Assay in serum of exchangeable copper and total copper using inductively coupled plasma mass spectrometry (ICP-MS): development, optimisation and evaluation of a routine procedure

Abstract

The assay in serum of non-caeruloplasmin copper, as exchangeable copper after complexation with EDTA (ExCu) and total copper has been evaluated and compared in patients with varying c-reactive protein(CRP).

Measurement of ExCu and total copper, range 0.2–47.2 µmol/L, was developed using ICP-MS. The chelating agents EDTA and TEPA were compared over 0.0–10 g/L after incubation with serum for 60 mins followed by ultrafiltration with Amicon 10 kDa filter. The assay for ExCu was optimised with EDTA 3 g/L (8.1 mmol/L) maintained at pH 7.0–8.0 before ultrafiltration. TEPA was not as selective in chelation of copper. Patients n = 82 were studied in relation to changes in inflammatory marker CRP and a group of patients n = 37 with normal CRP.

The ExCu assay gave excellent recoveries (94–102 % but poor recovery for free uncomplexed copper), good repeatability, limit of quantitation 0.19 µmol/l with a provisional reference range 0.48 to 1.63 µmol/L (n = 37 patients). The range for relative exchangeable copper (exchangeable copper divided by total serum copper) was 2.49 to 9.96 %. ExCu was elevated in conditions with increased CRP greater than 100 mg/L suggesting an effect of inflammation on the free copper fraction.

A reliable and reproducible assay for ExCu and total copper has been developed. The upregulated inflammatory state increases the ExCu suggesting excess free copper.

Keywords:

• Free copper
• non-caeruloplasmin copper
• exchangeable copper
• total copper
• relative exchangeable copper
• ultrafiltration
• ultrafiltrable copper
• caeruloplasmin
• ICP-MS

Acknowledgments

We would like to thank colleagues in the department of Clinical Biochemistry of The Royal Liverpool and Broadgreen Hospitals Trust for encouragement during this study. Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission. Research funding: None declared. Employment or leadership: None declared.Honorarium: None declared.

Disclosure statement

The funding organisation(s) played no role in the study design; in the collection, analysis, and interpretation of data; in the writing of the report; or in the decision to submit the report for publication.