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Comparison of renal cortical transit time (CTT) with diuretic drainage time parameters in antenatal hydronephrosis (AHN)
Ioana Fugaru, MD, MSc, Richard Liu, MD, Alexa Ehlebracht, MD, Sophie Turpin, MD, Roman Jednak, MD, Mohamed El Sherbiny, MD, John-Paul Capolicchio, MDCM.
McGill University, Montreal, QC, Canada.

Background:
Ureteropelvic junction obstruction (UPJO) is a frequent cause of antenatally detected hydronephrosis (AHN). There is no gold standard diagnostic test for UPJO. Serial diuretic renogram is often used as a surrogate for diagnosing UPJO. Cortical transit time (CTT) is a renogram parameter that might aid in decision-making in AHN. Our objective was to study the association of various CTT cut-offs with other diuretic renogram parameters, and ultimately with clinical management.
Methods:
We retrospectively reviewed 296 charts for AHN and pyeloplasty. We included 64 consecutive pyeloplasties (treatment group), who presented from 2010 to 2021, and 44 conservatively managed AHN with available diuretic renogram (conservative group), who presented from 2010 to 2016. Excluded were 55 patients who presented after 1 year of age and 133 patients with other urinary abnormalities or incomplete data. Indications for pyeloplasty were: worsening hydronephrosis (n=25), initial DRF ≤40% (n=16), decrease in DRF ≥5% (n=11), prolonged diuretic drainage time (n=11) and symptomatic (n=1). Initial and follow-up renal bladder ultrasounds (RBUS) were reviewed for HN grade and anterior-posterior diameter (APD). Initial and follow-up MAG-3 diuretic renograms were reviewed for differential renal function (DRF), CTT, T1/2 and global washout (GWO). Delta (∆) CTT between the affected and contralateral renal unit was calculated. Chi-square/Fisher and t-tests were used for categorical and continuous data. We performed receiver operating characteristic (ROC) curve analysis in order to evaluate the correlation of different CTT cut-offs with T1/2 and GWO. P-value was significant if <0.05.
Results:
Table 1. Initial demographic, ultrasound and renogram characteristics depending on management.

Table 1. Initial demographic, ultrasound and renogram characteristics depending on management.
ParameterPyeloplasty n=64Conservative n=44p-value
Gender
Malen=47 (73.4%)n=35 (79.6%)0.4657
Femalen=17 (26.6%)n=9 (20.5%)
Laterality
Leftn=46 (71.9%)n=33 (75.0%)0.7188
Rightn=18 (28.1%)n=11 (25.0%)
Differential renal function (DRF) of affected renal unit
Initial median DRF46%51%0.0012
Initial RBUS parameters
Initial median APD16 mm13 mm0.0090
SFU Grade 2n=3 (4.7%)n=00.0531
SFU Grade 3n=29 (45.3%)n=30 (68.2%)
SFU Grade 4n=32 (50.0%)n=14 (31.8%)
T1/2
Initial T1/2, median18.2 min5.0 min0.0046
T1/2 <5 minn=4 (6.3%)n=21 (47.7%)<0.0001
T1/2 5-75 minn=41 (64.1%)n=22 (50.0%)
T1/2 >75 minn=9 (14.1%)n=0
GWO
Initial GWO, median63.4%85.4%<0.0001
GWO >90%n=4 (6.3%)n=15 (34.1%)<0.0001
GWO 50-90%n=36 (56.3%)n=28 (63.6%)
GWO <50%n=23 (36.0%)n=0
CTT
Initial CTT, median5.0 min3.0 min0.0312
CTT > 3 minn=51 (79.7%)n=20 (45.5%)0.0004
CTT > 4 minn=33 (51.6%)n=9 (20.5%)0.0013
CTT > 5 minn=14 (21.9%)n=2 (2.3%)0.0135
∆ CTT ≥1 minn=42 (65.6%)n=9 (20.5%)<0.0001

Table 2. Sensitivity and specificity of various CTT and ∆ CTT cut-offs, depending on management.
Pyeloplasty (n=64)Conservative (n=44)Sensitivity (95% CI)Specificity (95% CI)P-value
CTT ≤ 3 min132479.7% (68.3-87.7)54.6% (40.0-68.3)0.0004
CTT > 3 min5120
CTT ≤ 5 min504221.9%(13.5-33.4)95.5% (84.9-99.2)0.0135
CTT > 5 min142
∆ CTT < 1 min223565.6% (53.4-76.1)79.6% (65.5-88.9)<0.0001
∆ CTT ≥ 1 min429
∆ CTT <3 min554314.1% (7.6-24.6)97.7%(88.2-99.9)0.2367
∆ CTT ≥ 3 min91

As illustrated in Table 2, a cut-off of > 3 min for prolonged CTT had the highest sensitivity for pyeloplasty (79.7%) but poor specificity (54.6%) (p=0.0004). CTT > 5 min had the best specificity (95.5%). A cut-off for ∆ CTT of ≥ 3 min had the highest specificity for pyeloplasty (97.7%). For T1/2, the cut-off with the highest area under the curve (AUC) on the ROC curves was CTT > 5 min (AUC = 0.8436; p=0.0001) and ∆ CTT ≥ 3 min (AUC=0.9115; p=0.0001). For GWO, the best cut-offs were the same: CTT > 5 min (AUC=0.8660; p<0.0001) and ∆ CTT ≥ 3 min (AUC=0.9073; p<0.0001).
Conclusions:
The previously suggested cut-off of CTT > 3 min is sensitive but not specific. We identify that initial CTT > 5 min and the presence of a ∆ CTT ≥ 3 min may represent indicators of severity for children presenting with AHN. These cut-offs may be useful for tailoring the frequency and severity of follow-up imaging and may be of benefit in counselling families. Further prospective studies are required to validate these findings.


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