Cortisol Assay Kits

Assay Principle

Cortisol is the primary glucocorticoid produced by the adrenal cortex under hypothalamic-pituitary-adrenal (HPA) axis control. A cortisol assay is available in several detection formats, differing mainly in throughput and specificity requirements. Radioimmunoassay (RIA) and ELISA-based immunoassays use antibody-based competitive binding chemistry with radioactive or colorimetric readout, respectively, and remain in use in some clinical and research reference laboratories. LC-MS/MS-based kits, often using a dilute-and-shoot sample preparation approach, provide the most specific quantification by physically separating cortisol from structurally related steroids such as cortisone and corticosterone, which can cross-react in antibody-based formats.

Protocol highlights and purchasing considerations

Protocol Highlights

RIA-based kits are typically validated for human serum, plasma, and urine, with standard ranges spanning roughly 21 to 2,360 nmol/L and analytical sensitivity around 5 nmol/L, and require only a small final sample volume with a straightforward one-hour incubation. LC-MS/MS dilute-and-shoot kits minimize sample preparation labor relative to traditional extraction-based steroid workflows, though they require access to a mass spectrometry platform, which is a meaningful infrastructure consideration distinguishing this category from most other assay kits in this series. Because cortisol has such a pronounced diurnal rhythm, standardizing collection time is often as important as the analytical method itself for producing interpretable results.

Assay Kit Purchasing Considerations

Method choice is the central decision for a cortisol assay kit: RIA and ELISA formats are generally more accessible for routine research use but carry a modest risk of cross-reactivity with structurally similar steroids, while LC-MS/MS offers superior specificity at the cost of requiring specialized instrumentation and more involved calibration. For studies involving samples with unusual steroid profiles (such as pregnancy, certain endocrine disorders, or exogenous corticosteroid treatment), the added specificity of an LC-MS/MS-based kit is often worth the added complexity. For routine screening or high-throughput applications where absolute specificity is less critical, an immunoassay-based kit's simpler workflow and lower per-sample cost are often the more practical choice. As with any activity assay, it's good practice to review the full protocol before deciding on a kit, since the manual often reveals practical details, such as incubation times, control requirements, and data analysis steps, that aren't captured in the product summary alone.