HIV Diagnostic Testing: ELISA and Western Blot

This set covers vocabulary related to HIV diagnostics, including the mechanisms of ELISA and Western Blot, false results, and specific viral proteins.

ELISA

The Enzyme-linked Immunosorbent Assay, which was the first screening test widely used for HIV due to its high sensitivity.

Secondary Antibody

An antibody chemically linked to an enzyme in advance that binds to other antibodies (anti-human immunoglobulin) during the ELISA and Western Blot processes.

Chromogen

A substrate that changes color or fluorescence when cleaved by the enzyme attached to the secondary antibody.

Cut-off point

The determined threshold value used to differentiate between a positive and a negative result in an ELISA.

False Positive

A result where an individual not infected with HIV tests positive, sometimes caused by antibodies against human leukocyte antigens ($\text{HLA}$) incorporated into the viral envelope.

False Negative

A result where an infected individual tests negative, which can occur during the window between infection and seroconversion.

Seroconversion

The transition period during which an individual's status changes from negative to positive as common antibodies to the virus develop.

Sandwich ELISA

A specific type of ELISA where the plate is first coated with a capture antibody, the sample antigen binds to it, and a detecting antibody followed by an enzyme-linked secondary antibody are added.

Optical Density

The unit in which ELISA data is expressed, typically measured at a wavelength of $450\,nm$ to indicate the concentration of bound antibodies.

Western Blot

Also called a protein immunoblot, this is a confirmatory test used to detect specific antibodies directed against individual viral proteins separated by electrophoresis.

SDS-PAGE

Sodium Dodecyl Sulfate - Polyacrylamide Gel Electrophoresis; a method used to separate native or denatured proteins by the length of the polypeptide or their 3-D structure.

Blocking

A step in Western Blotting where all sites on a solid membrane not containing blotted protein are covered to prevent non-specific antibody binding and false positive results.

Conjugate

Commonly used name for the secondary antibody when it is coupled to a reporter group, such as the enzyme alkaline phosphatase ($\text{AP}$).

gp160

An HIV viral envelope precursor protein ($\text{env}$) that appears as a band on a Western Blot.

gp120

An HIV viral envelope protein ($\text{env}$) that binds to $\text{CD4}$.

p24

An HIV viral core protein ($\text{gag}$) used as one of the criteria for confirming a positive Western Blot.

p31

An HIV reverse transcriptase protein ($\text{pol}$) used as one of the criteria for confirming a positive Western Blot.

Indeterminate (Western Blot)

A Western Blot interpretation for when bands are present, but the pattern does not meet the specific criteria for positivity (such as bands at either $\text{p31}$ OR $\text{p24}$ AND $\text{gp160}$ OR $\text{gp120}$).