ELISA kits and cell-based and biochemical assays
An ELISA kit is a microplate immunoenzymatic assay that quantifies an analyte by comparing the sample signal against a calibration curve built on standards of known concentration. Typical uses include the quantification of biomarkers in serum, plasma, tissue, supernatants and culture media. In the CABRU catalogue the category holds 12,602 references, 12,327 of them in the 96-well format. CABRU supplies the kits made by Cayman Chemical (462), FineTest (11,579), LDN (92), Affinity Biologicals for the haemostasis area, G Biosciences and BioVendor (391).
The assay format follows the analyte
The choice is not a matter of style: it is imposed by the molecule being measured. A sandwich assay needs two antibodies binding distinct epitopes of the same target at the same time, so it works on proteins and analytes large enough to expose at least two. A hapten — a steroid, an eicosanoid, a small molecule — has no room for two antibodies and has to be measured competitively: sample analyte and labelled analyte compete for a limited number of binding sites, and the signal falls as concentration rises. That is the most common misreading of a competitive assay: the curve runs downwards.
| Format | References declaring it | Typical analyte | Signal behaviour |
|---|---|---|---|
| Sandwich | 10,999 | Proteins, cytokines, biomarkers with at least two epitopes | Rises with concentration |
| Competitive | 862 | Haptens and small molecules: steroids, eicosanoids, metabolites | Falls as concentration rises |
| Indirect | not indexed | Antibodies in the sample, against a coated antigen | Rises with titre |
| Direct | not indexed | Coated antigen, detected by an already conjugated primary | Rises with concentration |
What decides whether a kit is usable
Three constraints come before any comparison. Species: a kit calibrated on the human analyte does not transfer to mouse without dedicated validation. Matrix: serum, plasma, urine, culture supernatant and cell lysate carry different matrix effects, and the datasheet states which ones were tested. Anticoagulant: EDTA, citrate and heparin are not interchangeable, because they can interfere with binding or with the enzymatic reaction. The declared measuring range then has to be compared with the concentration expected in the sample — if it falls outside the curve a dilution is needed, and the dilution must be made in the kit diluent, not in a generic buffer.
Formats, readout and storage
The prevailing format is the 96-well plate; there are variants with breakable strips, which allow only the wells needed to be used while the rest is kept, and whole-plate variants. The most common colorimetric detection uses HRP with TMB substrate and reading at 450 nm; fluorometric versions gain dynamic range for the same volume. Curves are not interpolated linearly: the correct fit for an immunoassay is a four- or five-parameter logistic function. Kits and components are kept at the temperature stated on the datasheet; reconstituted standards and conjugates are aliquoted, so that freeze-thaw cycles are not repeated on the same vial. For eicosanoid assays there is a deeper treatment in the eicosanoids guide; for coagulation-area kits, the Haemostasis and coagulation page.
Finding the right kit
Identifying a reference takes analyte, species and matrix; the rest is filtered in the product catalogue, by company or by application. If the analyte is not listed, CABRU checks availability among the manufacturers it represents.