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Original Investigation |

Impact of Minimally Invasive Surgery on Medical Spending and Employee Absenteeism FREE

Andrew J. Epstein, PhD1,2,3; Peter W. Groeneveld, MD, MS1,2,3; Michael O. Harhay, MPH2; Feifei Yang, MS2; Daniel Polsky, PhD2,3
[+] Author Affiliations
1Center for Health Equity Research and Promotion, Philadelphia Veterans Affairs Medical Center, US Department of Veterans Affairs, Philadelphia, Pennsylvania
2Division of General Internal Medicine, Department of Medicine, Perelman School of Medicine, University of Pennsylvania, Philadelphia
3Leonard Davis Institute of Health Economics, University of Pennsylvania, Philadelphia
JAMA Surg. 2013;148(7):641-647. doi:10.1001/jamasurg.2013.131.
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Published online

Importance  As many surgical procedures have undergone a transition from a standard, open surgical approach to a minimally invasive one in the past 2 decades, the diffusion of minimally invasive surgery may have had sizeable but overlooked effects on medical expenditures and worker productivity.

Objective  To examine the impact of standard vs minimally invasive surgery on health plan spending and workplace absenteeism for 6 types of surgery.

Design  Cross-sectional regression analysis.

Setting  National health insurance claims data and matched workplace absenteeism data from January 1, 2000, to December 31, 2009.

Participants  A convenience sample of adults with employer-sponsored health insurance who underwent either standard or minimally invasive surgery for coronary revascularization, uterine fibroid resection, prostatectomy, peripheral revascularization, carotid revascularization, or aortic aneurysm repair.

Main Outcomes and Measure  Health plan spending and workplace absenteeism from 14 days before through 352 days after the index surgery.

Results  There were 321 956 patients who underwent surgery; 23 814 were employees with workplace absenteeism data. After multivariable adjustment, mean health plan spending was lower for minimally invasive surgery for coronary revascularization (−$30 850; 95% CI, −$31 629 to −$30 091), uterine fibroid resection (−$1509; 95% CI, −$1754 to −$1280), and peripheral revascularization (−$12 031; 95% CI, −$15 552 to −$8717) and higher for prostatectomy ($1350; 95% CI, $611 to $2212) and carotid revascularization ($4900; 95% CI, $1772 to $8370). Undergoing minimally invasive surgery was associated with missing significantly fewer days of work for coronary revascularization (mean difference, −37.7 days; 95% CI, −41.1 to −34.3), uterine fibroid resection (mean difference, −11.7 days; 95% CI, −14.0 to −9.4), prostatectomy (mean difference, −9.0 days; 95% CI, −14.2 to −3.7), and peripheral revascularization (mean difference, −16.6 days; 95% CI, −28.0 to −5.2).

Conclusions and Relevance  For 3 of 6 types of surgery studied, minimally invasive procedures were associated with significantly lower health plan spending than standard surgery. For 4 types of surgery, minimally invasive procedures were consistently associated with significantly fewer days of absence from work.

As growth in US medical care expenditures continues to exceed overall economic growth,1 there has been an increasing emphasis placed on establishing the value of medical and surgical technologies. Technological innovation in medical care has played an important role on both sides of the value equation—affecting both benefits and costs. Thus far, comprehensive information about the value of many treatments has been limited, in part owing to a lack of available data. Most studies on the value of medical technology have focused on health care expenditures and clinical outcomes but have ignored other measures of benefit and cost such as the impact of medical innovation on worker productivity. Employers in particular are interested in measuring the effects of medical care on worker productivity because employers are the primary sponsors of health insurance for nonelderly employed individuals and their families, who comprise 55% of Americans.2

During the past 2 decades, many surgical procedures have undergone a transition from a standard, open surgical approach to a minimally invasive one using laparoscopic, endoscopic, and catheter-based techniques. The diffusion of minimally invasive surgery may have affected expenditures in terms of both the cost per procedure and the number of procedures, as has been shown for prostatectomy.3 As minimally invasive surgery typically permits shorter hospital stays and facilitates rapid postoperative recovery,4,5 it is likely that such procedures have affected worker absenteeism as well.

