Exchangeable serum copper: Adult and pediatric reference intervals and in vitro stability in a nordic cohort

Wilson disease (WD) is a rare genetic disorder characterized by copper overload, primarily affecting the liver and brain, and the organ damage is believed to be caused by non-ceruloplasmin-bound copper (NCC). Accurate and early diagnosis is important for prognosis. Recently, a method for the measurement of NCC, exchangeable serum copper (CuEXC), was developed and shown to be a promising marker of WD, especially as the fraction of total copper, relative exchangeable copper (REC). This study aimed to validate the CuEXC extraction method and establish reference intervals for CuEXC and REC, as well as to examine short- and long-term stability of CuEXC in serum samples. The adult reference interval for CuEXC was 0.61-1.62 µmol/L and for REC 3.0-9.7 % based on 120 blood donors. Based on 88 children, the reference intervals for CuEXC was 0.45-1.16 µmol/L. The intervals for REC were 1.8-5.8 % for children <10 years and 2.3-8.5 % for children ≥10 years. Regarding stability, CuEXC increased following a logarithmic scale in uncentrifuged serum and exceeded the permissible difference of 10 % after 4 h. With long-term freezing at -20 °C, CuEXC was stable for 1.7 months. In conclusion, reference intervals for CuEXC and REC were established and confirmed to be substantially lower in children. Accurate reference intervals are important to ensure timely diagnosis of WD. Finally, our findings on stability have important implications and highlight the need for standardization of the pre-analytical handling of CuEXC samples in order to obtain comparable results within and between laboratories both for clinical and research use.