Text Equivalent - HIV Screening and Testing Guide

Figure 1 - HIV Disease Progression

Figure 1 is a line graph comparing the CD4 lymphocyte count in cells per cubic milliliter and the HIV RNA copies per milliliter of plasma during the initial 12 weeks after primary detection and over the years as the disease progresses.

The chart shows that the CD4 lymphocyte count, which is an important marker used to assess immune system function, starts off above 1000 cells per cubic milliliter and then drops by half to 500 cells per cubic milliliter by six weeks into the infection. This is during the acute phase of HIV, when there is wide dissemination and seeding of the lymphoid organs with the virus.

The CD4 levels may rebound to a higher number during the clinical latency stage of the disease and through treatment of the infection. The levels will continue to decline through the years of infection, as the patient begins to show AIDS symptoms and other opportunistic diseases are able to invade the suppressed immune system. The CD4 levels will reach zero in death due to HIV.

The HIV RNA copies per milliliter are a measure of the viral load in the plasma and a marker of active viral replication. It begins at zero at the time of infection. During the acute phase of the HIV infection, this number can reach almost a million cells per milliliter at the time when the CD4 lymphocyte is plunging to its initial lowest level. Around the six month stage, the body responds to the acute infection and viral replication is controlled or reduced during the clinical latency stage. When the symptoms of AIDS emerge, the HIV RNA copies begin to soar to nearly 5 million cells per milliliter of plasma at the time of death.

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Figure 2 - Testing Types and Timing (Branson, 2007)

Figure 2 compares different test types used to identify HIV infection during a specific window of time by detecting the presence of the virus or antibody. Symptom onset may appear within two weeks after infection and taper off through the seventh week. During this time, the p24 antigen test can detect the virus between two and a half and six weeks after infection. HIV RNA viral load can first be detected between the first and second week and remains detectable throughout the infection. Third generation, IgM-sensitive enzyme immuno assay, or EIA, tests can start detecting the HIV antibody during the third and fourth week after HIV infection; whereas, second generation EIA and viral lysate EIA tests do not become positive until later on.

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Figure 3 - Laboratory Testing Algorithm

A typical laboratory testing algorithm starts by screening with an enzyme immune assay (EIA) test. If the EIA is non-reactive, then no HIV infection is present and no further testing is done.

If the initial EIA is reactive, then the EIA test should be repeated two additional times. If neither of the additional EIA tests is reactive, then the test is considered non-reactive, with no evidence of HIV infection.

If at least two of the three EIA results are reactive, then a confirmatory test such as the Western Blot assay is done. If this test is positive, HIV infection is confirmed. If the test is negative, the client is considered to have had a false reactive EIA. Retesting is required if it is likely the client is in the HIV window period. If it is likely the client is outside of the window period, then no HIV infection is present and no further testing is done.

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Figure 4 - Western Blot Assay

Figure 4 is an illustration of an HIV virus cell with corresponding arrows pointing to positive reactions on a Western blot assay strip. The Western blot assay uses individual components of the whole virus embedded on a strip to bind patient antibody. The bound patient antibody is detected with an enzyme-labelled anti-antibody that produces a coloured reaction. Samples with no bands are considered negative. In general, samples with a reaction for both envelope (any of gp41, gp120, gp160) and core proteins (p24) are considered positive; those with one or more bands, but not enough to be considered positive, are classified as "indeterminate". The presence of envelope antibodies alone is sufficient to confer HIV antibody positive status if there is a clinical diagnosis of AIDS. This illustration shows coloured reactions of p18, p24, p33, gp41, p66, gp120 and gp160 with arrows leading from different portions of the virus cell to corresponding reactions on the Western blot strip.

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Figure 5 - Antigen/Antibody Detection Periods

Figure 5 is a detailed diagram showing the days elapsed, from zero to 360, since the start of HIV infection. The diagram is divided into a sliding scale of four time periods:

The timelines associated with the window period have changed with the evolution of more sensitive antibody screening tests. The chart shows HIV RNA (plasma) viral detection begins around day 10, peaks around day 30 and then levels off around day 45. HIV p24 antigen appears around day 17, peaks around day 30 and then declines down to zero around day 48. HIV antibody appears around day 22, peaks around day 40, drops down for a few days and then begins a steady continual climb through day 360.

The seroconversion window runs from zero to approximately 60 days. This is the period of time between the onset of HIV infection and when antibodies are produced and reach detectable levels. Antibody concentrations will continue to rise during this period, known as the acute infection stage, leaving an individual highly infectious during this stage.

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