Accordingly, we sought to assess the effects of switching from standard to minimally invasive surgery on both medical care expenditures and worker absenteeism in a large national sample of patients for 6 types of surgery that have both standard and minimally invasive options. It would be difficult to generalize from only 1 type of surgery, as each has a unique set of attributes. Conversely, across the 6 types of surgery we examined, there is substantial variation in many dimensions, including the disease being treated, the specialty of the operating physician, and the level of clinical complexity. Thus, studying this group of surgery offers a broad picture of the economic effects of minimally invasive procedures.

Study Sample

We identified a sample of adults aged 18 to 64 years who were enrolled in employer-sponsored health insurance and underwent 1 of 6 types of surgery. The sample was drawn from the 2000 to 2009 Truven MarketScan Research Databases, which include insurance claims data and employer-provided absenteeism data and are collected from self-insured firms and health plans around the United States. The numbers of firms and plans contributing data increased steadily during the study period; for the 3 most recent years, data were collected from 150 firms and 21 health plans covering all 50 states and Washington, DC.

We analyzed 6 types of surgery, each with both standard and minimally invasive treatment options, listed in descending order of sample size: (1) coronary revascularization (coronary artery bypass graft surgery vs percutaneous coronary interventions); (2) uterine fibroid resection (open abdominal hysterectomy and myomectomy vs vaginal/laparoscopic hysterectomy, vaginal/laparoscopic myomectomy, and uterine artery embolization); (3) prostatectomy (radical vs laparoscopic [with or without robotic assistance]); (4) revascularization of peripheral arterial occlusive disease (open surgery vs endovascular treatment); (5) carotid revascularization (carotid endarterectomy vs carotid arterial stenting); and (6) aortic aneurysm repair (open vascular surgery vs endovascular treatment). Diagnosis and procedure codes that define each surgery cohort are listed in eTable 1 in Supplement.

Patients were included in the study cohort based on their having an insurance claim indicating receipt of one of the study’s types of surgery between January 1, 2001, and December 31, 2008, accompanied by a relevant study diagnosis in the MarketScan Commercial Database. For individuals with multiple claims for a given surgery, the earliest date was selected as the index surgery date. Individuals were excluded if they did not have continuous insurance coverage in the same plan for the 12 months before and after their index surgery date or if their total spending was less than $1, which implied erroneous data (eTable 2 in Supplement).

Because absenteeism data were available only for individuals who were employees of self-insured firms, we limited the analysis of absenteeism to this subset of patients. We linked data about paid time off and short-term disability workplace absences from the MarketScan Health and Productivity Management Database. We excluded patients who had 0 days of absence, as this implied erroneous data.

Outcomes

We assessed 2 outcomes: (1) health plan expenditures on medical care, including both medical and pharmacy costs; and (2) days absent from work, including vacation, sick leave, and short-term disability. Spending was adjusted to 2009 real dollars using the general Consumer Price Index. Following prior conventions,6 we divided the time around the procedure index date into 3 periods: baseline (−380 days to −15 days), perioperative (−14 days to +28 days), and postoperative (+29 days to +352 days). Our outcomes reflect spending and absenteeism during the combination of the perioperative and postoperative periods.

Statistical Analysis

Patient characteristics—including surgery type, age, sex, number of Elixhauser comorbidities,7 US census region, urban vs rural residence, and health plan type—were compared in the 6-surgery pooled sample according to whether the patients had undergone standard or minimally invasive procedures, using t tests or χ² tests as appropriate. Comparisons were performed for both the full sample and the subsample of patients with absenteeism data.

