Reassessment of a classical single injection ⁵¹Cr‐EDTA clearance method for determination of renal function in children and adults. Part I: Analytically correct relationship between total and one‐pool clearance

Abstract

Background. Total plasma clearance of ⁵¹Cr‐EDTA, Cl, is widely used as a measure of GFR. Commonly, only the final part of the plasma concentration curve is measured, and a one‐pool clearance (slope‐intercept clearance), Cl₁, is computed. Empirically determined second‐order polynomials of the general form Cl = b⋅Cl₁+c⋅Cl₁² are usually used to estimate Cl from a measured Cl₁. However, theoretical considerations indicate that such corrections underestimate Cl at high values. Aims. To derive an analytically correct relationship between Cl and Cl₁ and determine the parameters involved for children and adults. Material and methods. Cl was determined in 149 subjects (M/F/children: 71/46/32) from a complete plasma concentration curve followed for 4–5 h after injection of ⁵¹Cr‐EDTA (range of clearance: 8–183 mL/min/1.73 m²). Plasma volume, PV and the “missing” area under the plasma fraction curve, a (minutes), not used for determination of Cl₁, were measured. Results. The true relationship between Cl and Cl₁ is given by Cl = Cl₁/(1+f⋅Cl₁), where f = a/PV. For men, women and children alike, the equation f = 0.0032⋅BSA^−1.3 was applicable (BSA = body surface area in m²). Estimation errors on clearance were within ±8 % for adults and ±13 % for children (95 % limits of agreement). Conclusions. The true relationship between Cl and Cl₁ of ⁵¹Cr‐EDTA is given, resulting in a common correction equation applicable for children and adults. The new equation has better mathematical behaviour than quadratic equations on very high values of clearance and takes into account dependence on body size.

• ⁵¹Cr‐EDTA
• glomerular filtration rate
• plasma clearance
• renal function
• slope‐intercept technique