DRAWBACKS OF THE CONSTANT-INFUSION TECHNIQUE FOR MEASUREMENT OF RENAL-FUNCTION

We investigated the validity of the steady-state constant infusion met hod (CIM), in which quantitative urinary recovery and constant plasma concentrations of the solute infused are required. Successive 3-h clea rances of inulin and p-aminohip-puric acid (PAH) were determined for 2 7 h in 25 patients with renal disease. Results were compared with the standard method of bladder clearance (StM) and with a modified CIM (Mo dCIM). The 24-h urinary recovery was incomplete for both inulin and PA H. Mean 24-h ModCIM inulin clearance overestimated StM by 4.5 ml . min (-1) . 1.73 m(-2) (range 0-9, P < 0.001) independent of the extent of renal impairment and pointed to slow distribution and/or extrarenal cl earance of inulin. For PAH, the difference between ModCIM and StM clea rance was related to the average PAH clearance by ModCIM and StM (r = 0.78). Furthermore, neither plasma inulin nor PAH became completely co nstant, because of the circadian rhythm in renal function. In conclusi on, the conditions of the steady-state CIM technique are not fulfilled , and the method is not suitable for accurate measurement of inulin an d PAH clearance, especially when the clearance is low.