Comparison of GFR values from the commonly used Cockcroft-Gault and CKD-EPI equations to the ^99mTc-DTPA modified Russell two-point method

Objectives: Determination of glomerular filtration rate (GFR) is commonly used in the assessment of kidney function. Accurate determination of GFR is necessary in several clinical situations including possible kidney donation, monitoring patients with chronic kidney disease, and calculating/adjusting doses of medications with a narrow therapeutic window. The objective of our study is to compare the GFR values calculated using two commonly used formulas: the Cockcroft-Gault (CG) formula and the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation, to the ^99mTc-DTPA modified Russell two-point values.

Methods: Northwestern Memorial Hospital uses the modified Russell two-point method with ^99mTc-DTPA to routinely screen patients for potential kidney donation. This study consists of collecting blood samples at 60 and 180 minutes post injection and measuring the plasma clearance of ultra-filtered (protein free) samples to determine the GFR value. For each of the 35 patients in the study, GFR values were also calculated using both the CG formula, CLcr=(140-Age) [asterisk](Weight (Kg))/72(Cr_serum) ([asterisk]0.85 for females), as well as the CKD-EPI equation. The CKD-EPI equation is based on patient race, gender, age, and serum creatinine. All GFR values were normalized to 1.73 m² by multiplying the GFR value by 1.73/BSA. The BSA was calculated according to Du Bois and Du Bois: BSA = (Body Weight (kg)^0.425 [asterisk] Height (cm)^0.725) [asterisk] 0.007184. In preparation for the study, all patients were instructed to hydrate by drinking 32 oz of clear liquid one hour prior to study, to avoid caffeine starting at 10:00 pm the night prior, and to follow a low-protein diet. No patients tested were <18 years of age or suspected of having compromised renal function.

Results: Patient characteristics are shown in Table 1 and GFR results are shown in Table 2.

Conclusions: When evaluating patients for potential kidney donation, accurate assessment of renal function is critical. Based on the study, estimated GFR values using both the CG and CKD-EPI formulas were much lower than those determined from the modified Russell two-point method (-25.6% and -36.1% respectively).

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