Data sources

To estimate the number of women who will undergo surgery for SUI and POP in the future, we utilized 2 critical components of data: (1) inpatient and outpatient surgery rates for SUI and POP; and (2) US population growth estimates. To be as comprehensive as possible, we evaluated both inpatient as well as outpatient procedures. Inpatient data were based on the 2007 Nationwide Inpatient Sample (NIS), Healthcare Cost and Utilization Project, Agency for Healthcare Research and Quality. The 2007 NIS represents the largest publicly available database of hospital discharges in the United States and provides weights to account for its complex sampling design to calculate national estimates.

Outpatient data were based on the 2006 National Survey of Ambulatory Surgery (NSAS), National Center for Health Statistics, Centers for Disease Control and Prevention. The NSAS was conducted annually from 1994 through 1996 and then again in 2006 and includes surgical procedures performed on an outpatient basis in a hospital-based or freestanding ambulatory surgery center but does not include physician office-based procedures.

The second critical component was the US population projections, which is provided by Census Bureau Population Projections Program. The 2008 projections were based on the 2000 census and were produced using a cohort-component method. These projections are based on key assumptions for future births, deaths, and net international migration, and they estimate population growth from 2010 through 2050 by age, sex, race, and ethnicity. Because the primary set of population projections are based on middle estimates for all 3 parameters (fertility rate, life expectancy, and net immigration), these data are also referred to as the middle series. The middle series predicts that the total population will grow from 310.2 million in 2010 to 439.0 million in 2050.

This study was exempt from institutional review board review because only publicly available data were analyzed.

Analysis

The first step in the analysis was to determine the annual number of women who underwent both inpatient and outpatient procedures for SUI and POP. For inpatient SUI procedures, cases were identified by searching the NIS for any discharge associated with both a diagnosis for UI and a SUI procedure using a question diagram. For SUI, diagnoses and procedures were based on International Classification of Diseases, Ninth Revision, Clinical Modification ( ICD-9-CM ) codes. UI ICD-9-CM diagnoses codes included 625.6, 599.82, and 788.30-788.39 and procedure codes included 59.3-59.79. For prolapse, the ICD-9-CM diagnoses codes were 618.0-618.8, and the procedures codes ranged from 68.3–68.9, 69.2–69.29, and 70.4–70.92 and included all routes of surgery (vaginal, abdominal, and minimally invasive). Similarly for outpatient procedures, ICD-9-CM codes were used to identify women who underwent SUI or POP surgery in the NSAS.

We accounted for the complex sampling design and used the sampling weights, strata, and cluster information provided by the 2007 NIS to determine national estimates for SUI and POP surgery using STATA 10.0 (StataCorp, College Station, TX). For outpatient data, we used sampling weights provided by the 2006 NSAS to obtain national estimates of SUI and POP procedures using SAS 9.1.3 (SAS Institute Inc, Cary, NC) and STATA 10.0.

Next, we calculated the age-specific rates for SUI and POP surgery. Rates were calculated by dividing the number of women who had surgery in a specific age group in that year by the total number of women in that same age group in the same year. We calculated the rates of surgery for women in 20-year age cohorts (20-39, 40-59, 60-79, ≥80) to determine the age-specific rates for each type of surgery. We also estimated the 95% confidence intervals (CIs) for inpatient and outpatient SUI and POP surgery by 20-year age cohort; however, for several of the age groups in outpatient surgeries, we were unable to calculate the 95% CI due to the relatively low frequency of the procedures. Since we did not have consistent 95% CI data for all age cohorts for both SUI and POP surgery, we did not use the 95% CI in our sensitivity analysis.

We then applied the rates for both inpatient and outpatient SUI and POP surgery to the population projections. We multiplied the age-specific surgery rates by the number of women in each age cohort, for each decade from 2010 through 2050. Given that we cannot accurately predict changes in PFD surgery rates in the future, we assumed that the rates for both inpatient and outpatient SUI and POP surgery remain unchanged from 2010 through 2050.

For our primary analysis, we used the 2008 national population projections middle series. In addition to the middle series, the US Census Bureau also estimates the lowest and highest population projections. Because the lowest and highest series combine the extreme values of all 3 major components that favor the lowest and highest population growth, respectively, these extreme projections do not represent likely scenarios. However, they represent the extremes between which the most likely outcomes should occur. Projections based on the lowest series estimate a total population of 291.4 million in 2010 to 313.5 million in 2050. In contrast, the highest series predicts the population to grow from 310.9 million in 2010 to 552.8 million in 2050. Given that the lowest and highest series represent the most extreme scenarios for population growth, these estimates were ideal for sensitivity analyses. We applied the inpatient and outpatient surgery rates for SUI and POP procedures to the lowest and highest series to present the widest range for future predictions.