Study Objective. To evaluate the bias and precision of three methods of mea
suring glomerular filtration rate (GFR) relative to a standard method.
Design. Prospective, outpatient study.
Setting. University-affiliated general clinical research center.
Patients. Twenty-six patients with various degrees of renal function (GFR r
ange 25-151 ml/min/1.73 m(2)).
Interventions. Each patient received iothalamate twice during the study vis
it, first as a bolus injection and then as a priming dose followed by a con
stant-rate infusion for 2.5 hours.
Measurements and Main Results. Plasma (Clp(IVB)) and renal clearances (Clr(
IVB)) after bolus injection and plasma clearance during constant-rate infus
ion (Clp(INF)) were compared with standard renal clearance during constant-
rate infusion (Clr(INF)). All three measures were highly correlated with Cl
r(INF) (r>0.90, p<0.001). The mean Clr(IVB) was not significantly different
from Clr(INF) (106.3 +/- 30.4 vs 104.2 +/- 28.5 ml/min/1.73 m(2)) and prov
ided a precise (8.8%, 95% CI 6.5-11.1%) and unbiased measure of GFR. Both C
lp(IVB) and Clp(INF) were positively biased; values exceeded Clr(INF) by 11
.8 +/- 11.1 (p=0.0001) and 10.5 +/- 12.5 ml/min/1.73 m(2) (p=0.0003), respe
ctively. Use of a nonrenal correction factor of 9.8 and 10.5 ml/min/1.73 m(
2) for infusion and bolus plasma clearance values, respectively, eliminated
bias and improved the precision of these methods.
Conclusions. Iothalamate renal clearance after bolus injection is a simple,
accurate, and precise measurement of GFR and may be a useful alternative t
o the standard infusion method in clinical investigations. The corrected pl
asma clearance provides a simple index of GFR for clinical practice.