RECONSTRUCTIVE SURGERY The Whitaker Test in the Follow-up of Complex Upper Urinary Tract Reconstruction: Is It Clinically Useful or Not Xinfei Li1, Kunlin Yang1, Weijie Zhu1, Yuke Chen1, Yang Yang1*, Peng Zhang2, Juan Wu3, Pengjie Wu4, Shiliang Wu1, Xuesong Li1*, Liqun Zhou1 Purpose: To evaluate the feasibility and guiding significance in postoperative management of the Whitaker test after complex reconstruction of the upper urinary tract. Materials and Methods: Patients who underwent complex ureteral reconstruction and received the Whitaker test after surgery between December 2018 and December 2019 were included. We judged it abnormal that the renal pelvis pressure was higher than 22 cmH 2 O or the pressure difference was greater than 15 cmH 2 O. The results were used as a reference for removing the nephrostomy tube. Based on whether the renal pelvic pressure was higher than 22 cmH 2 O, the patients were divided into the elevated pelvis pressure group and the normal group. Follow ups at 1 month and every 3 months were collected. Results: A total of 19 patients were included. Fifteen patients did not present obvious abnormalities. One patient suffered from contrast infiltrating into the renal parenchyma, and the pressure was higher than 15 cmH2O. Ureteral stent implantation was performed. The other 3 patients had either elevated pelvis pressure or insufficient image, 2 of which prolonged the duration of nephrostomy tubes. The median follow-up time was 12.6 months. CTU/MRU after removing nephrostomy tubes indicated improved/stable hydronephrosis in all patients. The creatinine in the elevated pelvis pressure group was higher than that in the normal group (91.4 ± 27.6 vs 86.7 ± 16.5 μmol/L, P = .782), and the eGFR was lower (76.0 ± 14.0 vs 81.8 ± 24.1 mL/min/1.73m2, P = .695), but without significant difference. The change in creatinine during follow-up in the elevated renal pelvic pressure group was significantly different from that in the normal group (-13.6 ± 1.0 vs -0.2 ± 10.6 umol/L, P = .047). Conclusion: Postoperative Whitaker test can help judge whether nephrostomy could be removed. Elevated pres- sure in upper urinary tract after reconstruction suggests the need to prolong the time of the nephrostomy tube or even re-intervene. Proper management for patients with elevated renal pelvis pressure can help restore the renal function. Keywords: dilation; ureteral reconstruction; diagnostic techniques; urodynamic study; renal function; the Whitak- er test INTRODUCTION Complex ureteral stricture can lead to severe dila-tion and hydrops of the urinary tract. Upper urinary tract reconstructive surgery aims to restore the conti- nuity of the urinary tract and protect renal function.(1) The main reconstructive strategies included Boari flap, appendix, oral mucosa, ileal ureter replacement and au- totransplantation,(2-4) However, patients often receive ureteral stent or retain nephrostomy tube for the pro- tection of the reconstructive upper urinary tract, and the majority need a relatively long time to recovery. Routinely, patients are suggested to undergo ultra- sound, intravenous urography, diuretic renography, 1Department of Urology, Peking University First Hospital. Institute of Urology, Peking University. National Urological Cancer Center, Beijing, P.R. China. 2Department of Urology, Emergency General Hospital, Beijing, P.R. China. 3China rehabilitation research center, Beijing Boai hospital, Beijing, P.R. China. 4Department of Urology, Beijing Hospital, National Center of Gerontology; Institute of Geriatric Medicine, Chinese Academy of Medi- cal Sciences, Beijing, P.R. China. *Correspondence: Department of Urology, Peking University First Hospital. Institute of Urology, Peking University. National Urologi- cal Cancer Center, Beijing, China. Tel:+86-10-83575101, Fax:+86-10-66551726, E-mail: goldflamingo@126.com; pineneedle@126.com Xinfei Li, Kunlin Yang and Weijie Zhu contributed equally to this article Received May 2020 & Accepted January 2021 computed tomography urography (CTU) and magnetic resonance urography (MRU) at follow-up. The inter- pretation relies on the degree of hydrops and urinary tract lumen dilation to determine whether the urinary tract is unobstructed. Whereas, dilatation or hydrops does not always equate with obstruction.