A practical method of measuring glomerular filtration rate by Iohexol clearance using dried capillary blood spots

Background: Exogenous tracer-based methods of measuring glomerular filtration rate (GFR) are difficult to perform, whilst creatinine-based estimation formulae are inaccurate. Methods: We assessed a new technique of measuring iohexol clearance using timed dried capillary blood spots. A reference GFR was measured in 81 subjects (GFR 15 - 124 ml/ min/1.73 m(2)) by iohexol clearance using three venous samples (2, 3 and 4 h after an intravenous bolus). GFR was estimated by six test methods; iohexol clearance using (i) 3 blood spots (2, 3, 4 h); (ii) 2 blood spots (2, 4 h) and (iii) 1 blood spot ( 4 h); (iv) the Modification of Diet in Renal Disease (MDRD) formula; ( v) the Cockcroft-Gault formula, and ( vi) a formula estimating GFR from serum cystatin C concentration. For each test method the bias and precision were calculated as the mean and standard deviation (SD) of the 'GFR differences' ( test method GFR - reference GFR). Results: The limits of agreement (bias +/- 1.96 x SD; in ml/min/1.73 m(2)) were: ( i) 1.1 +/- 15.1 for 3-spot iohexol clearance; ( ii) 0.6 +/- 14.9 for 2-spot iohexol clearance; ( iii) 4.5 +/- 21.2 for 1-spot iohexol clearance; ( iv) - 15.7 +/- 33.3 for the MDRD formula; ( v) - 9.6 +/- 32.9 for the Cockcroft-Gault formula, and ( vi) - 12.1 +/- 31.7 for the Cystatin C formula. The accuracy of all six test methods was similar among individuals with GFR < 60 ml/min/ 1.73 m(2); however, in individuals with GFR >= 60 ml/min/ 1.73 m(2), the MDRD, Cockcroft-Gault and Cystatin C formulae were all imprecise and systematically underestimated GFR. Conclusions: Blood spot iohexol clearance provides a potentially practical method of estimating GFR accurately in large-scale epidemiological studies especially among individuals without established chronic kidney disease. Copyright (c) 2007 S. Karger AG, Basel.