For each procedure cohort, we used analysis of variance to compare mean medical expenditures by receipt of standard vs minimally invasive procedures for the baseline period and the combination of perioperative and postoperative periods (ie, from −14 to +352 days around the index procedure date). Parallel comparisons were made for absenteeism using analysis of variance with multiple imputation, as described in the eAppendix in Supplement.8

To assess the independent effects of undergoing minimally invasive surgery on medical expenditures and absenteeism, for each procedure cohort we estimated a series of regression models in which the outcome was either spending or absenteeism during the perioperative plus postoperative period. The baseline-adjusted model specification controlled for the patient-specific value of the outcome (ie, spending or absenteeism) during the baseline period. This approach is preferred to modeling a change score because it explicitly accounts for preexisting differences in baseline values of spending or absenteeism among patients receiving different surgical approaches.9 The fully adjusted model specification added controls for year of procedure and the patient’s propensity score for receiving minimally invasive surgery. The propensity score was derived from a cohort-specific logistic regression of surgical approach (ie, standard vs minimally invasive) as a function of patient age, sex, 29 indicators for Elixhauser comorbidities,7 US census region, urban vs rural residence, and health plan type. We used a propensity score rather than multivariable regression adjustment because some surgical cohorts had small samples and some comorbidities were rare. For the expenditure outcomes, we estimated generalized linear models with a log link and used the modified Park test to identify the optimal distribution for the dependent variable.10 To facilitate interpretation of model coefficients, we converted the regression results to the original (dollar) scale by computing the average partial effect of receiving minimally invasive surgery using the method of recycled predictions.11 Confidence intervals were calculated from a nonparametric bootstrap with 1000 iterations. For the absenteeism outcome, we estimated linear regression models with heteroskedasticity-robust standard errors. We combined multiple imputation with auxiliary information on the number of days with medical care use to address the lack of complete data for this outcome (eAppendix in Supplement).8

To help understand the national impact of minimally invasive surgical technology on spending and workplace absenteeism among individuals with employer-sponsored insurance, we then projected our results to the national level using a previously developed approach.12,13 We calculated the number of individuals with employer-sponsored insurance undergoing the 6 types of surgery nationally in 2009 by counting the number of claims for each cohort in the MarketScan Commercial Database and then weighting to the national level. Data from the household component of the 2009 Medical Expenditure Panel Survey were used to estimate the number of individuals with employer-sponsored insurance nationally. To match the MarketScan data, the Medical Expenditure Panel Survey data were stratified by respondents’ US census region, age group, sex, urban vs rural location, and insurance policyholder status (policyholder or spouse/dependent). For each unique combination of strata, weights were calculated as the ratio of the total number of individuals with employer-sponsored insurance (from the Medical Expenditure Panel Survey) to the total number of individuals enrolled in plans (from MarketScan). The relevant weight was then applied to each patient in each cohort based on the applicable combination of strata.

To derive the total reductions in health plan spending and absenteeism attributable to the use of minimally invasive surgery among those with employer-sponsored health insurance, we multiplied the national projection of the number of individuals in each cohort undergoing minimally invasive surgery by the regression-based estimated effect. For the absenteeism estimates, to convert from days to years we divided by 250 (assuming 50 five-day workweeks per year). We repeated the calculation using the projected number of individuals receiving standard surgery nationally in 2009 to derive the potential national reductions in spending and worker absenteeism if patients who received standard surgery had instead received minimally invasive surgery.

All analyses used Stata version 12.1 statistical software (StataCorp LP). Two-tailed P < .05 was considered statistically significant. This study was approved by the University of Pennsylvania Institutional Review Board.

Of the 321 956 patients in the 6 procedure cohorts, 196 700 (61.1%) received minimally invasive surgery and 125 256 (38.9%) received standard surgery (Table 1). There were 23 814 patients in the absenteeism subsample, because absenteeism data were available only for employees of self-insured firms. Of these, 8953 (37.6%) received standard surgery and 14 861 (62.4%) received minimally invasive surgery. Patients who underwent minimally invasive procedures were more often older, were more often male, had higher numbers of comorbidities, more often lived in rural areas, and were covered by less restrictive health plans such as comprehensive and preferred provider organization plans. Patient characteristics by surgery type are shown in eTable 3 in Supplement for the overall sample and eTable 4 in Supplement for the absenteeism subsample.