(5,6) More diffi- cult to interpret is the dilatation that remains after relief of obstruction. Different from imaging examination, renogram utilized radioisotopes to assess the excreto- ry function through radioisotopes and has been widely used since it was proposed. However, in a kidney with impaired function or associated with an extremely large system the test may be invalid, and the response to a diuretic is in doubt and different according to whether Urology Journal/Vol 19 No. 1/ January-February 2022/ pp. 56-62. [DOI: 10.22037/uj.v18i.6277] Vol 19 No 1 January-February 2022 57 the damage mainly is glomerular or tubular. Moreo- ver, with a renogram no information is achieved about compliance.(7) Since radionuclide imaging presents a blurred image of anatomical structure, the result of di- uresis renogram cannot reflect the changes in the shape of the upper urinary tract and the degree of difficulty of deformation during urine transportation. More im- portantly, all the present examinations are affected by a ureteral stent and a nephrostomy tube. The uncertain outcome after removing protective ureteral stents or ne- phrostomy tubes is a concern. In 1973, Whitaker advocated the use of a constant perfu- sion flow study to distinguish whether urinary tract dil- atation is caused by obstruction.(8) Currently, there are seldom articles discussing the role of the Whitaker test in the postoperative evaluation of complex upper uri- nary tract reconstruction. In addition, previous studies investigating the relationship between the Whitaker test and renal function focused almost on the pelvis-blad- der pressure difference, and the result is still controver- sial. This study presents the postoperative results of the Whitaker test in reconstructive upper urinary tract. We also explore the renal function after the Whitaker test guided management, aiming to evaluate the instruction- al significance of the Whitaker test in complex recon- structive upper urinary tract. MATERIALS AND METHODS Study population A retrospective study of complex upper urinary tract re- construction in 19 patients was conducted from Decem- ber 2018 to December 2019. All patients were indicated to a surgical management by severe hydronephrosis, flank pain, and/or poor renal function. Complex recon- struction was defined as (1) long ureter defects longer Table 1. Clinical characteristics and surgical information Variables N (%) Gender Male 7 (36.8%) Female 12 (63.2%) Age, years, mean ± SD 41.6±9.8 BMI, kg/m2, mean ± SD 24.2±3.2 Side Left 6 (31.6%) Right 10 (52.6%) Bilateral 3 (15.8%) Lesion Upper 5 (26.3%) Middle 3 (15.8%) Lower 11 (57.9%) Etiology Ureteroscopic holmium laser lithotripsy 9 (47.4%) Radiotherapy 4 (21.1%) Cervical cancer surgery 4 (21.1%) Idiopathy 2 (10.5%) Surgery Ileal ureter 11 (57.9%) Boari flap 3 (15.8%) Ureteroneocystostomy 2 (10.5%) Appendiceal onlay flap 2 (10.5%) Lingual mucosal 1 (5.3%) Mean preoperative serum creatinine (μmol/L) 92.9 ± 27.0 Mean preoperative eGFR (mL/min/1.73m2) 79.2 ± 23.7 Figure1: Normal postoperative result of the Whitaker test (lingual mucosal graft). Figure 1A &1B. Imaging of renal pelvis, upper urinary tract and bladder. Figure 1C. The record of pressure during perfusion. Co: Cough, Fi: First time feeling of urination, UU: Urgency of urination; P: Point of imaging time. The Whitaker test after complex reconstruction-Xinfei Li et al. Abbreviations: eGFR, estimated glomerular filtration rate. Reconstructive Surgery 58 than 5 cm that cannot be managed with simple resection and anastomosis, (2) secondary reconstruction and (3) severe ureteral injuries such as avulsion or rupture. Data on patient characteristics, etiology, laboratory data, im- aging studies, surgery information and perioperative data were collected from our Reconstruction of Urinary Tract: Technology, Epidemiology and Result (RECUT- TER) database. The present study was approved by the Institutional Ethics Committee of our hospital. In- formed consent was obtained from all