Abstract
Measurements of renal glomerular filtration rate using sodium iothalamate-131I by sample counting method in 39 cases and external counting method in 20 cases were investigated and compared with thiosulfate clearance by a single injection method performed simultaneously.
First, in sample counting method, after intravenous injection of SI-131I, samples of blood were obtained at 30 and 40 minutes, sample of urine collected from 20 to 50 minutes. The activity of 131I in plasma and urine were measured in well type scintillation counter in counts per milliliter per minute. All clearances were calculated by the formula C=UV/P, where C in the plasma clearance in ml/min, V the urine volume in ml/min, U the urine activity and P the plasma activity (epm) .
Second, in external counting method, plasma disappearance curve obtained by external monitoring over the head or the heart during 60 minutes.
This curve could be described by two components satisfying the equation Y=Ae-b1t+Be-b2t, where Y is the concentration of isotope at time t, A the time zero intercept of terminal slow component with slope b1, B the time zero intercept of early rapid component with slope b2. Also the above equation conducts being based on assumed two compartment model, then the dose of SI-131I injected into compartment one is mixed and distributed. Next diffused into compartment two and consist in excretory and intercompartmental clearances in compartments system. Clearance (C) was calculated from the plasma curve by the formula derived by Sapirstein and co-workers.
C=I×r1r2/Ar2+Br1ml/min.
Where 131I equals injected radioactivity of dose and A, γ1, B and γ2 is obtained from the slopes and intercepts of the components of the corrected curve as plotted on semilogarithmic graph paper.
Correlation coefficient for sample counting method and simultaneous thiosulfate clearance was 0.89, and for external counting method similarly 0.86, These counting methods seem as useful clinical examination of measurement of glomerular filtration rate, especially external counting method without urine collection and multiple venepunctures.
