Optimum Time of Blood Sampling for Determination of Glomerular Filtration Rate by Single-Injection [⁵¹Cr]EDTA Plasma Clearance

Abstract

We have investigated the influence on reproducibility of total [⁵¹Cr]EDTA plasma clearance (E) of various times and number 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, sc, was determined as part of total variation, St. 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, S_c did not contribute significantly to sr, and t(E) was about 2 h. For patients with low renal function sc contributed significantly to st, and t(E) increased steeply with decreasing E. The relative error of sc from fixed E types 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 180, 240, and 300 min after injection.

Key Words:

• [⁵¹Cr]EDTA plasma clearance
• glomerular filtration rate
• monoexpo-nential functions
• reproducibility