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Nadir creatinine in posterior urethral valves: How high is low enough?
Robert Coleman, FRACS (Urology), Thomas King, MRCS, Cezar D. Nicoara, MRCS, Muhammad Bader, MRCS, Liam McCarthy, FRCS (Paed), Harish Chandran, FRACS (Paed), Karan Parashar, FRCS.
Birmingham Children's Hospital, Birmingham, United Kingdom.

BACKGROUND:
Nadir creatinine (lowest creatinine during the first year following diagnosis) is the recognised prognostic indicator for renal outcome in posterior urethral valves. A cut-off value of 100umol/L as a risk factor for development of chronic renal insufficiency (CRI) has traditionally been used, without scientific validation. We conducted a statistical analysis of nadir creatinine in PUV patients in order to identify the optimal cut-off level as a prognostic indicator for CRI.
Materials and Methods:
Medical records for patients treated by endoscopic valve ablation at our institution between 1993 and 2004 were reviewed. Data collected included nadir creatinine and current renal function. Chronic renal insufficiency was defined as CKD2 or higher according to Kidney Disease Outcomes Quality Initiative guidelines.
Outcomes in test groups were analysed by Fisher exact test. Receiver operating characteristic (ROC) curve analysis was used to demonstrate test significance and to identify optimal cut-off criterion of nadir creatinine as a prognostic indicator. Statistical significance was defined as P < 0.05.
RESULTS:
A total of 129 patients were identified. Of these, progress notes were available for 120 patients. Nadir creatinine was recorded in 103 patients. Length of follow up was 8.42 (±0.47) years.
Receiver operating characteristic (ROC) analysis revealed nadir creatinine to have a high diagnostic accuracy with AUC of 0.852 (P=<0.001). Renal impairment occurred in 9 of 10 (90.0%) patients with nadir creatinine greater than 100umol/L compared with 20 of 93 (21.5%) patients with nadir creatinine less than 100umol/L, (P=<0.0005). As a test for future renal impairment, nadir creatinine cut-off of 100umol/L gave a specificity of 98.6% but a poor sensitivity of only 31.0%.
Using this same analysis, a more suitable nadir creatinine cut-off was identified at 75umol/L. Renal impairment occurred in 14 of 15(93.3%) patients with nadir creatinine greater than 75umol/L, compared with 15 of 88 (17.4%) patients with nadir creatinine equal to or less than 75umol/L (P=<0.0005). As a test for future renal impairment, nadir creatinine cut-off of 75umol/L gave a preserved specificity of 98.6% and an improved sensitivity of 48.3%, with positive and negative predictive values of 93.3% and 83.0% respectively.
CONCLUSIONS:
In posterior urethral valve patients, nadir creatinine higher than 100umol/L is highly specific for future development of renal impairment but sensitivity is low at this cut-off level. Nadir creatinine cut-off of 75umol/L has a higher sensitivity but remains highly specific, with similar positive and negative predictive values. Patients with nadir creatinine higher than 75umol/L should be considered at risk for development of CRI.


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