Investigation of copper status can be a diagnostic challenge. The non-caeruloplasmin-bound copper (NCC) has deficiencies; accordingly, the copper:caeruloplasmin ratio has been suggested as an alternative index of copper status. A reference interval for this index was derived. In addition to making the interpretation of copper easier, the copper:caeruloplasmin ratio should also enable adjustment for relatively high caeruloplasmin concentrations without recourse to producing gender- and age-derived intervals. The copper:caeruloplasmin ratio has weaknesses similar to those identified for NCC in that immunological methods used for caeruloplasmin can cross react with apocaeruloplasmin and there is no standardised method for caeruloplasmin. Caeruloplasmin assays also have uncertainty from precision, bias and specificity and, accordingly, method-related differences may have a large effect on the copper:caeruloplasmin ratio in a manner similar to the NCC.

Objectives: To compare percentage non-caeruloplasmin bound copper (%NCC) to generally accepted values.Design and methods: Using data for 338 normal individual patients, we calculated the % NCC.Results: The %NCC ranged (median) from -59.75% to 51.98% (8.75%) respectively. The non-parametric reference interval was -23.44% to 22.99% with 43.8% of the %NCC values > 10%.Conclusions: We found the %NCC to be higher than generally accepted. This may be due to uncertainty from precision, bias and specificity. (c) 2007 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.