MEASUREMENT OF GLOMERULAR-FILTRATION RATE USING NONRADIOACTIVE IOHEXOL - COMPARISON OF 2 ONE-COMPARTMENT MODELS

Radioisotopic methods for the determination of the glomerular filtrati on rate (GFR) are highly accurate but require the collection of multip le blood and urine samples and are costly to perform due to personnel, material, and analysis costs. Nonradioactive methods of GFR determina tion have the potential of minimizing procedure costs while preserving accuracy. We determined the GFR simultaneously by I-125-iothalamate a nd nonradioactive iohexol clearance methods in 41 adults. The study gr oup consisted of 54% males, with a mean age of 50.7 (range 28-79) year s and a mean GFR by I-125-iothalamate clearance of 66.5 +/- 28.3 (rang e 10-118) ml/min. The iohexol concentrations were measured by a simpli fied high-performance liquid chromatography method that did not requir e sample preparation. The iohexol plasma clearance was calculated by b oth a new one-compartment model as well as by Jacobsson's one-compartm ent model. Using Jacobsson's single-sample model and data from the 240 -min point, there was an excellent correlation between I-125-iothalama te and nonradioactive iohexol clearance values: r(2) = 0.95, standard error of the estimate = 11.4 ml/min, and intrapatient coefficient of v ariation = 16.9%. However, this formula tended to overestimate GFRs <3 0 ml/min and to underestimate GFRs >80 ml/min. The new one-compartment model is a modification of Bubeck's model, originally used for the de termination of renal plasma blood flow. Using this modified model, the re was an excellent correlation between I-125-iothalamate and nonradio active iohexol clearance values at all levels of GFR tested: r(2) = 0. 95, standard error of the estimate = 9.2 ml/min, and intrapatient coef ficient of variation = 13.7%. In conclusion, the determination of the plasma clearance of iohexol by a nonradioactive technique and a monoex ponential model is a simple and accurate method of determining the GFR in patients with varying degrees of renal impairment.