individual partic- ipants included in the study. Whitaker test All patients had a nephrostomy tube before surgery. In order to protect the reconstructed upper urinary tract, the nephrostomy tube was intermittently clipped after surgery instead of being removed immediately. Patients were followed up at 1 and 3 months after surgery to determine whether to remove the nephrostomy tube. The process of perfusion was the same as the meth- od Whitaker described in 1973. Patients took a prone position. A urinary catheter was placed before the ex- amination. Nephrostomy tubes and urinary catheters were connected to the pressure transducer respectively. The contrast medium was diluted at 50 % with saline. The initial perfusion rate was 10 mL/min (decreasing to 5 mL/min in 3 equivocal cases). The perfusion was continued until a steady state was reached at which the pressure did not change. The pressure was recorded si- multaneously from the renal pelvis and the bladder. If the patient had a bladder distention or the elevated pres- sure did not ease, the perfusion should be suspended or terminated. We judged it abnormal when the renal pelvis pressure was higher than 22 cmH 2 O or the pressure difference was greater than 15 cmH 2 O.(8-10) At the same time, X-ray was used to evaluate the morphology of the upper urinary tract. Hold-up of contrast medium at anywhere in the ureter may be suspected. A significant rise in the pressure difference across the suspected obstruction al- lows the diagnosis, whereas free drainage of contrast at low pressure excludes obstruction. Based on whether the renal pelvic pressure was higher than 22 cmH 2 O, the patients were divided into 2 groups for further com- parison. namely the elevated pelvis pressure group and the normal group. Follow-up After removing the nephrostomy tubes, patients came to the clinic every three months. Physical examination, blood serum creatinine tests, urine routine tests and urinary ultrasound were performed at each visit. CTU/ MRU was performed to evaluate the morphology of the reconstructive urinary tract. Based on the anteroposteri- or diameter of the pelvis in B-ultrasound, hydronephro- sis is defined as normal (<5 mm), mild (5-9 mm), mod- erate (10-14 mm) and severe (≥15 mm). The estimated glomerular filtration rate (eGFR) is calculated by the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation. Successful nephrostomy removal was defined as no symptoms and improved or stable hydronephrosis. Data analysis All analyses were performed with SPSS® Statistics (version 20.0). The Kolmogorov-Smirnov test was used to check whether the data (age, BMI, pre- and postop- erative creatinine, pre- and postoperative eGFR, chang- es in the creatinine and eGFR, pressure difference, follow-up time) were normally distributed. Independ- ent-sample t-test was used to compare the difference of average value (creatinine and eGFR) between the elevated renal pelvic pressure group and the normal pressure group. Pearson’s analysis was used to test the correlation of perfusion volume and pressure difference with the creatinine and eGFR. A two-sided p < .05 was taken to indicate statistical significance. RESULTS The clinical characteristic and surgical information Table 2. The detailed information of the Whitaker test in patients with abnormal results Patients Surgery Perfusion speed Perfusion volume Pelvis pressure Final perfusion Pressure difference Imaging (mL/min) (mL) (cmH 2 O) volume (mL) (cmH 2 O) 1 Ileal ureter 20 113 54 211 -6 Clear 2 Ileal ureter 10 71 46 205 -5 (1) Poor imaging at ureterovesical junction; (2) No contrast in the bladder until 86 ml; (3) Lower ureter was not visualized. 3 Uretero neocystostomy 5 21 155 24 49 (1) Contrast penetrated into the renal parenchyma; (2) Lower ureter was not visualized. 4 Ureteroneocystostomy 5→10 / Stable 366 0 Lower ureter and ureterovesical junction was visualized insufficiently the Whitaker test / Ultrasound No Mild Moderate Severe Total P Normal 5 5 2 4 16 .225 Abnormal 0 1 2 0 3 Total 5 6 4 4 19 Table 3. Comparative analysis between Whitaker test and ultrasound The Whitaker test after complex reconstruction-Xinfei Li et al. Vol 19 No 1 January-February 2022 59 were shown in Table 1. All patients received a ne- phrostomy preoperatively. All patients completed the Whitaker test successfully. No one experienced perfu- sion-related symptoms. The median perfusion volume at the end of the test was 227 mL (range 24 to 366 mL). 