Urine calcium and volume predict coverage of renal papilla by Randall's plaque

Background. Renal papillary plaques are common in calcium stone formers. We hypothesized that plaque should increase directly with urine calcium excretion, and inversely with urine volume. To test this, we measured papillary plaque areas in both idiopathic calcium oxalate stone formers and nonstone formers and examined 24-hour urine data to identify significant correlations. Methods. Fourteen stone formers and four nonstone forming controls underwent papillary mapping with flexible nephroscopy. For each papillum, representative still images and moving pictures expert group (MPEG) movies were used to identify plaque extent and papillary borders. The mean fractional plaque coverage for each polar region (upper, inter, lower) and per papillum was calculated. The relationship of the plaque coverage data to urine measurements was assessed with general multivariate linear modeling. Results. Mean polar fractional plaque coverage was higher in the calcium oxalate stone formers (7.4% vs. 0.5%, P=0.012) as was mean fractional plaque per papillum (7.6% vs. 0.6%, P=0.011). When correlating mean polar plaque coverage to urine data, urine volume and calcium excretion were the only measurements with independent relationships to plaque (P=0.002, adjusted multiple R-2=0.521), with higher calcium and lower volume increasing coverage. The same relationships hold for mean plaque per papillum, except that urine pH also becomes an independent factor (P=0.001, adjusted multiple R-2=0.606). Conclusion. Utilizing advanced digital video and endoscopic equipment, we have achieved the most accurate estimation of papillary plaque coverage to date. Our findings support the idea that urine volume and calcium are the main correlates of plaque coverage.