Differentiation of Obstructive and Non-Obstructive Hydronephrosis by Using Renal Artery Resistance Indices taking CT-KUB as the Gold Standard

Hydronephrosis is characterized by the dilation of the pelvicalyceal system due to impaired urine flow, often resulting from compromised urinary drainage.1 Obstructive hydronephrosis refers to structural and functional changes in the kidneys caused by difficulties in urine flow (difficulties in urinating).2 Non-obstructive hydronephrosis, on the other hand, describes the dilation of the renal pelvis and calyces without any physical blockage in the urinary tract. This condition usually arises from functional or physiological factors, such as increased urine production, vesicoureteral reflux, or transient changes in urinary flow dynamics, rather than mechanical obstruction.3 Hydronephrosis, particularly ureteral and renal pelvic dilation, is more commonly observed on the right side and occurs in 80–90% of pregnant women.4.

 Non-invasive imaging techniques, such as intrarenal arterial Doppler ultrasonography, have become increasingly important. This method allows the assessment of renal hemodynamic changes during acute obstruction by measuring parameters such as the Resistive Index (RI). Elevated RI values indicate increased vascular resistance, which is often associated with acute obstruction.5 Conventional abdominal radiography (KUB) is also commonly used to evaluate hydronephrosis, helping detect kidney size, shape, and the presence of calculi that may suggest obstruction.6

 Previous studies have shown the diagnostic value of RI in differentiating obstructive from non-obstructive hydronephrosis. Solomon et al. (2018) reported a sensitivity of 71.1%, specificity of 81.2%, and overall accuracy of 75.4%, with statistically significant results (p = 0.003).7 Nadzri et al. (2015) found that the mean RI was significantly higher in obstructive hydronephrosis (0.78) compared to non-obstructive cases (0.70, p < 0.05), with Doppler ultrasound demonstrating a sensitivity of 100% and specificity of 53%.8 Safdal et al. (2019) reported that the sensitivity and specificity of RI using color Doppler ultrasonography were 93.15% and 94.81%, respectively; the positive predictive value and negative predictive value for RI were 94.44% and 93.59%. Out of 150 patients, 72 (48%) were positive on both RI CDUS and CT-KUB.8

 To the best of the candidate's knowledge, no local research has been conducted on this topic. The findings will provide valuable local data, supporting the use of RI as a reliable and non-invasive diagnostic tool, potentially reducing the need for expensive or invasive imaging tests. Furthermore, these results may guide future studies to improve diagnostic methods and patient care, ultimately enhancing management and treatment outcomes for patients with hydronephrosis. To determine the diagnostic accuracy of Doppler ultrasound resistive index of the interlobar renal artery in differentiating obstructive from non-obstructive hydronephrosis, taking CT-KUB as the gold standard.

The study was designed as a cross-sectional survey and was conducted in the Department of Diagnostic Radiology at CMH, Lahore. The duration of the study was from May 2025 to October 2025 following the approval of the synopsis. The sample size of 141 cases was calculated based on a 95% confidence level, with an expected sensitivity of 71.1%, specificity of 81.2%, and an 11% margin of error, considering a disease prevalence of 48%.7,9 After obtaining approval from the Ethical Review Committee of the hospital, 141 patients presenting to the Radiology Department for imaging at CMH Lahore and meeting the inclusion criteria were enrolled in the study. Written informed consent was obtained from each participant. Demographic details, including age and gender, were recorded on a pre-designed proforma.

 Patients of both genders aged between 18 and 70 years who were referred to the radiology department with suspected hydronephrosis, as defined by the operational criteria, and who underwent abdominal ultrasound followed by surgical exploration, were included in the study. Additional inclusion criteria were patients with clinical suspicion of renal obstruction (such as flank pain, hematuria, or impaired renal function), those with accessible Doppler ultrasound evaluation of the interlobar renal artery, and patients who were willing to provide informed consent for participation in the study. Exclusion criteria were diagnosed cases of abdominal malignancy based on history and clinical records, patients with congenital abnormalities of the renal vasculature or urinary tract, those with a baseline estimated glomerular filtration rate (eGFR) <30 ml/min/1.73 m² or documented end-stage renal disease (ESRD), pregnant patients due to potential radiation exposure during CT-KUB, and patients with active urinary tract infections or sepsis at the time of evaluation.