15 patients did not present obvious abnormalities (Figure 1). Among them, 4 patients showed a negative pressure difference that was lower than 0 cmH 2 O. One patient had only insufficient image at the lower ure- ter. One patient showed elevated pelvis pressure with- out abnormal pressure difference or poor imaging. An unusual rise in renal pelvis pressure during perfusion together with poor imaging at the ureterovesical junc- tion was observed in 1 patient. In detail, perfusion fluid did not appear in the bladder until perfusion to 86 mL. Particularly, one patient showed obvious elevated pel- vis pressure and suffered from contrast penetrating into the renal parenchyma (Figure 2). Consequently, the perfusion was temporarily suspended. After the pres- sure dropped, the perfusion was continued, and the pel- vis-bladder pressure was higher than 15 cmH2O (Table 2). The results of B-ultrasound and perfusion test were consistent negative in 5 patients. In contrast, 14 patients had remained different degree of dilation in ultrasound after surgery, but the Whitaker test showed a velocity of 10 mL/min perfusion could be tolerated in 11 of 14 patients (Table 3). 16 patients removed the nephrostomy tube after the confirmation of the Whitaker test. Two patients with elevated pressure prolonged the duration of the ne- phrostomy tube. One patient with obvious abnormality during perfusion underwent ureteral stent implantation, and the tube was removed after 3 months. The median follow-up time was 12.6 months (range 8.6 to 17.3 months). All the patients were free of symptoms. The mean postoperative serum creatinine was 90.6 ± 25.3 μmol/L, and the mean eGFR was 80.9 ± 22.6 mL/ min/1.73m. Urine routine tests showed positive white Variables Elevated pelvis pressure group Normal pressure group P Serum creatinine, μmol/L 91.4 ± 27.6 86.7 ± 16.5 .782 eGFR, ml/min 1.73m2 76.0 ± 14.0 81.8 ± 24.1 .695 ΔCreatinine, μmol/L -13.6 ± 1.0 -0.2 ± 10.6 .047 ΔeGFR, ml/min 1.73m2 12.7 ± 4.2 -0.4 ± 12.7 .101 Table 4. The comparative results of creatinine and eGFR between the elevated pelvic pressure group and the normal pressure group Figure 2. Obstructive result of the Whitaker test. A. Contrast penetrating into renal parenchyma. B. Lower ureter was not visualized. C. The record of pressure during perfusion, the elevated renal pelvis pressure decreased when the perfusion was suspended, but it was still higher than the initial basic pressure. The final pressure difference was 49 cmH 2 O. SP: Supine position, S: Suspension, LP: Lithotomy position. Abbreviations: Δ, change; eGFR, estimated glomerular filtration rate. The Whitaker test after complex reconstruction-Xinfei Li et al. Reconstructive Surgery 60 blood cells in 8 patients. There were no obvious abnor- malities in the urine routine of the other patients. Ultra- sound showed that hydrops was improved in 5 patients, meanwhile, the degree of dilation remained stable in the other 14 patients. CTU/MRU confirmed no dilation or mild dilation of the collecting system that was im- proved than before. For 3 patients with abnormal imag- ing urodynamic results, ultrasound found that hydrops disappeared in one patient and remained stable in the other 2 cases. CTU/MRU showed only mild dilation in these 3 patients (Figure 3). The creatinine in the elevated pelvis pressure group was higher than that in the normal group (86.7 ± 16.5 vs 91.4 ± 27.6 μmol/L, P = .782), and the eGFR was lower (76.0 ± 14.0 vs 81.8 ± 24.1 mL/min/1.73m2, P = .695), but without significant difference. Figure 4 pre- sented the changing trend of the creatinine and eGFR in the normal and elevated renal pelvis pressure group, respectively. The change in creatinine during follow-up in the elevated renal pelvic pressure group was signif- icantly different from that in the normal group (-13.6 ± 1.0 vs -0.2 ± 10.6 