Establishing Reference Intervals for Serum and Salivary Cortisol in a Selected Population in Kano, Nigeria

Serum cortisol has long been used in the assessment of disorders of the hypothalamic-pituitary-adrenal axis. The reference interval for cortisol in both serum and saliva depends on the analytical methodology and the population studied; hence, a locally derived population-based reference interval is recommended. To our knowledge, there are no studies on reference interval determination in the study area, raising concerns about the use of reference intervals established in European and North American populations. This work aimed to establish reference intervals for baseline serum and salivary cortisol levels among healthy adults in Kano, Nigeria, using methods available in our laboratory.

A cross-section of 148 apparently healthy reference individuals aged 16 to 67 years were recruited from a local community in Kano, Nigeria, using a systematic sampling technique. Baseline serum cortisol was analyzed using highly sensitive and specific electrochemiluminescence quantitative measurements on an automated immunology analyzer. Salivary cortisol levels were measured using Salimetrics' competitive enzyme-linked immunosorbent assay test kits. Parametric methods with a 95% confidence interval were used to calculate reference intervals.

The reference intervals for cortisol in serum and saliva were 72.0 nmo/L to 554.0 nmol/L and 0.40 nmol/L to 18.0 nmol/L, respectively. There was a weak positive correlation between serum and salivary cortisol values, but this association was not statistically significant.

The development of locally derived adult reference intervals can improve the diagnostic utility of serum and salivary cortisol assessment and strengthen the reliability of adrenal insufficiency diagnoses in our population.