A generalized graph of the relationship between<unk>  copies (viral load) and CD4 counts over the average course of untreated<unk>  infection; any particular individual's disease course may vary considerably. Many<unk> -positive people are unaware that they are infected with the virus. For example, in 2001 less than 1% of the sexually active urban population in Africa had been tested, and this proportion is even lower in rural populations. Furthermore, in 2001 only 0.5% of pregnant women attending urban health facilities were counselled, tested or receive their test results. Again, this proportion is even lower in rural health facilities. Since donors may therefore be unaware of their infection, donor blood and blood products used in medicine and medical research are routinely screened for<unk> .<unk> -1 testing is initially done using an enzyme-linked immunosorbent assay (ELISA) to detect antibodies to<unk> -1. Specimens with a non-reactive result from the initial ELISA are considered<unk> -negative, unless new exposure to an infected partner or partner of unknown<unk>  status has occurred. Specimens with a reactive ELISA result are retested in duplicate. If the result of either duplicate test is reactive, the specimen is reported as repeatedly reactive and undergoes confirmatory testing with a more specific supplemental test (e.g., a polymerase chain reaction (PCR), western blot or, less commonly, an immunofluorescence assay (IFA)). Only specimens that are repeatedly reactive by ELISA and positive by IFA or PCR or reactive by western blot are considered<unk> -positive and indicative of<unk>  infection. Specimens that are repeatedly ELISA-reactive occasionally provide an indeterminate western blot result, which may be either an incomplete antibody response to<unk>  in an infected person or nonspecific reactions in an uninfected person. Although IFA can be used to confirm infection in these ambiguous cases, this assay is not widely used. In general, a second specimen should be collected more than a month later and retested for persons with indeterminate western blot results. Although much less commonly available, nucleic acid testing (e.g., viral RNA or proviral DNA amplification method) can also help diagnosis in certain situations. In addition, a few tested specimens might provide inconclusive results because of a low quantity specimen. In these situations, a second specimen is collected and tested for<unk>  infection. Modern<unk>  testing is extremely accurate, when the window period is taken into consideration. A single screening test is correct more than 99% of the time. The chance of a false-positive result in a standard two-step testing protocol is estimated to be about 1 in 250,000 in a low risk population. Testing post-exposure is recommended immediately and then at six weeks, three months, and six months. The latest recommendations of the US Centers for Disease Control and Prevention (CDC) show that<unk>  testing must start with an immunoassay combination test for<unk> -1 and<unk> -2 antibodies and p24 antigen. A negative result rules out<unk>  exposure, while a positive one must be followed by an<unk> -1/2 antibody differentiation immunoassay to detect which antibodies are present. This gives rise to four possible scenarios: * 1.<unk> -1 (+) &<unk> -2 (−):<unk> -1 antibodies detected * 2.<unk> -1 (−) &<unk> -2 (+):<unk> -2 antibodies detected * 3.<unk> -1 (+) &<unk> -2 (+): both<unk> -1 and<unk> -2 antibodies detected * 4.<unk> -1 (−) or indeterminate &<unk> -2 (−): Nucleic acid test must be carried out to detect the acute infection of<unk> -1 or its absence.
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