Develop formulas for accurately measuring GFR by renal dynamic imaging

Abstract

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Objectives: Gates’ method is most common used in measuring the glomerular filtration rate (GFR) by renal dynamic imaging, but it underestimated GFR according to many reports. Taking creatinine clearance (24hCcr) as reference, Gates’ formula was derived by making regressive analysis between the renal uptake percentage and 24h Ccr, and the renal uptake percentage was calculated by the measured renal radioactivity corrected by renal depth which was calculated by Tonnesen formula derived from B ultrasound measured value divided by injected radioactivity. To increase accuracy to calculate GFR, we used the GFR measured by two-plasma as the reference and the formula derived from CT measured value to calculate the renal depth, developed GFR formulas by both linear and nonlinear regressive analyses made between the renal uptake percentage and two-plasma GFR.

Methods: 147 cases who undertook abdomen CT scanning were studied to develop the renal depth formula by making a multiple linear stepwise regression analysis. 367 patients were selected and the GFRs with both kidney dynamic imaging and two-plasma method were measured. With the GFR of two-plasma as reference value, two equations were obtained respectively from the linear and nonlinear regressive analyses made between the renal uptake percentage and two-plasma GFR. The 367 cases were divided into two groups randomly, of which the first group was used to derive the regressive formulas, and the second to verify the formulas. Finally, all the cases together were studied to derive the formulas to calculate the GFR. The comparison of our formulas with Gates’ formula was conducted by Bland-Altman method.

Results: The depth regression equations were as followed: right kidney depth (cm)=15.449×(weight/height)+0.009637×age+0.782, left kidney depth (cm)=16.772×(weight/height)+0.01025×age+0.224.(weight: kg, height: cm). The linear and nonlinear GFR formulas were: GFR (ml/min/1.73m2) = (774.341× renal uptake percentage -2.83)×1.73/BSA (BSA, body surface area), and, GFR (ml/min/1.73m2)= (-1856.7004×renal uptake percentage2+1135.6409×renal uptake percentage-17.2033)×1.73/BSA, respectively. The biases of GFRs calculated by the linear and nonlinear formulas and Gates’ formula from GFR by two-plasma method were -3.3±24.6, -2.9±24.6 and -7.4±25.0, respectively.

Conclusions: GFR calculated by our new formulas correlate better with the reference GFR than Gates’ formula and GFR measured by the nonlinear formula are more accurate than that by the linear formula.