COPD Medication Adherence and Persistence Among Managed Medicare Enrollees

ABSTRACT

Objectives: To compare adherence to and persistence with chronic obstructive pulmonary disease (COPD) maintenance medications between Medicare Advantage Prescription Drug (MAPD) plan enrollees and Medicare prescription drug (PD)-only plan enrollees.

Study Design: This was a retrospective, cross-sectional study using Medicare health plan eligibility and pharmacy claim information collected from the Optum Research Database. The study period was January 1, 2011, to June 30, 2013.

Methods: Descriptive and multivariable analyses of adherence and persistence were performed. Adherence was quantified as the proportion of days covered (PDC) by any COPD maintenance medication, and persistence was appraised as the duration of therapy.

Results: The study population included 130,305 patients, with 21% and 79% enrolled in MAPD and PD-only plans, respectively. PD-only plan enrollees were significantly more adherent (PDC ≥80%) than MAPD plan enrollees (40.5% vs 38.7%; P <.001), based on descriptive and multivariable analysis (odds ratio, 1.09; 95% CI, 1.03-1.15; P = .002). PD-only plan enrollees had a similar likelihood of discontinuation compared with MAPD plan enrollees based on multivariable analysis (hazard ratio, 1.00; 95% CI, 0.96-1.05; P = .877).

Conclusions: MAPD plan enrollees do not have better adherence to medication regimens or persistence with COPD-maintenance medication than PD-only plan enrollees, despite financial incentive of MAPD health plans to improve medication adherence. Understanding the basis for this will be relevant for policy researchers and decision makers in developing strategies to improve patient outcomes through persistent medication use.

Am J Pharm Benefits. 2016;8(1):14-21

PRACTICAL IMPLICATIONS

This study compared adherence and persistence patterns between Medicare Advantage Prescription Drug (MAPD) plan and prescription drug (PD)-only plan enrollees for chronic obstructive pulmonary disease maintenance medication. Findings presented will be relevant for policy makers to develop strategies for improving patient compliance to medications.

• Adherence was significantly higher for PD-only plan enrollees compared with MAPD plan enrollees (40.5% vs 38.7%; P <.001).
• Higher adherence of PD-only plan enrollees was also noted in multivariable analysis (odds ratio, 1.09; 95% CI, 1.03-1.15; P = .002)
• A similar likelihood of discontinuation between PD-only and MAPD plan enrollees was observed, based on multivariable analysis (hazard ratio, 1.00; 95% CI, 0.96-1.05; P = .877).

Chronic obstructive pulmonary disease (COPD) represents a substantial disease burden. Approximately 14.8 million Americans have a COPD diagnosis, and COPD is currently the third-leading cause of death in the United States.¹ COPD also has significant economic consequences, with 2010 medical costs estimated at $32.1 billion, which is projected to increase to $49 billion by 2020.²

A large proportion of patients with COPD are 65 years or older and, therefore, are Medicare beneficiaries in many cases.^3,4 In 2013, approximately 68% of Medicare beneficiaries were enrolled in Part D, an optional prescription drug coverage program available to all individuals with Medicare.^4,5

Among those with Part D coverage, two-thirds have stand-alone prescription drug (PD) plans that are available to patients with the traditional fee-for-service Medicare plan. The remaining one-third have Medicare Advantage Prescription Drug (MAPD) plans, which are offered by insurance companies and cover hospital (Medicare Part A), medical (Medicare Part B), and prescription (Medicare Part D) costs.^4,5

All Part D plans must meet standard criteria, but may offer varied premiums, benefit designs, gap coverage, formularies, and management rules, as well as enhanced benefits.⁶ Based on 2011 data from CMS, premiums are an average of $25 higher on PD-only than on MAPD plans.⁶ The vast majority of PD-only and MAPD plans operate a tiered cost-sharing structure during the initial coverage period, differentiating among generic, preferred, and nonpreferred brand-name drugs and specialty drugs.

In 2011, PD plans
generally
charged
greater co-pays
than
MAPD plans for generic and preferred brands, but lower co-pays for nonpreferred or specialty brands.⁶ By law, PD plans must cover 7% of the costs of generics during the coverage gap, whereas pharmaceutical companies are obliged to offer a 50% price discount on brand-name drugs during the coverage gap. In addition to this legislation, some plans offer additional gap coverage, reducing total out-of-pocket costs incurred by patients during the coverage gap, although this is mostly for generics only.

A greater share of MAPD plans offer gap coverage than PD-only plans (67% vs 6%).6 Furthermore, PD-only and MAPD plans may offer enrollees the option of an “enhanced plan,” which can widen the range of drugs covered, remove the annual deductible, or lower co-pay rates.

Since MAPD plans cover both medical services and prescription drugs, these health plans have an incentive to control total cost of care; therefore, they often expend greater effort than PD-only plans to ensure adherence, with the assumption of greater offsets in medical care for chronic conditions, including COPD.⁷ However, in a study of MAPD plan enrollees, patients with COPD had one of the lowest observed rates of adherence and some of the highest pharmacy costs compared with patients with other chronic diseases (PD-only plan enrollees were not evaluated in this study).⁸

In an effort to understand factors that affect medication adherence, differences in the structure of MAPD and PD-only plans represent an intriguing policy variable—one previously explored for statin usage.⁷ These factors are particularly relevant since patient adherence to COPD maintenance medication is associated with fewer hospitalizations and lower Medicare costs.^9,10

Identifying whether components of health management offered by an insurer could be a driver of adherence in patients with COPD could ultimately lead to improved medication use in elderly patients with chronic conditions. Therefore, this study was undertaken to evaluate adherence to and persistence with COPD maintenance medications among patients enrolled in MAPD or PD-only plans.

