In determining the cause of hydronephrosis, the location of obstruction can be determined with a Whittaker (or pressure perfusion) test, wherein the collecting system of the kidney is accessed percutaneously, and the liquid is introduced at high pressure and constant rate of 10ml/min while measuring the pressure within the renal pelvis. A rise in pressure above 22 cm H2O suggests that the urinary collection system is obstructed. When arriving at this pressure measurement, bladder pressure is subtracted from the initial reading of internal pressure. (The test was first described by Whittaker in 1973 to test the hypothesis that patients whose hydronephrosis persists after the posterior urethral valves have been ablated usually have ureters that are not obstructed, even though they may be dilated.)