ELISA, EIA and RIA kits answer one question: how much of a given peptide is present in a sample. Each kit in the assay kits section bundles a matched antibody, a calibrated standard, a detection system and a validated concentration range into a single validated procedure — which is why buying a kit is almost always faster and more reproducible than assembling antibody, tracer and standards yourself. The kit's specifications, not its brand, determine whether it will work on your samples.
ELISA, EIA and RIA: the three formats compared
| Format | Detection | Typical design | Practical notes |
|---|---|---|---|
| ELISA | Enzyme (HRP or AP) with colorimetric substrate | Sandwich (two antibodies) for larger peptides; competitive for short ones | Plate reader only; no licensing; the default choice for most targets |
| EIA | Enzyme, usually competitive format | Competitive: sample peptide competes with labeled peptide for limited antibody | The term is often used interchangeably with competitive ELISA; suits short peptides |
| RIA | I-125 radiolabeled tracer, gamma counter | Always competitive | Highest sensitivity for some targets; requires licence, shielding and waste route; tracer decays |
Glossary definitions: ELISA and RIA. The format choice is driven mostly by peptide size. A sandwich assay needs two antibodies binding simultaneously to non-overlapping epitopes, which a ten-residue peptide cannot accommodate — so short peptides are measured competitively, and competitive assays have an inverse standard curve where high signal means low analyte.
The specifications that decide whether a kit fits
Standard range and sensitivity come first. Compare the kit's range with your expected concentrations before anything else: a plasma kit calibrated in the picogram range will saturate on concentrated cell-culture supernatant, and a wide-range kit may not resolve differences at the low end of a rodent plasma measurement. Sensitivity, usually quoted as the minimum detectable concentration, sets the floor.
Sample type is second. Kits are validated on specific matrices — serum, EDTA or heparin plasma, urine, CSF, tissue extract, cell-culture supernatant — and matrix effects are real: components of serum can interfere with antibody binding in ways buffer standards do not reproduce. Using a validated matrix, or running a spike-recovery and linearity-of-dilution check on an unvalidated one, is the difference between numbers and artefacts.
Species is third, and it is the most frequent cause of unusable data in metabolic work. A human-calibrated insulin or leptin kit systematically misreports rodent samples. Check species reactivity on the kit, not on the analyte.
Analyte definition is fourth and easily missed. Total versus active GLP-1, intact versus C-terminal FGF-23, acylated versus des-acyl ghrelin, BNP versus NT-proBNP — these are different measurements, and the kit will report exactly what its antibody pair recognises. Cross-reactivity tables in the kit insert are the specification here, particularly for families like proglucagon products where sequence overlap is extensive.
Sample collection is part of the assay
More kit failures trace to collection than to the kit. Several peptides degrade within minutes of blood draw unless the sample is protected: active GLP-1 and GIP require a DPP-4 inhibitor; acyl-ghrelin requires acidification and protease inhibition; vasopressin is unstable enough that copeptin is measured instead. Anticoagulant choice matters, because a kit validated on EDTA plasma may behave differently in heparin or serum. Freeze–thaw of samples degrades many analytes, so aliquot samples as well as reagents. The kit insert states the required collection protocol; treating it as optional invalidates the standard curve you paid for.
Running the assay well
Four practices carry most of the reproducibility. Run standards and samples in duplicate and treat a coefficient of variation above roughly 15% between replicates as a pipetting or washing problem, not noise. Bring all reagents to the stated temperature before starting — cold plates and cold substrate produce edge effects and drifting kinetics across a plate. Wash thoroughly and consistently; incomplete washing is the single most common cause of high background in enzyme formats. And fit the standard curve appropriately: competitive assays are sigmoidal and need a four-parameter logistic fit, not a linear regression through the middle points. Where a target's expected concentration is unknown, run a dilution series on a pilot sample before committing the plate.
Storage, shelf life and kit supplies
Kits ship as multi-component sets and their components have different requirements — most are stored at 2–8 °C, with standards, controls or conjugate often supplied lyophilised or frozen. Follow the insert per component rather than storing the box uniformly. Once a standard is reconstituted its stability is limited, usually to days at 2–8 °C or a single freeze in aliquots; a re-frozen standard is the quiet cause of a curve that no longer matches the previous run. Coated plates are sensitive to moisture, so reseal unused strips with desiccant. Kit expiry is set by the least stable component, typically the conjugate. Consumables that are not included — plate sealers, wash buffer, diluent — sit under kit supplies, and bead-based multiplex formats use magnetic beads instead of coated plates. Our ELISA and RIA guide and the explainer on how these kits quantify peptides cover the underlying chemistry.
Where assay kits fit against the other product types
Order a kit when the question is concentration in a sample. Order an antibody when the question is localisation or identity on a blot, and a peptide standard when you need to apply a known quantity, spike a recovery control or absorb an antibody. Labeled peptides supply the tracers used in competitive formats and in custom assay development. Start from the catalog hub or a topic such as diabetes, cardiovascular or Alzheimer's, where kit selection carries most of the analytical weight. All catalog kits are research immunoassays supplied for in-vitro and approved animal research only — none is a clinically validated diagnostic.