HIV Diagnostic Testing: ELISA and Western Blot

Overview and Mechanisms of the HIV ELISA Screening Test\n\nThe ELISA (enzyme-linked immunosorbent assay), also referred to as an HIV enzyme immunoassay (EIA), was the first screening test widely utilized for HIV because of its high sensitivity. This assay is the most basic test used to determine if an individual is positive for a specific pathogen like HIV. In the ELISA procedure, a person's serum is diluted $400\text{-fold}$ and applied to a plate where partially purified, inactivated HIV antigens have been pre-coated. If antibodies to HIV are present in the serum, they bind to these HIV antigens. Following this, the plate is washed to eliminate all other serum components. A secondary antibody, which is prepared to bind to other antibodies (anti-human immunoglobulin), is then applied to the plate. This secondary antibody is chemically linked in advance to an enzyme. After another washing step, a substrate or chromogen is applied to the plate. Catalysis by the enzyme causes the substrate to change color or fluorescence when cleaved. The result is reported as a numerical value, and the most controversial aspect of the assay is the determination of the cut-off point between a positive and a negative result. Results for this test can be obtained within $1\text{ to } 5\,hrs$. \n\n# Physical Dimensions and Laboratory Setup of the ELISA\n\nThe HIV ELISA is typically performed in a plastic plate measuring $8\,cm \times 12\,cm$. These plates contain a matrix of $96$ wells arranged in an $8 \times 12$ grid. Each individual well measures approximately $1\,cm$ in height and $0.7\,cm$ in diameter. The plate contains HIV antigens pre-attached to the surfaces of these wells. During the test, various controls are used, including a positive control, a negative control, and an assay control to ensure the reagents are functioning correctly. \n\n# Variations in ELISA: The Sandwich ELISA Method\n\nA variation known as the sandwich ELISA follows a specific five-step sequence: (1) the plate is first coated with a capture antibody; (2) the sample is added to the plate, and any antigen present in the sample binds to the capture antibody; (3) a detecting antibody is added, which also binds to the antigen; (4) an enzyme-linked secondary antibody is then added to bind to the detecting antibody; (5) finally, a substrate is added and is converted by the enzyme into a detectable form. While this is a standard variation, some lab notes suggest it is used less frequently in modern screening compared to direct antibody detection methods.\n\n# Interpretation of ELISA Data and Results Thresholds\n\nELISA data is expressed as optical density (OD) measured at a wavelength of $450\,nm$. Specific numerical thresholds define the status of a patient: a cutoff value of $0.500$ or higher indicates a positive result; values between $0.300$ and $0.499$ are considered indeterminate and necessitate retesting; and values below $0.300$ are classified as negative. For example, clinical data with a cutoff of $0.500$ classified Patient A at $0.412$ as indeterminate, Patient B at $1.999$ as positive, and Patient C at $0.123$ as negative. Control values in such a study might include a positive control at $1.689$ and negative controls at $0.153$ and $0.055$. Normally, if a serum gives a positive result, the patient is retested; if the retests are consistently positive, the person is then tested using Western Blotting analysis.\n\n# Factors Affecting ELISA Accuracy: False Positives and Negatives\n\nIt is possible for an individual not infected with HIV to produce a positive ELISA result, known as a false positive. This can occur because individuals, particularly women who have had multiple pregnancies, may possess antibodies directed against human leukocyte antigens (HLA). These HLA are present on the host cells used to propagate HIV for the test; as HIV buds from host cells, it incorporates host HLA into its own viral envelope. Conversely, false negatives can occur during the window period between the initial infection and the point where the body produces a detectable antibody response, a process known as seroconversion. This transition from negative to positive immunity usually takes several weeks.\n\n# The Western Blot as a Confirmatory Analytical Technique\n\nThe Western blot, or protein immunoblot, is a more specific and sensitive analytical technique used as a confirmatory test for HIV following a positive ELISA result. While ELISA measures antibodies to the whole virus, the Western blot allows the visualization of antibodies directed against each individual viral protein. This technique uses gel electrophoresis (specifically SDS-PAGE) to separate native or denatured proteins by polypeptide length (denaturing conditions) or by $3\text{-D}$ structure (native conditions). After separation, the proteins are transferred to a solid membrane, typically nitrocellulose or PVDF, where they are probed using antibodies specific to target proteins. This process generally takes approximately $24\,hrs$. \n\n# Methodological Steps in Western Blotting\n\nWestern blotting involves several key procedural steps: (1) HIV-infected cells are lysed; (2) proteins are subjected to SDS-PAGE and blotted onto a membrane using an electric current so they transfer in the same pattern they formed during electrophoresis; (3) all sites on the membrane not containing blotted protein are non-specifically blocked (often with BSA) to prevent antibodies from binding non-specifically and causing false positives; (4) the membrane is often cut into strips to facilitate multiple tests; (5) a primary antibody (patient serum) is added; (6) a secondary anti-immunoglobulin antibody coupled to a reporter group, such as the enzyme alkaline phosphatase (e.g., goat anti-human IgG-alkaline phosphatase), is added. This is known as the conjugate. (7) Finally, a substrate is added which precipitates upon reacting with the conjugate, resulting in visible bands where the primary antibody bound the protein. \n\n# Identification of Specific HIV Viral Proteins\n\nTo confirm HIV positivity, the Western blot must detect antibodies against specific viral proteins, which are classified by their origin within the virus: gp (glycoprotein, containing sugars) and p (protein). Key diagnostic proteins include: \n- $gp160$: The viral envelope precursor (env).\n- $gp120$: The viral envelope protein (env), which specifically binds to $CD4$ cells.\n- $p24$: The viral core protein (gag).\n- $p31$: Reverse transcriptase (pol).\nIn the laboratory, these proteins are often derived from HIV-infected cells that are lysed and separated to create the positive control and diagnostic test strips.\n\n# CDC Interpretation Criteria for Western Blot Results\n\nIn $1987$, the Centers for Disease Control (CDC) and other organizations established standard criteria for the serologic interpretation of HIV Western blots: \n- Negative: No bands are present on the strip.\n- Positive: Bands must be present at either $p31$ OR $p24$, AND bands must be present at either $gp160$ OR $gp120$.\n- Indeterminate: Bands are present, but the pattern does not meet the specific criteria for positivity. \nAdditionally, some guidelines suggest that for a conclusive HIV-positive diagnosis, a Western blot must display $5$ horizontal stripes. It is noted that an HIV-positive diagnosis is not the same as an AIDS diagnosis.\n\n# Clinical Progression and Sample Preparation Methods\n\nThe progression from infection to a conclusive positive result is tracked during the window period. This period lasts from Day $0$ (initial infection) to approximately Day $30$, at which point the person has a conclusive HIV infection. Although HIV cannot be cured, medical treatment can reduce the viral load to zero, which helps stop transmission and preserve health. Preparation for testing involves cell culture followed by cell lysis using SDS detergent. SDS binds to the proteins, and heat is applied to linearize them before they are loaded onto a gel for size separation in an electric current. High-resolution techniques like NEPHGE 2DE may also be used, followed by blotting to PVDF and blocking with BSA. Advanced detection frequently uses FastRed with alkaline phosphatase (AP) to stain local areas where proteins are identified based on spot position. While the transcript mentions a \"Western Blot Method\" involving RNA separation on an agarose gel and blotting onto a nylon membrane with radiolabeled probes exposed on film, this is technically a description of Northern blotting despite the slide title.