Effect of renal depth estimation on GFR calculation in dynamic renal scintigraphy in patients with hydronephrosis

Objectives: The aim of this study was to analyze the effect of renal depth, which was estimated by CT measurement and standard Tonnesen formula, on glomerular filtration rate (GFR) calculation in dynamic renal scintigraphy in patients with hydronephrosis.

Methods: Fifty-four patients diagnosed with hydronephrosis by Ultrasound or CTU during January to December 2018 in the General Hospital of NingXia Medical University were included in the study. All the patients underwent 99mTc-DTPA SPECT/CT dynamic renal scintigraphy with a low-dose abdominal CT scan. Gate’s method was used to calculate the total and split renal GFR. There were sixty-one hydronephrosis kidneys and Forty-seven non-hydronephrosis kidneys. The difference of renal depth between CT mesurement and Tonnesen formula were compared in hydronephrosis kidneys group and non-hydronephrosis kidneys group. The corresponding GFRs of split kidney were calculated and compared according to the different renal depth estimation. CKD-EPI_cr formula was used as the gold standard to calculate the total GFR of the patients. Statistical differences among EPI-GFR, Tonnesen-GFR and CT-GFR were compared. SPSS 22.0 was used in statistical analysis. Paired t-test and ANOVA were used in the comparison of data.

Results: There was significant difference of the renal depth between CT measurement and Tonnesen formula in whatever hydronephrosis kidneys group or non-hydronephrosis kidneys group (P<0.01). The distance of renal depth between CT measurement and Tonnesen formula in hydronephrosis kidneys group was (1.57±1.26) cm, while in non-hydronephrosis kidneys group it was (1.11±0.67) cm. The similar results were seen when compared the split GFRs corrected by different renal depth in the two groups. The distance of split GFR in hydronephrosis kidneys group was (6.60±11.07) mL/min, while in non-hydronephrosis kidneys group it was (11.33±7.47) mL/min. When compared with the gold standard EPI-GFR, there was no statistical difference between CT-GFR and EPI-GFR (P<0.05). Tonensen-GFR was significant lower than either EPI-GFR or CT-GFR formula in unilateral hydronephrosis kidneys group (P<0.05). However, CT-GFR was much higher than EPI-GFR, while Tonnensen-GFR was much lower than EPI-GFR in bilateral hydronephrosis group (P<0.05).

Conclusions: The measurement of renal depth using CT may improve accuracy of the GFR estimation for patients with hydronephrosis patients, especially unilateral renal hydronephrosis patients. Tonnensen-GFR obviously underestimated the GFR, whereas CT-GFR overestimated the GFR in patients with bilateral renal hydroneprosis.