Exchangeable copper for patients with Wilson disease at follow-up: Rethinking normal ranges or changing methodology.

Background And Aim: Determining suitable copper parameters for monitoring Wilson disease remains a topic of ongoing discussion. International recommendations currently rely on the combination of urinary copper excretion and nonspecific liver markers when considering therapy and time elapsed since diagnosis. The emergence of exchangeable copper (CuEX) as a novel measurement reflecting the "free copper pool" held promise as a valuable target to ensure metabolic stability during follow-up, although the validation of target ranges remains unknown. We aimed to evaluate CuEX quantification in repeated samples from 92 real-world patients with Wilson disease during a 2-year period.

Approach: Patients were classified as "stable" if a diagnosis had been made more than 1 year before and were compliant with stable anti-copper drug and dose. Otherwise, patients were classified as "nonstable."

Results: Two hundred and thirteen CuEX samples were obtained per clinical practice. Overall, 57% of CuEX measurements fell below the reference "range of normality," whereas only 34% were within and 9% were above normal levels. There was no association of CuEX levels with therapy, elapsed time from diagnosis, or clinical stability, although most of the samples above normality corresponded to nonstable patients. Only 23.4% of the CuEX samples were aligned with data obtained from concomitant urinary copper excretion.

Conclusions: Our findings suggest that CuEX is a suboptimal tool for assessing copper homeostasis when used alone and should be used with caution if no additional information is available. Normal reference intervals for Wilson disease-treated patients should be redefined, as most CuEX quantifications fell in the lower range, with no sign of overtreatment in these patients.