Bladder diary measurements in asymptomatic females: functional bladder capacity, frequency, and 24-hr volume

To study the effects of age and 24-h volume (V(24)) on bladder diary measurements of voiding frequency (F(24)) and functional bladder capacity (FBC) from an asymptomatic female population. Also, to use these data to develop clinical reference values. (We use FBC as a generic term for bladder diary volume-per-void measurements.)

Computer-processed 3-day bladder diaries were collected from 161 females (median age: 46.6 years; range = 19.6-81.8 years) claiming no urological symptoms, previous pelvic surgery and diseases and medications effecting urologic function. Regression analysis was used to investigate relationships among age, FBC and V(24).

Both FBC and F(24) increase as V(24) increases (P < 0.0005). With aging, F(24) increases (P = 0.026) and FBC may decrease slightly (P = 0.02-0.08). There is a concave downward, curvilinear relationship between age and V(24). We used multiple regression to generate tables of FBC and F(24) "normal limits" adjusted for these simultaneous influences of V(24) and age. Removing their relationships to age reduces the variability of FBC and F(24) reference values by 50% and 20%, respectively.

Our finding, supported by others, that, with increasing V(24), FBC increases more than F(24) suggests an adaptive mechanism that adjusts FBC to urine production to minimize changes in voiding frequency. We illustrate adjustment of reference values for age and V(24) by calculating traditional clinical "normal limits." However, the probable large overlap between "normal" and "abnormal" suggests that it may be more useful to report bladder diary measurements as reference population percentiles rather than to designate them "normal" or "abnormal".