THE 15-minute phenolsulfonphthalein (PSP) excretion test is a widely used clinical test of renal function. Fifteen minutes after the intravenous injection of 6 mg. of phenolsulfonphthalein the patient empties his bladder, and the amount of dye excreted in the urine specimen is determined and expressed as a percentage of that injected. The excretion of 25 per cent of the injected dye within 15 minutes is generally considered to be the lower limit of normal renal function.1 The 15-minute post-injection urine sample has generally been considered to be the ideal standard, since, in longer periods of time, even poorly functioning kidneys will be able to excrete a good percentage of the injected dye.
Frequently, under office and hospital conditions it is impossible to obtain an exactly timed 15-minute urine specimen. This may be due to neglect of the examiner to make the collection or to inability of the patient to