Can the Bonn Risk Index be replaced by a simple measurement of the urinary concentration of free calcium ions?

It has been shown that a strong hyperbolic relationship exists between the urinary concentration of free Ca ions ([Ca]) and the amount of ammonium oxalate (Ox) that must be titrated in a standardized procedure to a urine sample to induce CaOx crystallization. The ratio of [Ca] to (Ox) is termed the Bonn Risk Index (BRI). Most data plot around a hyperbola described by the formula, [Ca] x (Ox) = constant. Due to the high relationship between [Ca] and (Ox) one may argue that determining only 1 of the 2 BRI parameters, preferably [Ca], is sufficient to describe the urine crystallization risk.

Based on 195 urine samples taken from CaOx stone formers and healthy subjects we compared the sensitivity and specificity of BRI, and its corresponding [Ca] value by calculating ROC curves. Furthermore, ROC curves of the established risk indexes, namely the model value of urinary supersaturation and the model value of the urine activity product, are presented.

Our results clearly demonstrate that 1) BRI has the highest sensitivity and specificity of the tested indexes, 2) (Ox) cannot be reliably predicted from [Ca] and 3) determining [Ca] alone revealed a meaningful first estimate of urine CaOx crystallization risk according to BRI.

To avail ourselves of the high quality of BRI in patient treatment the additional determination of (Ox) is required.