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External Quality Assessment of Urinary-Free Cortisol Measurement in the UK against a Gas Chromatography Mass Spectroscopy Reference Method

The performance of methods used for the measurement of urinary-free cortisol (UFC) has been monitored in the UK by an external quality assessment scheme (EQAS). Bias and variation of the bias (VAR) from the all laboratories trimmed mean (ALTM) show that mean bias ranged from −15·5% to + 83·1% and mean VAR from 9·3% to 53·2%. The dominant kit methods had a bias range of −4·2% and + 56·9% and VAR between 11·4% and 18·4%. Recovery of added cortisol ranged from 74·9% to 207·2%. The precision of the estimate of urinary-free cortisol was different for different methods and changed with concentration. When methods were compared with a gas chromatography mass spectroscopy (GCMS) target there was a linear correlation between per cent bias from the ALTM and from the GCMS result with the ALTM averaging 63·1% greater than the GCMS value.