Caeruloplasmin oxidase activity: measurement in serum by use of o-dianisidine dihydrochloride on a microplate reader.

Background The enzymatic method of caeruloplasmin measurement is based on copper-dependent oxidase activity. The advantage of the oxidase determination is that it has a much lower detection limit compared with immunoassay-based methods. It has found its application in both the diagnosis of Wilson's disease and also in the monitoring of patients' response to treatment. Methods The method previously described in literature was adapted for use on a 96-well plate. Caeruloplasmin oxidase activity results were derived from the equation: caeruloplasmin oxidase activity = (A-A) × 185 U/L. Results Repeatability (intra-batch) imprecision ranged from 6 to 15% and intermediate (inter-batch) imprecision varied from 7 to 16% for caeruloplasmin oxidative activities of 14, 29, 45 and 99 U/L. Between 3 and 92 U/L, the assay appeared linear with a regression coefficient R= 0.9958. The lower limit of quantification was 4 U/L. Samples were stable over a five-week period at 4℃ and for at least four freeze-thaw cycles. There was a statistically significant difference between the areas under ROC curve for copper-to-caeruloplasmin ratios between caeruloplasmin oxidative activity and immunoassay-based methods ( P < 0.0171). The reference interval for caeruloplasmin activity was determined to be 12-166 U/L. Conclusions Using the oxidative assay provides a cost-effective means of estimating caeruloplasmin concentrations. The method is easily adaptable to a 96-well plate format that facilitates high throughput of samples in a busy laboratory. The enzymatic method is more sensitive and specific for differentiating between Wilson's and non-Wilson's when compared with immunoassay-based methods.