Urinary Free Cortisol: Interference in the Determination
Introduction: The excretion of urinary free cortisol (UFC) is a sensitive indicator of hypercortisolism. However, the low concentration of UFC compared with its total urinary metabolites and the molecular similarity between them, make a reliable UFC measurement dif cult. A typical example is the increase in the measurement of UFC secondary to an increase in 6β-hydroxycortisol (6β-OHF), an unconjugated metabolite of cortisol. Since 6β-OHF excretion may increase up to 60 more by the administration of drugs whose metabolism is also carried out by p450 cytochrome, exists the possibility of interference with importance for the diagnosis.
This problem has largely been overcome, by submitting the urine sample to a previous extraction with an organic solvent. Nevertheless, some immunoassay methods dispense the extraction step and choose to indicate reference values obtained without extraction.
Aim: (1) To evaluate the in uence of dichloromethane extraction in UFC levels; (2) To evaluate the interference of 6β-OHF in the measurement of UFC using ve immunoassays; (3) To con rm the absence of 6β-OHF in the organic phase after extraction.
Methods: The UFC concentration was determined by immunoassay, before (D) and after (E) dichloromethane extraction, in patient’s urine samples and in one urine sample after addition of known amounts of 6β-OHF. The presence of 6β-OHF was evaluated by gas chromatography analysis. Results: (1) The difference between UFC results before and after extraction showed a variation between -7 and 878%; (2) The UFC levels in samples without exogenous 6β-OHF, showed a variation of 56-90 μg/24 h (E) and 71-671 μg/24 h (D). The range of UFC values after the addition of increasing amounts of 6β-OHF was 53-231 μg/24 h (E) and 71-2842 μg/24 h (D); (3) After extraction, gas chromatography analysis only showed the presence of 6β-OHF in the aqueous phase.
Conclusion: It was demonstrated the existence of variable degrees of interference induced by the presence of 6β-OHF in CLU measurement by immunoassay. Each laboratory should con rm the need to submit the urine samples to a previous extraction with an organic solvent.
This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
The authors, when submitting this paper for publication, transfer to “Revista Portuguesa de Endocrinologia, Diabetes e Metabolismo” the rights and copyrights of the article.