Total clearance and extracellular volume with 125I-iothalamate in peritoneal dialysis

To assess treatment adequacy by calculating total clearance (CL) in patients on peritoneal dialysis (PD), fluids must be collected over 24 hours, a laborious procedure and prone to inaccuracy. CL calculation from the plasma disappearance curve of an injected tracer has none of the above inconveniences. We therefore compared creatinine clearance CR-CL with CL calculated from the plasma disappearance curve of 125I-iothalamate (IO) injected in a single intravenous bolus. Twelve patients aged 63 (44-80) years and on PD for four (1-44) months were studied in hospital. Nine plasma samples were taken for IO-CL after a bolus of 21 kBq/kg of tracer. The least-squares biexponential fitting according to Gauss-Newton-Raphson was then carried out: [Aexp(-at) + Bexp(-bt)], and clearance was calculated by the formula, IO-CL = dose/AUC, where AUC = A/a + B/b. Both CLs were normalized for 1.73 m2 of body surface. Agreement (r = 0.746, p = 0.005) for the CR-CL (6.8 +/- 1.9 mL/min) and IO-CL (7.6 +/- 1.9 mL/min) was good, with a difference of 0.9 +/- 1.4 mL/min (t = 2.182, p = 0.052). Extracellular volume (ECV), calculated from the IO plasma disappearance curve with the formula, ECV = dose/ bAUC, and including the endoperitoneal fluid, was 19.8 +/- 2.9 L (29.5 +/- 6.2% body weight).