umol/L, P = .047), but there was no difference in the change in eGFR (12.7 ± 4.2 vs -0.4 ± 12.7, P = .101). Neither the pressure difference show correlation with creatinine (r = -.056, P = .819) or eGFR (r = -.109, P = .657), nor did the perfusion volume showed correlation with creatinine (r = .205, P = .401)) or eGFR (r = -.040, P = .870). DISCUSSION Ureters are slender ducts that drainage urine from the kidney to the bladder. Urine transport relies on two ma- jor mechanisms: the active contraction of the smooth muscles, and passive flow driven by hydrostatic pres- sure. An obstruction that results in a decrease in urine output can cause dilation. Relaxed smooth muscle of the collecting system owing to long term hydronephro- sis can also lead to dilation. Therefore, it is not enough to observe the dilation of the urinary tract, which is the shortcoming of the existing imaging examinations. The reasons behind the dilation also need to be brought to the forefront. While investigating the dilation of upper urinary tract after reconstruction, whether the obstruction still exists is the focus of attention. The renogram not only judges how well the isotope can pass through the urinary sys- tem but also gives information on split renal function. However, dilated renal pelvic and ureter, particular- ly if associated with a poorly functioning kidney, can cause stasis to give a false impression of obstruction. (7) In addition, protective ureteral stent or nephrostomy tube may lead to better imaging results than it is. The Whitaker test uses a constant perfusion flow study to evaluate the function of the upper urinary tract.(8) The Whitaker test provides a quantitative assessment of out- flow resistance, and it has the advantage that it does not rely on renal function or a diuretic response. During the past decade, researchers have passed dif- ferent judgments on the Whitaker test. Djurhuus et al. Figure 3. Pre- and postoperative CTU of the patient with elevated renal pelvis pressure and abnormal pressure difference. a. Preoperative cross section CTU; b. Preoperative coronary section CTU; c. Cross section CTU after removing the percutaneous nephrostomy tube; d. Coronary section CTU after removing the percutaneous nephrostomy tube. The Whitaker test after complex reconstruction-Xinfei Li et al. Vol 19 No 1 January-February 2022 61 have shown that the resting pelvis pressure may show considerable overlap in a hydronephrotic or in a nor- mal one.(11) Wahlin et al. also agreed the specificity of the method was limited.(12) However, proponents of the Whitaker test have approved its values for the diagno- sis of obstruction and exclusion of unobstructed dilata- tion. Johnston proposed the Whitaker test was useful in evaluating patients with skeptical ureteropelvic or ure- terovesical junction obstruction.(13) Lupton and George concluded that the Whitaker test contributed to the clin- ical management in 84% of the suspected upper urinary tract obstruction.(9) Except for these arguments, there are seldom articles reporting the use of the Whitaker test for postoperative evaluation of complex urinary tract reconstruction. The Whitaker test can help clarify whether the urinary tract is still obstructed and determine when percutane- ous nephrostomy tubes could be safely removed, espe- cially for those patients who showed no improvement in radiographic appearances. Previous studies under- lined that about 60% of the patients showed no clear improvement in ultrasound after reconstruction.(14) In the present study, we found 14 patients retained urinary tract dilation in ultrasound, while the negative Whitaker test in 11 of 14 patients provided evidence for nephros- tomy removal. CTU or MRU prompted satisfactory im- aging results of the reconstructive upper urinary tract after removing the nephrostomy tube. No patients suf- fered from renal function deterioration. Therefore, with normal results of the Whitaker test, no need to worry too much that ultrasound changes recovered slowly.