ELISA
Summary
ELISA (Enzyme-Linked Immunosorbent Assay) is a laboratory technique used to detect and quantify antigens or antibodies in a sample using enzyme-linked reactions producing a measurable color change. It is widely used for infectious disease screening (e.g., HIV), autoimmune workups, and hormone assays due to its high sensitivity.
Detail
ELISA works by immobilizing a specific antigen or antibody on a plate, then adding the sample and an enzyme-conjugated secondary antibody. A substrate is added that reacts with the enzyme to produce a color change proportional to the amount of target analyte present, which is measured spectrophotometrically. There are several formats: direct, indirect, sandwich, and competitive ELISA, each suited to different detection needs.
Clinically, ELISA is classically used as a screening test for HIV due to its high sensitivity (few false negatives), though it has a notable false-positive rate; positive results are traditionally confirmed with Western blot or, more recently, HIV RNA PCR or a fourth-generation antigen/antibody combination assay. ELISA is also used to detect antibodies in autoimmune diseases (e.g., anti-CCP in rheumatoid arthritis, anti-dsDNA in lupus), to measure hormone levels (e.g., beta-hCG in pregnancy tests), and to diagnose various infections (e.g., Lyme disease screening, Toxoplasma antibodies).
Key testing principle: sensitivity vs specificity trade-off — ELISA is designed as a high-sensitivity screening tool (minimizing false negatives) at the potential cost of specificity, making confirmatory testing essential in positive results for high-stakes diagnoses like HIV. Understanding the math behind sensitivity, specificity, PPV, and NPV in the context of screening (low prevalence) versus confirmatory testing (high pretest probability) is heavily tested on Step 1 and Step 2.
Sources
- First Aid for the USMLE Step 1
- Kaplan USMLE Step 1 Lecture Notes: Immunology
- Robbins Basic Pathology
- UpToDate: Diagnostic Testing for HIV Infection
Reviewed by AnkiBoss editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.