Acute HIV infection – Acute human immunodeficiency virus (HIV) infection is a highly infectious phase of disease that lasts approximately two months and is characterized by nonspecific clinical symptoms. Acute HIV infection contributes disproportionately to HIV transmission because it is associated with a high level of viraemia. HIV infection may not be detected on HIV assays that employ antibody detection only. Persons who are in the phase of acute HIV infection often have flu-like symptoms, and may be more infectious than persons with chronic HIV infection.

Client-initiated HIV testing and counselling – a type of HIV testing and counselling in which persons actively seek HIV testing and counselling, often at a facility that offers these services.

Concentrated HIV epidemic – HIV has spread rapidly in a defined subpopulation, but is not well established in the general population. This type of epidemic suggests active networks of people with high-risk behaviours within the subpopulation. The future course of the epidemic is determined by the nature of the links between subpopulations with a high HIV prevalence and the general population. Numerical proxy: HIV prevalence is consistently over 5% in at least one defined subpopulation but is below 1% in pregnant women attending antenatal clinics.

Discordant test results – when one HIV test result in an individual is reactive and another test result using a different HIV assay in the same individual is non-reactive.

Generalized HIV epidemic – HIV is firmly established in the general population. Although subpopulations at high risk may contribute disproportionately to the spread of HIV, sexual networking in the general population is sufficient to sustain the epidemic. Numerical proxy: HIV prevalence is consistently over 1% in pregnant women attending antenatal clinics.

Key populations at higher risk of HIV exposure – persons likely to be at higher risk of HIV exposure because of behavioural risk factors include people who inject drugs and their sex partners; persons who provide sex in exchange for money, goods, drugs or other benefits (sex workers); sex partners of HIV-infected persons; and men who have sex with men (MSM). Persons at high risk may also include heterosexual persons who have engaged in unprotected sex or who have had sex with someone who has engaged in unprotected sex since their most recent HIV test.

HIV-indeterminate status – the HIV status of an individual in whom the test results can not lead to a definitive diagnosis, meaning that no clear HIV status (either HIV positive or HIV negative) can be assigned.

HIV testing algorithm – Algorithms are defined as the combination and sequence of specific assays used in HIV testing strategies.

Low-level HIV epidemic – HIV may have existed for many years, but has never spread to substantial levels in any subpopulation. Recorded infection is largely confined to individuals with higher-risk behaviour, e.g. sex workers, people who inject drugs and MSM. Numerical proxy: HIV prevalence has not consistently exceeded 5% in any defined subpopulation or 1% in pregnant women attending antenatal clinics.

Non-reactive – refers to an HIV antibody or HIV antigen/antibody test result that does not show a reaction to indicate the presence of HIV antibody and/or antigen.

Partner – a person with whom an individual has had sex at least once or with whom the individual shares injecting equipment.

Provider-initiated HIV testing and counselling – HIV testing and counselling that is recommended by health-care providers to persons attending health-care facilities as a standard component of medical care.

Reactive – refers to an HIV antibody or HIV antigen/antibody test result that shows a reaction to indicate the presence of HIV antibody and/or antigen.

Repeat testing – refers to a situation where additional testing is performed for an individual immediately following a first test during the same testing visit due to inconclusive or discordant test results; the same assays are used and, where possible, the same specimen.

Re-testing – refers to a situation where additional testing is performed for an individual after a defined period of time for explicit reasons, such as a specific incident of possible HIV exposure within the past three months, or ongoing risk of HIV exposure such as sharing injecting equipment. Re-testing is always performed on a new specimen and may or may not use the same assays (tests) as the one at the initial test visit.

Sensitivity – the probability that an HIV test will correctly identify all individuals who are infected with HIV.

Seroconversion – when a sufficient quantity of HIV antibodies are produced by an individual to become detectable on a given HIV antibody and/or antigen assay.

Specificity – the probability that an HIV test will correctly identify all individuals who are not infected with HIV.

Trimester – three-month intervals of a woman's pregnancy. The first trimester is the period of pregnancy from the first day of the last normal menstrual period through the completion of 14 weeks gestation. The second trimester is the period of pregnancy from the beginning of the 15th through the 28th week of gestation. The third trimester is the period of pregnancy from the beginning of the 29th through the 42nd completed week of gestation.

Window period – the period of time from when a person is suspected to have been infected with HIV to when HIV antibodies can be detected by a given assay. The window period varies from person to person, and also depends on the assay used. The mean time from infection to development of antibodies is about one month. Most people (95%) will develop detectable antibodies by 3–4 months.1, 2