Real-time estimation of nephron activity with a linear measurement system (RENAL-MS) predicts postoperative estimated glomerular filtration rate

Abstract

Objective: To determine whether a simple point-of-care measurement system estimating renal parenchymal volume using tools ubiquitously available could be used to replace nuclear medicine renal scintigraphy (NMRS) in current clinical practice to predict estimated glomerular filtration rate (eGFR) after nephrectomy by estimating preoperative split renal function.

Patients and methods: We performed a retrospective review of patients who underwent abdominal cross-sectional imaging (computed tomography/magnetic resonance imaging) and mercaptoacetyltriglycine (MAG3) NMRS prior to total nephrectomy at a single institution. We developed the real-time estimation of nephron activity with a linear measurement system (RENAL-MS) method of estimating postoperative renal function via the following technique: renal parenchymal volume of the removed kidney relative to the remaining kidney was estimated as the product of renal length and the average of six renal parenchymal thickness measurements. The utility of this value was compared to the utility of the split renal function measured by MAG3 for prediction of eGFR and new onset Stage 3 chronic kidney disease (CKD) at ≥90 days after nephrectomy using uni- and multivariate linear and logistic regression.

Results: A total of 57 patients met the study criteria. The median (interquartile range [IQR]) age was 69 (61-80) years. The median (IQR) pre- and postoperative eGFR was 74 (IQR 58-90) and 46 (35-62) mL/min/1.73 m² , respectively. [Correction added on 29 December 2023, after first online publication: The data numbers in the preceding sentence have been corrected.] Correlations between actual and predicted postoperative eGFR were similar whether the RENAL-MS or NMRS methods were used, with correlation using RENAL-MS being slightly numerically but not statistically superior (R = 0.82 and 0.76; P = 0.138). Receiver operating characteristic curve analysis using logistic regression estimates incorporating age, sex, and preoperative creatinine to predict postoperative Stage 3 CKD were similar between RENAL-MS and NMRS (area under the curve 0.93 vs. 0.97). [Correction added on 29 December 2023, after first online publication: The data numbers in the preceding sentence have been corrected.] CONCLUSION: A point-of-care tool to estimate renal parenchymal volume (RENAL-MS) performed equally as well as NMRS to predict postoperative eGFR and de novo Stage 3 CKD after nephrectomy in our population, suggesting NMRS may not be necessary in this setting.

Keywords: clinical nomogram; kidney cancer; kidney function; nuclear medicine renal scan; radical nephrectomy; renal mass.

Fig 1.

RENAL-MS. Anterior, posterior, and lateral parenchymal thickness measurements from the superior and inferior poles at the last axial image with collecting system present. Renal parenchymal thickness was measured with linear measurement function within basic imaging software and renal length was measured by counting the number of images with renal parenchyma present in the axial series multiplied by the slice thickness. (For example, if the kidney were present on 20 axial images with 0.5cm slice thickness, the renal length was estimated to be 20 × 0.5 =10cm)

Fig 2a.

Scatter plot comparing SRF estimated by MAG-3 (y-axis) vs. RENAL-MS (x-axis). Moderate correlation with R=0.69, p<0.001

Fig 2b.

Actual >90 day post-nephrectomy eGFR (mL/min/1.73m2) vs postoperative eGFR predicted by univariable models using SRF estimated by RENAL-MS (R=0.82, y=−9.9+1.5x, SE=0.08) and NMRS methods (R=0.76, y=−6.5+1.3x, SE=0.09). Each patient has two estimated values (NMRS and RENAL-MS) and reported in ascending order according to actual postoperative eGFR (x-axis).

Fig 3.

Receiver operating curves assessing ability of RENAL-MS (AUC=0.95, 95% CI 0.88–0.99) and NMRS (AUC=0.98, 95% CI 0.93–1.00) multivariable models incorporating preoperative eGFR, age, and sex to predict new onset eGFR ≤ 45 ml/min/1.73m² after nephrectomy. The DeLong’s test for two correlated ROC curves indicated p-value=0.24