Free serum cortisol: quantification applying equilibrium dialysis or ultrafiltration and an automated immunoassay system

Background: Quantification of bioactive, free serum cortisol
concentrations can characterize adrenocortical function more
appropriately compared to total serum cortisol measurement.
Ultrafiltration or equilibrium dialysis of serum samples allow
direct measurement of free serum cortisol concentrations but
respective methods have poorly been validated so far. The aim of
our study was to investigate the analytical performance of free
serum cortisol measurement employing equilibrium dialysis and
ultrafiltration. Methods: Two commercially available
ultrafiltration devices and self-assembled dialysis cells,
respectively, were studied. Cortisol was quantified in filtrate or
dialysate using an automated immunoassay system. Using two serum
pools, the inter-assay coefficient of variation was determined for
the three methods and a method comparison was performed. Results:
Inter-assay coefficients of variation (n=10) between 3.2% and 14.8%
were observed in the imprecision study. Method comparison
demonstrated close agreement between free serum cortisol results
obtained by ultrafiltration and equilibrium dialysis, respectively
(equilibrium dialysis = 1.2x ultrafiltration+3.9 nmol/L; r=0.99;
n=35). Conclusions: Direct quantification of free serum cortisol
after equilibrium dialysis or ultrafiltration of the samples offers
acceptable reproducibility and results in close agreement can be
obtained. Both methods can potentially be introduced into a routine
laboratory setting.