Purpose: Exclusive use of the half-time of clearance of a filtration marker in the bolus injection, single compartment technique for measuring glomerular filtration rate (GFR) is a convenient approach that gives GFR (GFR(SO)) already scaled for extracellular fluid volume (ECV). It has been criticized as less accurate than the conventional, slope-intercept technique (GFR(SI)). The aim of the study was to compare the respective levels of agreement of GFR(SI) and GFR(SO) with GFR based on plasma creatinine (eGFR) used as an independent arbiter.
Methods: GFR was measured with both 51Cr-EDTA and iohexol, simultaneously injected into opposite arms. Plasma obtained bilaterally 20, 40, 60, 120, 180 and 240 min after injection was assayed for marker injected contra-laterally, 51Cr-EDTA by well-counting and iohexol by X-ray fluorescence. ECV and GFR, scaled to body surface area (BSA), were formally measured from six samples and GFR(SI) (scaled to BSA) and GFR(SO) from the last three.
Results: Disagreement between GFR(SO) measured with 51Cr-EDTA and eGFR was not significantly higher than the corresponding disagreement between eGFR and GFR(SI). Disagreement between GFR(SO) measured with iohexol and eGFR was significantly higher than between eGFR and GFR(SI). GFR(SI) and GFR(SO) correlated more closely when measured with 51Cr-EDTA than with iohexol. Individual differences between GFR(SI) and GFR(SO) using one marker correlated significantly with ECV measured with the other.
Conclusions: GFR(SO) is critically dependent on the accuracy of half-time measurement and, measured with iohexol, appeared less reliable than GFR(SI). GFR(SI) and GFR(SO) measured with 51Cr-EDTA, however, have similar levels of reliability.