The plasma phenolsulfonphthalein index (PSPI) of renal function: I. Theoretical considerations and investigative studies.

A simple method is described for the assessment of renal function, without collection of urine. Phenolsulfonphthalein (PSP) is injected intravenously in a dosage of 1 mg./kg. body weight, and blood samples are taken at timed intervals. Only one venepuncture is required. The index is based on the rate of decrease in concentration of PSP in the plasma 15 to 35 minutes after injection, during which time this rate is exponential. The methodology and normal range, and the influence of dose, urine flow rate, age and body surface are defined. The extrarenal component of the PSP index and the distribution of PSP in body fluids are discussed.