SIGNIFICANT quantities of residual urine are conducive to urinary-tract infection. Retention of this pool of urine also makes treatment of the infection difficult, and predisposes to reinfection. At times, there is a reluctance or a relative contraindication to use of a catheter simply to determine the quantity of residual urine. Phenolsulfonphthalein (PSP) excretion can be used, without catheterization, to screen patients for residual urine and by modifying the method can measure the volume of residual urine.
The fractional PSP excretion test is widely used to assess renal function. In this test, urine collections are made 15, 30, and 60 minutes after the dye is given. Since the rate of dye excretion by the kidney steadily decreases, the amount of dye in the urine collections should also decrease. If it does not, it is an indication of clinically significant residual urine.
If a known amount of dye is added to an