The technique described is a Western blot, the standard confirmatory test for HIV. It works by detecting host antibodies (from patient serum) that bind to specific viral proteins (e.g., p24, gp41, gp120) that have been separated by gel electrophoresis. Detecting viral RNA or proviral DNA requires nucleic acid amplification tests like PCR, while reverse transcriptase activity assays are primarily used in research.

A Southern blot is used to detect specific sequences within a DNA sample. The key steps are restriction enzyme digestion of DNA, separation by size via gel electrophoresis, transfer to a membrane, and hybridization with a specific DNA probe. This method is classically used to identify the large CGG repeat expansions in the FMR1 gene characteristic of Fragile X syndrome. Northern blot analyzes RNA, Western blot analyzes protein, and ELISA detects antigens or antibodies.

An indirect Enzyme-Linked Immunosorbent Assay (ELISA) for HIV screening works by detecting host-produced antibodies (IgG and IgM) against HIV. The test wells are coated with viral antigens. If the patient's serum contains anti-HIV antibodies, they will bind to these antigens. A secondary, enzyme-linked antibody that binds to human antibodies is then added to produce a detectable signal. Detection of p24 antigen uses a different type of ELISA (sandwich ELISA). HIV RNA is detected by RT-PCR. CD4+ cells are quantified by flow cytometry.

Polymerase chain reaction (PCR) is the diagnostic method of choice for HSV encephalitis. It is highly sensitive and specific, capable of detecting minute amounts of HSV DNA in the cerebrospinal fluid (CSF) within hours. Viral culture is too slow for this emergent condition. Tzanck smear lacks sensitivity and is used for skin lesions. Serology is not useful for diagnosing acute CNS infection as antibody presence does not distinguish between acute and past infection.

The detection of RNA viruses like influenza requires Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR). The key first step is the use of the enzyme reverse transcriptase to create a complementary DNA (cDNA) copy from the viral RNA template. This cDNA is then amplified using standard PCR methodology. Standard PCR, Southern blot, and RFLP all operate on DNA templates and do not involve reverse transcription.

Fluorescence in situ hybridization (FISH) is a molecular cytogenetic technique that uses fluorescent probes to detect specific DNA sequences on chromosomes. It is ideal for identifying submicroscopic deletions or duplications, such as the 22q11.2 deletion in DiGeorge syndrome, which are often too small to be seen on a standard or even high-resolution karyotype.

The described process of culturing cells, arresting them in metaphase, and staining and arranging the chromosomes is known as karyotyping. It provides a visual representation of the entire chromosome complement of an individual and is used to detect large-scale numerical (e.g., trisomy 21) or structural abnormalities.

In sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), the detergent SDS denatures proteins and coats them with a uniform negative charge, eliminating the influence of their native charge. As a result, when an electric current is applied, the proteins migrate through the gel matrix solely based on their size (molecular weight), with smaller proteins moving faster and further.

Isoelectric focusing (IEF) is a technique that separates proteins based on their isoelectric point (pI). Proteins migrate through a pH gradient in an electric field until they reach the pH equal to their pI, at which point they have no net charge and stop moving. This is particularly useful for separating hemoglobin variants that may differ by a single amino acid, altering their overall charge.

Affinity chromatography is a method of separating a specific molecule from a mixture based on a highly specific interaction, such as that between an enzyme and substrate, or an antigen and antibody. In this case, the GST-tagged protein specifically binds to the immobilized glutathione ligand on the column resin, while all other proteins wash through. The purified protein can then be eluted by adding free glutathione.