Usefulness of measuring renal papillae in Hounsfield units in stone - forming patients

The aim of this work is to study the density of the renal papillae in stone-forming patients and to determine its usefulness.

This study included a total of 79 patients diagnosed with renal stones and on whom a computed tomography without contrast was performed from June 2014 to May 2015. The patients were divided into two groups: Group 1 (single ep¬isode) included 43 patients, and Group 2 (recurrent episodes) included 36 patients. The density of six renal papillae (3 per kidney) was measured, and the means obtained were compared between Groups 1 and 2. Statistical analysis was performed using SPSS 20.0.

The mean papillary density in Group 1 was 32.26 (SD 4.07) HU compared to 42.36 (SD 8.03) HU in Group 2 (P=00001). A ROC curve was constructed, obtaining an optimal cut-off point of 36.8HU [area under the curve, 0.881 (95% CI; 0.804-0.958); P=0001], with a sensitivity of 80% and a specificity of 90%. The relative risk was estimated at 40.3 (95% CI; 10.8-151.1), meaning that a patient with a mean papillary density greater than 36.8HU would have a 40 times greater risk of having recurrent renal stones. The positive predictive value (PPV) was 81% and the negative predictive value (NPV) was 90%.

The measurement of renal papillary density could be useful in predicting recurrent stone-formers. These results need to be confirmed in future studies with a greater number of patients and a longer follow-up.