(15) The obstructive results often indicated a prolonged time of nephrostomy and even the possibility of reinterven- tion. In our study, 3 cases with abnormal results were treated accordingly. In detail, 1 case who underwent ureteroneocystostomy had high pelvis pressure, high pressure difference and poor imaging, which together indicated the existence of obstruction. Marshall et al. reported a similar patient with left megaureter that was reimplanted into the bladder showed a pressure of 19 cmH2O in the Whitaker test. A transureteroureterosto- my and temporary left nephrostomy ensued. The pres- sure again was less than 8 cmH 2 O, and the patient was free of symptoms and deterioration.(16) For our patient, the renal pelvis pressure increased with the progress of perfusion, and the perfusion was temporarily suspended twice. After a brief stop and position change, the re- nal pelvis pressure decreased to a stable level. But the pressure was still higher than the initial basic pressure, and the final pressure difference was greater than 15 cmH 2 O. This patient subsequently underwent ureteral stent implantation. The other 2 patients showed high re- nal pelvis pressure with or without poor imaging. The pressure gradually decreased and remained stable as the perfusion went on. The reason may be the poor pelvic compliance. The Whitaker test simulates a high urine flow actually to evaluate the ability of the reconstruc- tive upper urinary tract to convey urine. Under phys- iological conditions, the urine production rate is 1-3 mL/min,(17) but it can increase up to 20 mL/min after diuresis.(9) Accompanied by moderate dilation in ultra- sound, these 2 patients were treated by prolonging the time of nephrostomy. The CTU/MRU during follow-up showed improved dilation in the collecting system. Negative pressure difference reminds urologists to be vigilant against reflux, but for patients undergoing il- eal ureter replacement, it may be a normal phenome- non. The ureter is contracted to close when transporting urine downward without reflux.(6) In this study, 4 pa- tients had negative pressure difference, and 3 of them underwent ileal ureter replacement. The wide intestine was difficult to be completely closed during peristalsis, allowing reflux confined to the bowel segment. These 3 patients were not given special treatment, and there were no recurrent urinary tract infections, aggravated hydronephrosis, or deterioration of renal function dur- ing the follow-up. Another patient underwent Boari flap surgery. The negative pressure difference was due to the displacement of the transducer, and the measured bladder pressure was lower than the actual pressure. Renal pelvis pressure was an informative factor. We found the creatinine in the elevated pelvis pressure group was higher than that in the normal group, and the eGFR was lower, but without significant difference. Figure 4. The changing trend of the renal function during follow-up. A. the trend of the creatinine in the normal and elevated renal pelvis pressure group. B. the trend of the eGFR in the normal and elevated renal pelvis pressure group. black arrow: the time of the Whitaker test and corresponding management. The Whitaker test after complex reconstruction-Xinfei Li et al. The changes of the creatinine and eGFR during fol- low-up were related to whether the renal pelvis pressure was elevated. The reason was that increased renal pelvic pressure indicated that the upper urinary tract had poor tolerance to hydrops, and the renal function could be impaired easily. However, the consequences were still reversible, so timely treatment helped restore the renal function. In the past, most researchers considered that abnormal results of the Whitaker test did not predict worse renal function. Djurhuus et al. pointed out the renal function in the high pressure group was the same as those in the low pressure group.(11,18) Identical with previous literature reports, pressure difference had no predictive value for postoperative renal function in our patients. However, few articles involved the pressure of the renal pelvis, which indicated the compliance of the upper urinary tract. Koff et al. thought renal pelvis pressure was an indicator determinant to progression.