The diversity of our large population based RRISK cohort provides a unique opportunity to examine differences in incontinence among 4 major racial/ethic groups within 1 study. To the best of our knowledge, this report is the first population based study with all major racial/ethnic groups to ascertain incontinence by type, thereby avoiding the problems inherent in comparing incontinence prevalence across studies. We found significant differences in the prevalence of incontinence in the last year with Hispanic women having the highest prevalence, followed by white, black and Asian-American women. After adjustment for multiple risk factors, white women appeared to have almost twice the risk of stress incontinence as Asian-American women, and almost 3 times the risk of black women, while differences in the risk of urge incontinence were relatively small and nonsignificant.
In contrast to our results, other recent studies have reported a lower prevalence of incontinence among Hispanic women.1,8,10
These discrepant findings between other studies and our study may be due to differences in the age of the populations studied, definitions of incontinence, or ascertainment of incontinence. Ethnic group composition may have also contributed to the variation in studies. For example, the composition of our Hispanic population was mostly of Mexican origin while in other studies the Hispanic women were from the Caribbean.8
Further studies of variations within subsets of the same racial/ethnic group would be helpful.
We found incontinence to be more prevalent in white women than in black women, a finding consistent with previous population based studies,2,3,6,7,9,10
including studies that used multivariate analysis to adjust for incontinence risk factors.6,7,9,10
This result is particularly noteworthy in that black women are more likely to have risk factors for incontinence, including obesity, diabetes, smoking and hysterectomy.
We found lower rates of stress incontinence among black women, but higher rates of urge incontinence compared to white women. This finding of a relative predominance of urge incontinence in black patients is consistent with the results of at least 1 other large epidemiological study6
and with studies of patients referred for urogynecological evaluation.4,18
This demonstrates the importance of characterizing incontinence by type in that simply comparing the overall prevalence of incontinence can be misleading when stress and urge incontinence differ in opposite ways.
A few studies have compared physiological parameters between black and white19
women to explain the lower prevalence of stress incontinence in black women. These studies have reported black women to have higher urethral closure pressure, greater urethral length and pubococcygeal muscle strength, larger urethral volume and, paradoxically, greater vesical mobility.19,20
The extent to which these differences, if confirmed, can explain differences in the prevalence of stress incontinence is not yet clear.
We found a substantially lower prevalence of stress and urge incontinence in Asian-American compared to white or Hispanic women. However, in the multivariate model this difference remained significant only for stress incontinence, with Asian-American women having a nearly 50% lower risk than white women. Two studies have reported Asian-American women as having a lower prevalence of incontinence compared to white women, but neither study examined incontinence by type or adjusted for other variables.8,10
While we cannot rule out the possibility of differential underreporting of incontinence among the race/ethnicity groups, we took several steps to minimize underreporting in general. Women were initially asked about accidental leakage of urine on the self-administered questionnaire to minimize underreporting due to embarrassment. Women who reported never having accidentally leaked urine were asked about urine leakage by the interviewer, using medical and vernacular terms, to be sure they understood the question. For women who did report incontinence, the questions to determine frequency and type of incontinence were asked by the interviewer, who clarified the questions if needed and checked answers for consistency.
Our study had several limitations that should be considered when interpreting the results. As in previous large epidemiological studies, incontinence in the current study was defined by self-report. While the questions used to distinguish between stress and urge incontinence have been shown to correlate reasonably well with urodynamic classification15
and to be reproducible,16
there are inevitable differences between self-report, urodynamic classification and clinical diagnosis in characterizing the presence, severity and type of urinary incontinence, as each are effectively measuring different, although related, phenomena. We do not know if using clinical or urodynamic definitions of stress and urge incontinence would result in the same associations with race/ethnicity.
Another limitation was a bias toward enrolling women with incontinence. Correcting for this bias moved the prevalence of incontinence in Hispanic women to that of white women, but it increased the differences between white women and Asian or black women, and did not affect our conclusions. Finally, we enrolled women who had been long-term members of a large prepaid health delivery system. Before initiating the study we determined that women who were members since age 18 were similar to all women members of the same age with respect to multiple characteristics, including the number of office visits in the last 27 months, prior hysterectomy and use of hormone replacement therapy. However, this aspect of our study should be considered in generalizing our results to other populations.