ELISA
Definition
Enzyme-Linked Immunosorbent Assay. Heterogeneous, plate-based, enzyme label (HRP, alkaline phosphatase), colour or chemiluminescent readout. Four standard formats: direct, indirect, sandwich, competitive.
- Full name
- Enzyme-Linked Immunosorbent Assay
- Format
- Heterogeneous, plate-based
- Common labels
- HRP, alkaline phosphatase
- Standard formats
- Direct, indirect, sandwich, competitive
Common questions
When would a forensic lab choose a sandwich ELISA over a competitive format?+
Sandwich ELISA is preferred when the target analyte is large enough to be bound by two different antibodies at once, giving higher sensitivity and specificity, while competitive ELISA is used for small molecules, such as many drugs, that cannot be sandwiched between two antibodies.
Why is ELISA described as heterogeneous rather than homogeneous?+
Heterogeneous means unbound reagents must be physically washed away from the plate between steps before the signal is read, unlike homogeneous immunoassays where bound and free label can be distinguished without a separation step, making ELISA more labour-intensive but often more sensitive.
Related terms
- Antibody
- An immunoglobulin (typically IgG) raised to bind a specific antigen. Source decides specificity: polyclonal antibodies recognise many epitopes, monoclonal antibodies recognise one.
- Antigen
- A molecule recognised by the adaptive immune system through specific binding to an antibody or T-cell receptor. Defined by immunogenicity (capacity to...
- Competitive vs Non-Competitive Format
- Competitive: labelled antigen and sample antigen compete for a limited number of antibody sites; signal is inversely proportional to analyte. Non-competitive (sandwich):...
- Epitope and Hapten
- Epitope is the small surface patch on an antigen the antibody actually binds. A hapten is a small molecule (most drugs of...
- Homogeneous vs Heterogeneous
- Homogeneous assays (EMIT, FPIA) need no wash step, so they run fast on clinical analysers. Heterogeneous assays (ELISA, RIA) require a wash...
- Label
- The reporter coupled to either the antigen or the antibody so the binding event becomes measurable. Three families dominate: radioisotopes (RIA), enzymes...
- Lateral-Flow Immunoassay
- A point-of-care competitive immunochromatographic format on a nitrocellulose strip. In the absence of drug, colloidal gold-labelled antibody is captured at a test...
- RIA
- Radioimmunoassay. Berson and Yalow, 1959. Heterogeneous, competitive, radioisotope label (most often 125-I). Highest sensitivity historically, now displaced in most Indian labs by...