That is not necessarily true for all IC patients. IC patients tend to urinate very frequently, both daytime and nighttime. Many patients urinate more at night because of their physiology. Sometimes fluid builds up in the lower extremities over the course of the day. When they lie down and the legs go up, the fluid comes out of the tissues; it gets back into the bloodstream, is filtered through the kidneys and you then urinate a lot. In order to determine whats going on, most patients fill out a "voiding diary". For two days patients are asked to write the time and the number of ounces voided. A typical IC patient will have recorded multiple 1 to 3 ounce voids, both day and night. Here and there, a patient who voids 16 to 25 ounces at a clip, multiple times during the day. These patients dont have IC; they either have some metabolic problem to account for this or theyre compulsive water drinkers. Patients whose voiding pattern varies from 3 ounces to 15 ounces usually have PFD as their biggest problem since very few patients with IC can hold 16 ounces in their bladders. Yes, a simple voiding diary can be a very helpful tool when making a diagnosis.