Intended for healthcare professionals

Rapid response to:

Practice Easily Missed?

Cushing’s syndrome

BMJ 2013; 346 doi: (Published 27 March 2013) Cite this as: BMJ 2013;346:f945

Rapid Response:

Re: Cushing’s syndrome

In this article, Prague et al advise us to interpret salivary cortisol results according to "normal reference range for the assay used". Some explanations about the limits of salivary cortisol assay have to be considered by clinicians :

(i) salivary cortisol is measured in clinical laboratories by immunoassays mainly. although a better method improves the measurement (liquid chromatography combined to mass spectrometry)

(ii) immunoassays lack of analytical sensitivity, and in this case the main analytical parameter to be accounted is limit of quantification (the lower concentration of an analyte measured with accuracy) and not limit of detection (the lower concentration significantly different from zero)

(iii) immunoassays lack of analytical specificity (ability of antibodies to measure the analyte alone, rather the others) because steroids are close to each other in terms of the structure.

Low diagnostic sensitivity and specificity because of a lack of analytical sensitivity or specificity, can occur, leading to false negative or false positive cases in Cushing's syndrome. Analytical parameters for each diagnostic test are very important to be kept in mind by clinicians in the interpretation of results, especially when laboratory tests are pillars of positive diagnosis.

Competing interests: No competing interests

16 April 2013
Julien Wils
Chemical Pathologist
University Hospital Rouen
1 rue de Germont