Comparison of laser-probe and photometric determination of the urinary crystallization risk of calcium oxalate.

A variety of equipment is used for the observation of precipitation processes which occur in urinary samples. The Bonn-Risk-Index, a measure of the calcium oxalate crystallization risk of human urine, has been developed with the use of an in-line laser-probe gauge. For basic research or in clinical laboratories, however, this instrument, which fulfills industrial requirements for the evaluation of particle size distributions, is not widely available. The evaluation of an alternative method to determine the Bonn-Risk-Index based on a more commonly available apparatus would therefore be useful. In vitro crystallization experiments with 124 native urine samples from stone-forming and non-stone forming individuals were performed in order to determine their crystallization risk according to the Bonn-Risk-Index approach. The onset of an induced urinary crystallization was detected by simultaneous sample monitoring with an in-line laser-probe and a conventional dip-in photometer. A decrease of the sample's relative light transmissivity from initially 100% to 98% was assumed to be a reliable photometer-based criterion to indicate that crystallization actually began. The laser-probe signal was set as the reference measure. Linear regression analysis of the results of the laser-probe and the photometer-based Bonn-Risk-Index determinations reveals a significant and close correlation between the two measures. Method comparison by statistical evaluation shows i) that no significant deviation from linearity exists and ii) that both methods are statistically identical. The differences in the results are small enough to be confident that the photometer can be used in place of the laser-probe for clinical purposes. The photometer is a reliable, easy-to-use and cost-effective method for the determination of a triggered crystallization event in a urine sample. The assumed 98% criterion allows the determination of the Bonn-Risk-Index with adequate accuracy.