A simple method for the estimation of glomerular filtration rate

A simple method is presented for indirect estimation of the glomerular filtration rate from two venous blood samples, drawn after a single injection of a small dose of [125I]sodium iothalamate (10 muCi). The method does not require exact dosage, as the first sample, taken a few minutes (t = 5 min) after injection, is used to normalize the value of the second sample, which should be taken in between 2-4 h after injection. The glomerular filtration rate, as measured by standard inulin clearance, may then be predicted from the logarithm of the normalized value and linear regression formulas with a standard error of estimate of the order 1-2 ml/min/1.73 m2. The slope-intercept method for direct estimation of glomerular filtration rate is also evaluated and found to significantly underestimate standard inulin clearance. The normalized 'single-point' method is concluded to be superior to the slope-intercept method and more sophisticated methods using curve fitting technique, with regard to predictive force and clinical applicability.