During the baseline period prior to the index surgery, differences in the unadjusted levels of health plan spending and days absent for standard and minimally invasive surgery were frequently statistically significant but nevertheless qualitatively close in magnitude for most cohorts (Table 2). In contrast, during the combined perioperative and postoperative period, unadjusted mean health plan spending was significantly lower among patients receiving minimally invasive surgery for 3 cohorts (coronary revascularization, uterine fibroid resection, and peripheral arterial revascularization) and significantly higher for 2 cohorts (prostatectomy and carotid revascularization). Unadjusted mean days absent during the combined perioperative and postoperative period were statistically lower for patients undergoing minimally invasive surgery in 5 cohorts (all but aortic aneurysm repair).

Table Graphic Jump LocationTable 2.  Mean Unadjusted Spending and Days Absent in the Baseline Period and Both Perioperative and Postoperative Periods

The differences in perioperative and postoperative spending and absenteeism were similar after adjustment for spending during the baseline period (Table 3). Mean health plan spending was significantly lower for minimally invasive surgery compared with standard surgery for coronary revascularization (−$31 418), uterine fibroid resection (−$1311), and peripheral revascularization (−$11 796) and significantly higher for prostatectomy ($1228) and carotid revascularization ($6229). Results were qualitatively unchanged when fully adjusted. Moreover, after adjusting for baseline differences in days absent, patients undergoing minimally invasive surgery missed significantly fewer days of work during the perioperative and postoperative periods compared with those undergoing standard surgery in 5 cohorts; the differences ranged from 6.3 fewer days (prostatectomy) to 37.4 fewer days (coronary revascularization). The results were similar after full adjustment, except that the difference between minimally invasive surgery and standard surgery was no longer statistically significant for carotid revascularization.

Table Graphic Jump LocationTable 3.  Adjusted Differences in Perioperative and Postoperative Spending and Days Absent by Standard vs Minimally Invasive Surgery

For 2009, we projected that across the 6 cohorts nationally there were 248 632 individuals with employer-sponsored health insurance who underwent standard surgery and 533 196 who underwent minimally invasive surgery. We combined these projections with our fully adjusted regression-based estimates to calculate the net impacts of minimally invasive surgery under the assumption that everyone with employer-sponsored health insurance was employed. In 2009, the use of minimally invasive procedures for the 6 types of surgery we studied was responsible for projected national reductions of $8.9 billion in health plan spending and 53 134 person-years in workplace absenteeism (Table 4 and Table 5). Furthermore, if standard surgery in 2009 had been replaced by minimally invasive surgery, we projected that health plan spending would have been reduced by an additional $2.3 billion and workplace absenteeism nationwide would have been further reduced by 19 635 person-years. These findings were driven largely by coronary revascularization, which accounted for 43.1% of all cases in 2009 and had the largest reductions from minimally invasive surgery in health plan spending (mean difference, −$30 850) and absenteeism (mean difference, −37.7 days).

Table Graphic Jump LocationTable 4.  Health Plan Spending National Projections, 2009
Table Graphic Jump LocationTable 5.  Absenteeism National Projections, 2009

Across a diverse set of 6 surgery types, the net impact of minimally invasive surgery was to reduce health plan spending and workplace absenteeism. In 2009 for these 6 types of surgery, we projected nationally that the use of minimally invasive procedures reduced health plan expenditures by more than $8.9 billion and workplace absenteeism by more than 50 000 person-years, which, at the 2009 national average wage of $40 712, was worth $2.2 billion.14 Furthermore, if minimally invasive procedures had been used for all patients undergoing any of the 6 types of surgery in the United States in 2009, spending would have decreased by an additional $2.3 billion and absenteeism by another 20 000 person-years, similarly worth $800 million. Altogether, the sum of these effects exceeds $14 billion.

The aggregate estimates mask the effects of minimally invasive procedures specific to each surgery. Minimally invasive procedures significantly reduced health plan spending for coronary revascularization, uterine fibroid resection, and peripheral revascularization but significantly increased costs for prostatectomy and carotid revascularization. Moreover, they significantly reduced workplace absenteeism for 4 types of surgery (coronary revascularization, uterine fibroid resection, prostatectomy, and peripheral revascularization) and increased it for none. These findings were robust to statistical adjustment.

