Optimum time of blood sampling for determination of glomerular filtration rate by single-injection [51Cr]EDTA plasma clearance

We have investigated the influence on reproducibility of total [51Cr]EDTA plasma clearance (E) of various times and numbers of blood samples in patients with normal (13 patients) and low (14 patients) renal function. The study aims at fixing a clinically useful procedure suitable for all levels of renal function. Six different types of E were evaluated with time periods for blood sampling between 3 and 5 h after tracer injection, and the variation from counting radioactivity, s1, was determined as part of the total variation, s2. Optimum mean time, t(E), for blood sampling was calculated as a function of E, as the mean time giving the least change in E for a given change in the 'final slope' of the plasma curve. For patients with normal E, s1 did not contribute significantly to s2, and t(E) was about 2h. For patients with low renal function s1 contributed significantly to s2, and t(E) increased steeply with decreasing E. The relative error in s1 from fixed Etypes was calculated for all levels of renal function. The results indicate that blood sampling individualized according to predicted E values is not necessary. A sufficient precision of E can be achieved for all function levels from three blood samples drawn at 180, 240, and 300 min after injection. (Auth.)