 Each patient underwent Doppler ultrasonography to assess the renal artery resistive index (RI). The ultrasound examinations were performed using a GE-LOGIQ P9 machine with a convex high-frequency (3–5 MHz) transducer. Imaging was carried out in the supine and lateral decubitus positions to ensure optimal visualization of the renal interlobar arteries.

 The renal artery resistive index (RI) was calculated for each patient using the standard formula:

RI = (Peak Systolic Velocity − End Diastolic Velocity) / Peak Systolic Velocity.

 An elevated RI (>0.7) was considered indicative of obstructive hydronephrosis, while a normal RI (<0.7) suggested non-obstructive hydronephrosis. Following Doppler ultrasonography, all patients underwent CT-KUB performed on a Siemens SOMATOM 16-slice CT scanner, which served as the gold standard diagnostic tool. CT-KUB findings were used to confirm the diagnosis of either obstructive or non-obstructive hydronephrosis according to the operational definitions. The results of Doppler ultrasonography were then compared with CT-KUB findings, and each case was categorized as true positive, false positive, true negative, or false negative. The diagnostic accuracy of Doppler ultrasonography was evaluated in terms of sensitivity, specificity, positive predictive value, and negative predictive value. To ensure standardization, all Doppler ultrasonography examinations were conducted by the same consultant radiologist with at least eight years of experience, while all CT-KUB scans were reviewed by another senior radiologist who was blinded to the ultrasound findings. Confounding variables were controlled through strict adherence to the exclusion criteria. All collected data were recorded in the attached proforma by the investigator.

 Suspected hydronephrosis was defined by the presence of at least two of the following: flank pain >3 on a 0–10 scale, hematuria (>5 RBCs/HPF or macroscopic), or impaired renal function (serum creatinine >1.5 mg/dl or eGFR <60 ml/min/1.73 m²). Hydronephrosis was classified as obstructive if the renal artery resistive index (RI) was >0.75 and non-obstructive if RI was <0.75. On CT-KUB, obstructive hydronephrosis was diagnosed with renal pelvis dilation >10 mm and visible obstruction, while non-obstructive cases had mild to moderate dilation (<10 mm) without obstruction. Diagnostic accuracy referred to Doppler ultrasound’s ability to differentiate obstructive from non-obstructive hydronephrosis based on RI. A true positive was defined as a patient with obstructive hydronephrosis who tested positive on both the renal artery resistive index (RI) and CT-KUB. A true negative was a patient with non-obstructive hydronephrosis who tested negative on both RI and CT-KUB. A false positive occurred when obstructive hydronephrosis was diagnosed on RI, but the patient was found to have non-obstructive hydronephrosis on CT-KUB. Conversely, a false negative was a case in which non-obstructive hydronephrosis was not detected on RI, but CT-KUB revealed obstructive hydronephrosis.

 All collected data were entered and analyzed using SPSS version 25. Numerical variables, including age, renal artery resistive index (RI), and duration of symptoms, were presented as mean ± standard deviation (SD). Categorical variables, such as gender, presence of obstructive and non-obstructive hydronephrosis, Doppler ultrasound findings, CT-KUB findings, diabetes mellitus, and hypertension, were presented as frequencies and percentages. A 2 × 2 contingency table was constructed to calculate the sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and overall diagnostic accuracy of Doppler ultrasonography in differentiating obstructive from non-obstructive hydronephrosis, using CT-KUB findings as the gold standard. The data were stratified according to age, gender, duration of symptoms, diabetes mellitus, and hypertension to account for potential effect modifiers. Following stratification, diagnostic performance measures were recalculated. Statistical significance was considered at p < 0.05.

