Background: Glomerular filtration rate is a key parameter in kidney disease. The Radionuclides in Nephrourology Committee has recommended a single-sample method with 99mTc-DTPA based on the mean sojourn time. This study was done to develop the method for use with iohexol making the method more available.
Methods: The single-sample formula was derived for group I (n = 48, Cl = 8-188 ml/min) and applied on group II (n = 47) and on group III (n = 123). In groups I and II, reference clearance was determined according to Sapirstein and in group III according to Brøchner-Mortensen.
Results: The formula Cls= (-b+sqrt b2-4ac)/(2a) (a = (-6.49 x 10(-6) x t + 8.85 x 10(-4)) x t, b = 1.143 x t and c = ln[(C(t))x(ECV/Q0)](ECV) was derived for patients with estimated Cl > 30 ml/min with the best result if the single sample was obtained between 4 and 5 h. Extracellular volume was estimated as ECV =9985 x BSA - 3431. The formula ClS(24 h) = -ln[(C(t)) x (ECV/Q0)](ECV)/(t) was developed for patients with estimated Cl <30 ml/min with a single sample at 24 h. With this combined approach SDdiff was 2.7 ml/min in group II and 3.1 ml/min in group III.
Conclusions: An accurate determination of iohexol clearance can be obtained from a single plasma sample applying the mean sojourn time approach. A separate formula must be used for patients with low clearance values. Body surface area (BSA), injected amount of iohexol (Q0), time when the single sample is drawn (t) and the concentration of iohexol [C(t)] in the sample are needed for the calculations.