Because we did not assess differences in clinical outcomes, our findings should be interpreted with caution. Regardless of their economic benefits, minimally invasive procedures clearly remain inappropriate for some patients. Nevertheless, our findings do draw attention to important differences between standard and minimally invasive surgical approaches in nonclinical outcomes. Given that inpatient lengths of stay for minimally invasive procedures were already recognized as being shorter on average than for their standard counterparts,4,5 it is perhaps not surprising that we found that total workplace absenteeism in the year after surgery was also much lower for patients who had undergone minimally invasive procedures. The full benefit to the patient of what is measured here as reduced workplace absenteeism may extend beyond economic productivity gains to include quality-of-life advantages from faster recovery times, which were not quantified in our analysis but would be relevant for patients regardless of their employment status.

Our findings have direct relevance for health care payers. For 4 types of surgery, we found that patients receiving minimally invasive procedures had nominally if not statistically significantly lower medical spending and workplace absenteeism on average. For the other 2 types of surgery, we can calculate the ratio of spending to days absent to obtain an average effective price per reduced day of absence using the fully adjusted results in Table 3. This yields an average price of about $140 per day for prostatectomy and $300 per day for carotid revascularization; average prices from the baseline-adjusted results are about $50 higher. Basic economic theory suggests that, all else equal, an employer should prefer a minimally invasive procedure for an employee when it has both lower costs and absenteeism than its corresponding standard surgery or when its effective price is lower than the employee’s wage.

The principal implication of our results for policy is that a focus on evaluating new medical technologies based on their clinical outcomes alone, or even an evaluation including direct medical costs, may be too narrow. Increased worker productivity is one important example of a benefit that follows from shorter surgical recovery times produced by new technologies but cannot be adequately measured using administrative claims or other data sources typically used for comparative effectiveness research. A more comprehensive accounting of the benefits and costs of medical innovation is necessary to ensure that manufacturers’ research and development strategies, payers’ coverage policies, and health care providers’ and patients’ treatment decisions are fully informed and are thus economically optimal.

Our study has several limitations. Most notable is that our data do not permit examination of clinical outcomes; our findings must therefore be interpreted in light of possible differences between standard and minimally invasive approaches in clinical outcomes. However, the standard and minimally invasive options for the 6 types of surgery chosen for this study have generally been demonstrated to produce comparable clinical outcomes. Second, patients who underwent standard procedures may have been different from those who underwent minimally invasive procedures in clinically important ways that may also have influenced spending and absenteeism. Despite our adjustment for observable factors, any omitted variable bias would likely exaggerate the benefits of minimally invasive surgery in situations where, for example, patients with more severe disease were selected for standard surgery. Third, our data and imputation strategy may have led to inaccurate measurement of absenteeism. Although there is little reason why this limitation would affect standard and minimally invasive procedures differently, it is important to recognize that data on absenteeism were available only for patients working at self-insured firms. Fourth, our national projections of the anticipated effects of switching from standard to minimally invasive surgery represent upper bounds, as they are based on unrealistic assumptions that all patients with employer-sponsored health insurance are workers and that minimally invasive surgery could have substituted for standard surgery for all patients. Fifth, our findings may not be representative of other types of surgery and our analysis was limited by the small samples of patients with absenteeism data, particularly for carotid disease and aortic aneurysm repair. Last, although we treat minimally invasive surgery uniformly, our data conceal potentially important differences among the technologies underlying various minimally invasive approaches; ideally, an individual technology should be evaluated on its own merits.

For 3 of 6 types of surgery studied, minimally invasive procedures were associated with significantly lower health plan spending than standard surgery. For 4 of the 6 types of surgery, minimally invasive procedures were consistently associated with significantly fewer days of absence from work. Although the effect size depends critically on the clinical setting, the net impact of minimally invasive surgery across the 6 types of surgery studied was to lower both health care spending and worker absenteeism.

