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Beverley E. Pearson Murphy, Commercial Radioimmunoassays Do Not Measure Urinary Free Cortisol Accurately and Should Not Be Used for Physiological Studies, The Journal of Clinical Endocrinology & Metabolism, Volume 88, Issue 8, 1 August 2003, Pages 4001–4004, https://doi.org/10.1210/jc.2003-030715
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To the editor:
In their recent article, Legro et al. (1) claim to have studied urinary free cortisol (UFC) in adolescent females using “established RIA methods that employ methanol extraction before assay.” They provide data for reproducibility (8%) and sensitivity (5 μg/24 h) and minimal data for cross-reactivity. No references are provided. It is not clear what the methanol extraction step consisted of or was meant to accomplish.
The assays used are presumably commercial RIAs, none of which has been shown to measure UFC accurately. Such assays are validated only for serum or plasma cortisol and give values that are much too high for UFC, as shown more than 20 yr ago and many times since when compared with data obtained after HPLC or other extensive chromatography (2), or more recently by liquid chromatography-tandem mass spectrometry (3). The values obtained by RIA are usually about three or more times higher than the true values, due to competition by large amounts of metabolites or other forms of interference. Although such RIA methods may have some clinical use in determining excessive cortisol production, they are unsuitable for physiological studies if one wishes to measure cortisol itself.
