ELISA and Immunoassays

ELISA and Immunoassays

1. Definition & overview

• Immunoassay = an analytical method that measures the concentration of an analyte (antigen or antibody) by exploiting the highly specific, high-affinity antigen–antibody (Ag–Ab) binding reaction, with the bound complex reported through a measurable signal (enzyme, radioisotope, fluorophore, or electrochemiluminescent label) (ICH M10 §1, §4 — defines the ligand-binding assay class to which ELISA belongs).
• ELISA (Enzyme-Linked ImmunoSorbent Assay) = the subclass of immunoassay in which the reporter is an enzyme covalently conjugated to one of the immunoreactants; the enzyme converts a colourless/low-signal chromogenic (or chemiluminescent/fluorogenic) substrate into a coloured/luminescent product whose intensity is proportional to the amount of bound analyte (Medhi Ch.2 — IL-6 cytokine estimation by the ELISA method, Flow Chart 2.2, p.68).
• ELISA is the dominant member of the ligand-binding assay (LBA) family in bioanalysis — the platform ICH M10 treats in a dedicated chapter (§4) as distinct from chromatographic assays (LC/LC-MS) for measuring large-molecule (biologic) drugs, hormones, cytokines and anti-drug antibodies in biological matrices (ICH M10 §4, p.17).
• Core attributes that make ELISA the workhorse of pharmacology bench-work and clinical drug development: high specificity (driven by the antibody), high sensitivity (enzymatic signal amplification), microvolume sample requirement (µL-scale — Medhi notes that ELISA, like HPLC/PCR/spectrophotometry, needs only microlitre blood volumes vs larger volumes for biochemistry), 96-/384-well microtitre-plate throughput, and a non-radioactive readout (Medhi Ch.1, p.31 — analytical-process sample volume; ICH M10 §4.2, p.18 — "most often microtitre plates are used for LBAs").
• The signal–concentration relationship in an immunoassay is non-linear and sigmoidal (mass-action binding with saturable antibody), so quantification requires fitting a standard (calibration) curve rather than a simple linear regression — a defining contrast with chromatographic assays (ICH M10 §4.2.3, p.19).