Corresponding Author: Andrew J. Epstein, PhD, Perelman School of Medicine, University of Pennsylvania, 423 Guardian Dr, Philadelphia, PA 19104 (eandrew@mail.med.upenn.edu).

Accepted for Publication: December 19, 2012.

Published Online: March 20, 2013. doi:10.1001/jamasurg.2013.131

Author Contributions: Dr Epstein had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Study concept and design: Epstein, Groeneveld, and Polsky.

Acquisition of data: Epstein and Harhay.

Analysis and interpretation of data: All authors.

Drafting of the manuscript: Epstein, Harhay, and Yang.

Critical revision of the manuscript for important intellectual content: Epstein, Groeneveld, Harhay, and Polsky.

Statistical analysis: Epstein, Groeneveld, Harhay, and Yang.

Obtained funding: Epstein and Polsky.

Administrative, technical, and material support: Groeneveld and Harhay.

Study supervision: Epstein and Polsky.

Conflict of Interest Disclosures: Dr Polsky has received consulting fees from GlaxoSmithKline.

Funding/Support: This work was supported by a research grant from the Institute for Health Technology Studies.

Role of the Sponsor: The sponsor had no role in the design or conduct of the study; collection, management, analysis, and interpretation of the data; or preparation, review, or approval of the manuscript.

Previous Presentation: This paper was presented at the 17th Annual International Meeting of the International Society for Pharmacoeconomics and Outcomes Research; June 5, 2012; Washington, DC.

Congressional Budget Office. The 2012 long-term budget outlook, June 2012. http://www.cbo.gov/sites/default/files/cbofiles/attachments/06-05-Long-Term_Budget_Outlook_2.pdf. Accessed November 30, 2012.
DeNavas-Walt  C, Proctor  BD, Smith  JC. Income, Poverty, and Health Insurance Coverage in the United States: 2010. Washington, DC: US Census Bureau; 2011.
Barbash  GI, Glied  SA.  New technology and health care costs: the case of robot-assisted surgery. N Engl J Med. 2010;363(8):701-704.
PubMed   |  Link to Article
Steiner  CA, Bass  EB, Talamini  MA, Pitt  HA, Steinberg  EP.  Surgical rates and operative mortality for open and laparoscopic cholecystectomy in Maryland. N Engl J Med. 1994;330(6):403-408.
PubMed   |  Link to Article
Escarce  JJ, Chen  W, Schwartz  JS.  Falling cholecystectomy thresholds since the introduction of laparoscopic cholecystectomy. JAMA. 1995;273(20):1581-1585.
PubMed   |  Link to Article
Carls  GS, Lee  DW, Ozminkowski  RJ, Wang  SH, Gibson  TB, Stewart  E.  What are the total costs of surgical treatment for uterine fibroids? J Womens Health (Larchmt). 2008;17(7):1119-1132.
PubMed   |  Link to Article
Elixhauser  A, Steiner  C, Harris  DR, Coffey  RM.  Comorbidity measures for use with administrative data. Med Care. 1998;36(1):8-27.
PubMed   |  Link to Article
Allison  PD. Missing Data. Thousand Oaks, CA: Sage; 2002.
Vickers  AJ, Altman  DG.  Statistics notes: analysing controlled trials with baseline and follow up measurements. BMJ. 2001;323(7321):1123-1124.
PubMed   |  Link to Article
Manning  WG, Mullahy  J.  Estimating log models: to transform or not to transform? J Health Econ. 2001;20(4):461-494.
PubMed   |  Link to Article
Kleinman  LC, Norton  EC.  What’s the risk? a simple approach for estimating adjusted risk measures from nonlinear models including logistic regression. Health Serv Res. 2009;44(1):288-302.
PubMed   |  Link to Article
Lee  DW, Gibson  TB, Carls  GS, Ozminkowski  RJ, Wang  SH, Stewart  EA.  Uterine fibroid treatment patterns in a population of insured women. Fertil Steril. 2009;91(2):566-574.
PubMed   |  Link to Article
Hawkins  K, Wang  S, Rupnow  MFT.  Indirect cost burden of migraine in the United States. J Occup Environ Med. 2007;49(4):368-374.
PubMed   |  Link to Article
US Social Security Administration. National average wage index. http://www.ssa.gov/oact/cola/AWI.html. Accessed November 30, 2012.

