Hounsfield unit attenuation value can differentiate pyonephrosis from hydronephrosis and predict septic complications in patients with obstructive uropathy

Abstract

We aimed to assess the role of computerized tomography attenuation values (Hounsfield unit—HU) for differentiating pyonephrosis from hydronephrosis and for predicting postoperative infectious complications in patients with obstructive uropathy. We analysed data from 122 patients who underwent nephrostomy tube or ureteral catheter placement for obstructive uropathy. A radiologist drew the region of interest for quantitative measurement of the HU values in the hydronephrotic region of the affected kidney. Descriptive statistics and logistic regression models tested the predictive value of HU determination in differentiating pyonephrosis from hydronephrosis and in predicting postoperative sepsis. A HU cut-off value of 6.3 could diagnose the presence of pyonephrosis with 71.6% sensitivity and 71.5% specificity (AUC 0.76; 95%CI: 0.66–0.85). At multivariable logistic regression analysis HU ≥ 6.3 (p ≤ 0.001) was independently associated with pyonephrosis. Patients who developed sepsis had higher HU values (p ≤ 0.001) than those without sepsis. A HU cut-off value of 7.3 could diagnose the presence of sepsis with 76.5% sensitivity and 74.3% specificity (AUC 0.79; 95%CI: 0.71–0.90). At multivariable logistic regression analysis, HU ≥ 7.3 (p ≤ 0.001) was independently associated with sepsis, after accounting for clinical and laboratory parameters. Measuring HU values of the fluid of the dilated collecting system may be useful to differentiate pyonephrosis from hydronephrosis and to predict septic complications in patients with obstructive uropathy.

Pyonephrosis Ultrasound and Computed Tomography Features: A Pictorial Review

“…Contrast layer inversion, representing contrast material overlying purulent fluid, is rarely encountered. Gas in the collecting system, in the absence of previous procedures, is the most accurate indicator of the presence of the infected fluid [ 28 , 29 , 30 , 33 , 34 , 44 ].…”

“…CT signs of pyonephrosis are considered indirect; in fact, in addition to features of obstruction, CT may demonstrate thickening of the renal pelvic wall (>2 mm), parenchymal or perinephric inflammatory changes, dilatation and obstruction of the collecting system, and gas–fluid or fluid–fluid levels in the intrarenal collecting system [ 28 , 29 , 30 , 31 ]. A caveat of CT evaluation is that it is often difficult to distinguish simple hydronephrosis from pyonephrosis by fluid attenuation measurements, although recent studies have demonstrated that patients with pyonephrosis had higher Hounsfiled Units (HU) levels in the dilated collecting system than in the case of simple non-complicated hydronephrosis, supporting its utility in measuring the density of urine in those patients [ 29 , 30 , 31 ]. This may be especially important in patients that cannot undergo contrast-enhanced CT.…”

“…In these patients, the evaluation of the HU of the obstructed collecting system may provide additional information to the clinician for a probable diagnosis of UTI. Using the HU in addition to the classical parameters (thickening of the renal pelvis and stranding of the perirenal fat) for pyonephrosis in CT-imaging allows responsible physicians to make earlier diagnoses and earlier interventions [ 30 ].…”

“…Ultrasound can rapidly confirm or exclude the presence of pyonephrosis, speeding up patient management and treatment, and demonstrate hydronephrosis with echogenic debris in the renal pelvis, which is considered highly suggestive of pyonephrosis [ 18 , 24 , 25 , 26 ]. Although computed tomography (CT) is considered more reliable, useful, and impactful in the critical care setting [ 22 , 23 , 27 , 28 , 29 , 30 , 31 ], the diagnosis can be missed on unenhanced CT because of the difficulty encountered in the radiologic differentiation of hydronephosis from pyonephrosis [ 31 ]. The direct assessment of purulent urine is easily performed in the case of the presence of air bubbles; recent studies demonstrated the HU measurement of urine may be helpful in the diagnosis of purulent urine [ 27 , 29 , 30 , 31 ].…”

“…Although computed tomography (CT) is considered more reliable, useful, and impactful in the critical care setting [ 22 , 23 , 27 , 28 , 29 , 30 , 31 ], the diagnosis can be missed on unenhanced CT because of the difficulty encountered in the radiologic differentiation of hydronephosis from pyonephrosis [ 31 ]. The direct assessment of purulent urine is easily performed in the case of the presence of air bubbles; recent studies demonstrated the HU measurement of urine may be helpful in the diagnosis of purulent urine [ 27 , 29 , 30 , 31 ]. At contrast-enhanced CT examinations, PN signs are characterized by the presence of signs of obstruction with associated renal pelvic wall thickening and perirenal fat stranding [ 27 , 28 , 32 , 33 ].…”