmed case now accommodate new multitest algorithms, including criteria for differentiating between HIV-1 and HIV-2 infection and for recognizing early HIV infection.
The surveillance case definition is intended primarily for monitoring the HIV infection burden and planning for prevention and care on a population level, not as a basis for clinical decisions for individual patients. A confirmed case can be classified in one of five HIV infection stages 0, 1, 2, 3, or unknown: If there was a negative HIV test within 6 months of the first HIV infection diagnosis, the stage is 0, and remains 0 until 6 months after diagnosis.
Otherwise, if a stagedefining opportunistic illness has been diagnosed, the stage is 3. Otherwise, the stage is determined by the CD4 test immunologic criteria shown in the following table: