Measurements of serum non-ceruloplasmin copper by a direct fluorescent method specific to Cu(II)

Background: Meta-analyses indicated the breakdown of copper homeostasis in the sporadic form of Alzheimer's disease (AD), comprising copper decreases within the brain and copper increases in the blood and the pool not bound to ceruloplasmin (non-Cp Cu, also known in the literature as "free" copper). The calculated non-Cp Cu (Walshe's) index has many limitations. Methods: A direct fluorescent method for non-Cp Cu detection has been developed and data are presented herein. The study included samples from 147 healthy subjects, 36 stable mild cognitive impairment (MCI) and 89 AD patients, who were tested for non-Cp Cu through the direct method, total serum copper, ceruloplasmin concentration and o-dianisidine ceruloplasmin activity. The indirect non-Cp Cu Walshe's index was also calculated. Results: The direct method was linear (0.9-5.9 mu M), precise (within-laboratory coefficient variation of 9.7% for low and 7.1% for high measurements), and had a good recovery. A reference interval (0-1.9 mu M) was determined parametrically in 147 healthy controls (27-84 years old). The variation of non-Cp Cu was evaluated according to age and sex. Non-Cp Cu was 1.5 times higher in AD patients (regarding the upper value of the reference interval) than in healthy controls. Healthy, MCI and AD subjects were differentiated through the direct non-Cp Cu method [areas under the curve (AUC) = 0.755]. Considering a 95% specificity and a 1.91 mu mol/L cut-off, the sensitivity was 48.3% (confidence interval 95%: 38%-58%). The likelihood ratio (LR) was 9.94 for positive test results (LR +) and 0.54 for negative test result (LR -). Conclusions: The direct fluorescent test reliably and accurately measures non-Cp Cu, thereby determining the probability of having AD. Keywords: Alzheimer's disease; ceruloplasmin; copper; metal; mild cognitive impairment; non-ceruloplasmin copper.