Figures

Tables

Table Graphic Jump LocationTable 2.  Mean Unadjusted Spending and Days Absent in the Baseline Period and Both Perioperative and Postoperative Periods
Table Graphic Jump LocationTable 3.  Adjusted Differences in Perioperative and Postoperative Spending and Days Absent by Standard vs Minimally Invasive Surgery
Table Graphic Jump LocationTable 4.  Health Plan Spending National Projections, 2009
Table Graphic Jump LocationTable 5.  Absenteeism National Projections, 2009

References

Congressional Budget Office. The 2012 long-term budget outlook, June 2012. http://www.cbo.gov/sites/default/files/cbofiles/attachments/06-05-Long-Term_Budget_Outlook_2.pdf. Accessed November 30, 2012.
DeNavas-Walt  C, Proctor  BD, Smith  JC. Income, Poverty, and Health Insurance Coverage in the United States: 2010. Washington, DC: US Census Bureau; 2011.
Barbash  GI, Glied  SA.  New technology and health care costs: the case of robot-assisted surgery. N Engl J Med. 2010;363(8):701-704.
PubMed   |  Link to Article
Steiner  CA, Bass  EB, Talamini  MA, Pitt  HA, Steinberg  EP.  Surgical rates and operative mortality for open and laparoscopic cholecystectomy in Maryland. N Engl J Med. 1994;330(6):403-408.
PubMed   |  Link to Article
Escarce  JJ, Chen  W, Schwartz  JS.  Falling cholecystectomy thresholds since the introduction of laparoscopic cholecystectomy. JAMA. 1995;273(20):1581-1585.
PubMed   |  Link to Article
Carls  GS, Lee  DW, Ozminkowski  RJ, Wang  SH, Gibson  TB, Stewart  E.  What are the total costs of surgical treatment for uterine fibroids? J Womens Health (Larchmt). 2008;17(7):1119-1132.
PubMed   |  Link to Article
Elixhauser  A, Steiner  C, Harris  DR, Coffey  RM.  Comorbidity measures for use with administrative data. Med Care. 1998;36(1):8-27.
PubMed   |  Link to Article
Allison  PD. Missing Data. Thousand Oaks, CA: Sage; 2002.
Vickers  AJ, Altman  DG.  Statistics notes: analysing controlled trials with baseline and follow up measurements. BMJ. 2001;323(7321):1123-1124.
PubMed   |  Link to Article
Manning  WG, Mullahy  J.  Estimating log models: to transform or not to transform? J Health Econ. 2001;20(4):461-494.
PubMed   |  Link to Article
Kleinman  LC, Norton  EC.  What’s the risk? a simple approach for estimating adjusted risk measures from nonlinear models including logistic regression. Health Serv Res. 2009;44(1):288-302.
PubMed   |  Link to Article
Lee  DW, Gibson  TB, Carls  GS, Ozminkowski  RJ, Wang  SH, Stewart  EA.  Uterine fibroid treatment patterns in a population of insured women. Fertil Steril. 2009;91(2):566-574.
PubMed   |  Link to Article
Hawkins  K, Wang  S, Rupnow  MFT.  Indirect cost burden of migraine in the United States. J Occup Environ Med. 2007;49(4):368-374.
PubMed   |  Link to Article
US Social Security Administration. National average wage index. http://www.ssa.gov/oact/cola/AWI.html. Accessed November 30, 2012.

Correspondence

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Supplement.

eAppendix. Methods

eReference

eTable 1. Cohort Coding Definitions

eTable 2. Cohort Exclusions by Indication

eTable 3. Patient Characteristics (Full Sample)

eTable 4. Patient Characteristics (Absent Days Sample)

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