Hypothesis / aims of study
There is growing interest in the promotion of bladder health and prevention of lower urinary tract symptoms (LUTS), particularly in women who are at higher risk for LUTS and associated bladder conditions. An essential step in health promotion efforts is to estimate bladder function parameters, such as daytime and nighttime urination frequencies, in women without LUTS to define the normal ranges and inform future interpretation of patient symptoms. Currently, there are no established reference ranges for urination frequencies in healthy women. A recent meta-analysis of 22 studies found wide heterogeneity across estimates of urination frequencies in healthy women, precluding generalization of reference ranges to all healthy women.1 Analyses were limited by differences in definitions of healthy women, and failure to present results separately by, or take into consideration demographic characteristics and other variables known to affect bladder function. Therefore, this study was undertaken to: 1) estimate normative reference ranges in daytime and nighttime urination frequencies in healthy women based on two operational definitions of “healthy,” and separately by age, race/ethnicity, and fluid intake; and 2) examine differences by age, race/ethnicity, and fluid intake.
Study design, materials and methods
This is a secondary analysis of data collected from the first follow-up interview of female respondents (ages 31-87 years) who participated in the Boston Area Community Health (BACH) Survey, a large, population-based study of Boston residents.2 We created two definitions of healthy participants using less restrictive (“healthy”) and strict (“elite healthy”) exclusion criteria. Exclusion criteria for healthy women were: pregnancy; congenital urinary tract abnormalities; urinary incontinence (UI); interstitial cystitis/painful bladder syndrome; use of LUTS medications; use of a chronic catheter; “bladder emptying problem as a result of nerve or muscle problem;” and progressive neurological disease. Additional exclusion criteria for elite healthy women were: poor to fair self-rated health; LUTS in the past 30 days; use of pads; previous UI treatment; self-report of “any bladder problem;” use of medications that could affect bladder function (“insulin or pills for sugar,” heart failure, or antidepressants); use of medications for pelvic pain; gynecological conditions or comorbidities that could affect bladder function (e.g., irritable bowel syndrome, pulmonary disease, heart failure, or stroke); prior bladder or UI surgery; genitourinary cancer; current treatment for any cancer other than skin cancer; or self-reported “a lot” of health-related activity limitations.
All analyses were weighted to account for the BACH sampling design. Descriptive statistics were used to describe daytime and nighttime urination frequencies for each healthy definition, which were also stratified by age group (31-44, 45-64, 65+ years) and race/ethnicity (Black, Hispanic, White). Normative reference values were identified by the quantiles corresponding to the middle 95% of the distribution of urination frequencies for the two definitions. Reference values were also calculated by age, race/ethnicity, and fluid intake. Unadjusted and adjusted generalized linear regression with a log-link was used to estimate the rate ratio of daytime and nighttime urination frequencies by age group, race/ethnicity, and fluid intake adjusting for body mass index, number of pregnancies, hormonal status, and smoking status. Total daily fluid intake of ≥ 50 oz and ≥ 75oz with the majority consumed after 5pm was also explored in relation to nighttime urination frequency
Of the 2534 women who completed the BACH follow-up survey, 1505 women met healthy eligibility criteria, while 300 women met the stricter elite healthy criteria. Overall, the reference range for elite healthy women was ≤9 voids/day and ≤2 voids/night, whereas those in the relaxed healthy definition voided ≤10 times/day and ≤4 times/night (Table 1). After adjustment, there were no significant differences in daytime or nighttime urinations by age or race/ethnicity except for Black women who had 11% less daytime voids than White women (Table 2). Women who reported fluid intakes of ≤ 49oz had 29% less daytime voids on average than those who consumed the recommended amount of fluid, whereas there were no significant differences noted for those who consumed ≥ 75oz. Consuming the majority of fluids after 5 PM was associated with a decrease in nighttime voids by 23% though not found to be statistically significant.
Interpretation of results
The normative reference range for daytime urination frequency was similar in women using strict and less restrictive health definitions, whereas, nighttime urinations differed. Findings suggest that voiding up to 10 times/day and 4 times/night may be “within normal limits” for women with comorbidities and certain medication use, whereas women with elite health void up to 9 times/day and 2 times/night. These results add further support to the prior meta-analysis1 conclusion that there is a wide range of normality in healthy women and challenge the current definition of overactive bladder with respect to 24-hour urination frequency. In healthy women, when adjustments are made for key covariates, there was little influence on urination frequencies by age or race/ethnicity, except for Black women, who voided less frequently than White women. Having low daily fluid intake was associated with lower daytime frequency but consuming the majority of fluids after 5 PM had no association with nighttime frequency.