The study included 141 participants with a mean age of 43.1 ± 14.9 years. Of these, 75 (53.2%) were male and 66 (46.8%) were female. Hypertension was present in 48 participants (34.0%) while 93 (66.0%) did not have hypertension. Diabetes mellitus was identified in 31 participants (22.0%), with the remaining 110 (78.0%) being non-diabetic. The mean duration of symptoms was 12.3 ± 7.6 days, with 87 participants (61.7%) experiencing symptoms for 14 days or less and 54 participants (38.3%) reporting symptoms for more than 14 days, indicating a majority of patients presented relatively early after symptom onset. These findings are summarized in Table 1. Regarding imaging findings, CT-KUB, used as the gold standard, identified obstructive hydronephrosis in 63 participants (44.7%) and non-obstructive hydronephrosis in 78 participants (55.3%). Doppler renal artery resistive index (RI) findings showed obstructive hydronephrosis in 71 participants (50.4%) and non-obstructive hydronephrosis in 70 participants (49.6%). These distributions indicate a close agreement in the proportion of obstructive and non-obstructive cases detected by both imaging modalities, as detailed in Table 2.

 The cross-tabulation of Doppler RI versus CT-KUB findings showed that out of 63 participants with obstructive hydronephrosis on CT-KUB, 56 (39.7%) were correctly identified as obstructive by Doppler RI (true positive), while 7 (5.0%) were missed (false negative). Among the 78 participants with non-obstructive hydronephrosis on CT-KUB, 63 (44.7%) were correctly identified as non-obstructive by Doppler RI (true negative), while 15 (10.6%) were incorrectly labeled as obstructive (false positive). Overall, Doppler RI classified 71 participants as obstructive and 70 as non-obstructive, indicating good concordance with CT-KUB findings (Table 3).

 The diagnostic accuracy of Doppler RI in differentiating obstructive from non-obstructive hydronephrosis was assessed using standard performance measures. Sensitivity was 88.9%, indicating a high ability to correctly identify obstructive cases. Specificity was 80.8%, reflecting good ability to correctly identify non-obstructive cases. The positive predictive value (PPV) was 78.9% and the negative predictive value (NPV) was 90.0%, while overall accuracy was 84.4%, demonstrating that Doppler RI is a reliable non-invasive tool for detecting hydronephrosis when compared to CT-KUB (Table 4).

Stratified analysis of diagnostic performance showed variations across different patient subgroups. Sensitivity and overall accuracy were higher in females (96.6% and 90.9%) than males (82.4% and 80.0%). Participants with hypertension had sensitivity of 87.0% and overall accuracy of 87.5%, while those without hypertension had slightly higher sensitivity (90.0%) but lower accuracy (83.0%). In diabetic patients, sensitivity was 86.7% and overall accuracy 87.1%, compared to 90.0% and 84.5% in non-diabetics. Duration of symptoms strongly affected diagnostic performance; patients with symptoms ≤14 days had perfect sensitivity (100%) and high overall accuracy (95.4%), whereas those with symptoms >14 days showed reduced sensitivity (62.1%) and overall accuracy (62.1%). These findings indicate that patient characteristics and symptom duration can influence the diagnostic reliability of Doppler RI (Table 5).

Table 1: Demographic Characteristics of Study Participants (n = 141)

Table 2: Distribution of CT-KUB and Doppler RI Findings (n = 141)

Table 3: Cross-Tabulation of Doppler RI versus CT-KUB Findings (n = 141)

Table 4: Diagnostic Accuracy of Doppler RI in Differentiating Obstructive and

Non-Obstructive Hydronephrosis (n = 141)

Table 5: Stratified Diagnostic Performance of Doppler RI (n = 141)

Doppler renal artery resistive index demonstrates high sensitivity and specificity in differentiating obstructive from non-obstructive hydronephrosis. Its use as a non-invasive diagnostic tool may reduce the need for CT imaging. Patient characteristics and symptom duration can influence its diagnostic accuracy.

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