(19) In general, it was necessary to deal with when the renal pelvis pressure was abnormal, and the outcome was not different from the normal group after timely treatment. There are still some limitations in our study. Firstly, contrast media have a greater viscosity than urine or saline and will produce higher pressure at a given flow rate. Moreover, with the high infusion rates used that are not physiological, the urinary tract may be over- stretched. Thus, follow-up needs to perfect 1-3 mL/min physiological velocity perfusion or even individualize the infusion rates, so that some of the measurement bias could be avoided. Secondly, we didn’t perform a diu- resis renogram for reference because the nephrostomy tube would affect the result of diuretic renogram. At last, large samples and long term follow-up result are necessary. CONCLUSIONS Despite the concerns and debates over its relative value, the Whitaker test can help judge whether nephrostomy tube could be removed. Abnormal results during per- fusion prompt reintervention or longer duration of ne- phrostomy tube. Proper management for patients with elevated renal pelvis pressure can help restore renal function. CONFLICT OF INTEREST The authors have no conflicts of interest to declare. REFERENCES 1. Jayanthi VR. Reconstructive surgery of the upper urinary tract. Curr Opin Urol. 1998;8:215-20. 2. Wang J, Xiong S, Fan S, et al. Appendiceal Onlay Flap Ureteroplasty for the Treatment of Complex Ureteral Strictures: Initial Experience of Nine Patients. J Endourol. 2020;34:874-81. 3. Fan S, Yin L, Yang K, et al. Posteriorly augmented anastomotic ureteroplasty with lingual mucosal onlay grafts for long proximal ureteral strictures: Ten cases of experience. J Endourol. Epub ahead of print. doi: 10.1089/ end.2020.0686. 4. Gu H, Chen S, Wu Y, et al. Improved Long Ureteral Reconstruction With Ileum By Longitudinal Clipping And Mucosal Stripping: An Animal Study. Urol J. 2020;17:198-203. 5. Vignoli G. Urodynamics of the Upper Urinary Tract. In: Vignoli G, editor. Urodynamics. Switzerland: Springer Cham; 2017. p. 175-84. 6. Farrugia MK, Whitaker RH. The search for the definition, etiology, and effective diagnosis of upper urinary tract obstruction: the Whitaker test then and now. J Pediatr Urol. 2019;15:18- 26. 7. Whitaker RH, BuxtonThomas MS. A comparison of pressure flow studies and renography in equivocal upper urinary tract obstruction. J Urol. 1984;132:1209-10. 8. Whitaker RH. Methods of assessing obstruction in dilated ureters. Br J Urol. 1973;45:15-22. 9. Lupton EW, George NJ. The Whitaker test: 35 years on. BJU Int. 2010;105:94-100. 10. George NJ, O'Reilly PH, Barnard RJ, Blacklock NJ. High pressure chronic retention. Br Med J. 1983;286:1780-3. 11. Djurhuus JC, Jorgensen TM, Norgaard JP, Nerstrom B, Hvid-Hansen H. Constant perfusion provocation in idiopathic hydronephrosis. Urology. 1982;19: 611-6. 12. Wahlin N, Magnusson A, Persson AE, Lackgren G, Stenberg A. Pressure flow measurement of hydronephrosis in children: a new approach to definition and quantification of obstruction. J Urol. 2001;166:1842-7. 13. Johnston RB, Porter C. The Whitaker test. Urol J. 2014;11:1727-30. 14. Amling CL, O'Hara SM, Wiener JS, et al. Renal ultrasound changes after pyeloplasty in children with ureteropelvic junction obstruction: long-term outcome in 47 renal units. J Urol. 1996;156:2020-4. 15. Fernandez-Ibieta M, Nortes-Cano L, Guirao- Pinera MJ, et al. Radiation-free monitoring in the long-term follow-up of pyeloplasty: Are ultrasound new parameters good enough to evaluate a successful procedure? J Pediatr Urol. 2016;12:230-1. 16. Marshall V, Whitaker RH. Ureteral pressure flow studies in difficult diagnostic problems. J Urol. 1975;114:204-7. 17. Weiss RM. Clinical implications of ureteral physiology. J Urol. 1979;121:401-13. 18. Djurhuus JC, Sorensen SS, Jorgensen TM, et al. Predictive value of pressure flow studies for the functional outcome of reconstructive surgery for hydronephrosis. Br J Urol. 1985;57:6-9. 19. Koff SA. Determinants of progression and equilibrium in hydronephrosis. Urology. 1983;21:496-500. The Whitaker test after complex reconstruction-Xinfei Li et al. Reconstructive Surgery 62