Objectives

This study had 2 objectives: 1) to compare adherence to and persistence with COPD maintenance medications in MAPD plan enrollees versus PD-only plan enrollees; and 2) to identify covariates that are significantly associated with medication adherence, especially those relevant to Medicare policy (eg, low-income subsidy [LIS], coverage gap).

METHODS

Study Design

This was a retrospective, cross-sectional database study using data from a large, geographically diverse, US managed care plan. Eligibility and pharmacy claims data were extracted from the Optum Research Database, which included approximately 2.1 million annual MAPD plan enrollees with medical and pharmacy coverage, and approximately 5.6 million annual PD-only plan enrollees. In accordance with the Health Insurance Portability and Accountability Act, the use of de-identified data did not require institutional review board approval or waiver of authorization.

Because diagnosis codes on medical claims cannot be used to identify the presence of COPD among PD-only plan enrollees, and because medications to treat COPD are used in asthma as well, an algorithm to identify patients with COPD for study inclusion based on pharmacy claims alone was required. This approach was previously successfully evaluated by Mapel and colleagues (2010) using outpatient pharmacy data.¹¹

The algorithm was developed and validated using the MAPD population. The sensitivity and specificity of various combinations of rescue and maintenance medications were examined for their ability to identify patients with COPD. This process resulted in the following algorithm for identifying COPD patients: a) filled 1 or more prescriptions for roflumilast (an alpha1-proteinase inhibitor), tiotropium, aclidinium, arformoterol, formoterol, or indacaterol; or b) filled 2 or more prescriptions for ipratropium or ipratropium-albuterol combination.

Inclusion and Exclusion Criteria

Patients who met the criteria identified in the algorithm between January 1, 2011, and June 30, 2012, were identified as patients with COPD. An index date was defined as the first date that a prescription for a COPD maintenance medication was filled between January 1, 2012, and June 30, 2012. The COPD medication filled on the index date was designated as the index medication.

Patients were required to have been eligible for Medicare in the year prior to the index date (aged ≥65 years); have COPD based on the pharmacy claims—based algorithm described above; have received at least 1 pharmacy claim for COPD maintenance medication from January 1, 2012, through June 30, 2012; and have been continuously enrolled with pharmacy coverage in an MAPD or PD-only plan for 12 or more months prior to, and 60 days or more after, the index date.

Patients were excluded if they received a prescription for a leukotriene receptor antagonist at any time during the study period, if they switched between MAPD and PD-only plans during the study period, or if their gender was unknown as of the index date. Outcomes, including adherence and persistence, were measured during a 365-day post index period (inclusive of the index date), or until death or disenrollment, in which case the post index period ranged from 60 to 365 days.

Primary Study Outcome Variables

Adherence was measured as the proportion of days covered (PDC) by any COPD maintenance medication. PDC was calculated as the number of days the patient had any type of COPD maintenance medication during the post index period, divided by the postindex period. Number of days of coverage was calculated using "days supplied" fields from pharmacy claims. The "days supplied" amounts indicate the number of days of therapy covered by the prescription fill.

This approach is commonly employed when pharmacy claims are used for adherence and persistence measurement. PDC values range from 0 to 1, with 1 indicating perfect adherence. Patients with a PDC value of less than 0.80 were defined as nonadherent with therapy.^12,13 This approach is similar to that used in Medicare star ratings.¹²

Persistence was measured as the duration of therapy (DOT).

This was defined as the number of days between the index date and the run-out date of the last COPD maintenance medication claim preceding a gap in therapy of 60 days or more. For patients without at least a 60-day gap in fills, the DOT was equivalent to the length of their post index period. Adherence and persistence were calculated using all COPD maintenance medications; switching between medications was considered as maintaining persistence.

Additional Study Variables

Demographics (including age, gender, geographic location, and socioeconomic factors) and clinical characteristics were captured within the 12-month pre-index period. Socioeconomic factors (including median household income, probability of having a college education, likelihood of being nonwhite, and rural/urban residence) were obtained by linking the enrollee’s zip code to 2010 US Census data.

Pharmacy benefit factors included pharmacy benefit type, as well as LIS status, which was obtained from enrollment information. True out-of-pocket costs were calculated from January 1, 2012, to the index date, for patient payments for deductibles and co-pays, payments by other payers, and reductions in payments due to LIS.

The clinical and medication-related characteristics evaluated, included coverage phase of index prescription (eg, coinsurance, gap, catastrophic); gap coverage (as defined in Table 16,14); prescribing physician specialty for index prescription; Medicare health-risk assessment level; fills for short-acting beta2-agonists, corticosteroids, or chronic conditions; continuing user status; and total drug spend (patient and plan spend) during the pre-index period.

The Medicare health-risk assessment level was based on the pre-index prescription drug hierarchical condition categories (Rx-HCC) score provided by Medicare.¹⁵ The Rx-HCC model uses demographic and disease information from a base year to predict a patient’s drug costs in the next year.¹⁵

Statistical Methods

Power analysis indicated that a sample size of 4288 patients with a ratio of 1:3 for enrollees in MAPD to PD-only plan cohorts, respectively, was needed to detect a 5% difference in PDC between cohorts with 80% power. Descriptive analyses were used to compare study variables, including covariates and outcome measures, between the MAPD and PD-only plan cohorts.

Adherence was modeled between MADP and PD-only plan cohorts using logistic regression analysis. Statistical significance was determined using either

χ

2 or Student’s t test. Kaplan-Meier analyses were conducted to compare the time to discontinuation of therapy for MAPD versus PD-only plans. A multivariable Cox proportional hazards model was employed to determine hazard ratio (HR) and P values for time to discontinuation. Fixed covariates utilized in the model included pre-index subject demographics, socioeconomic variables, clinical characteristics, and pharmacy benefit factors.

RESULTS

Demographic and Pre-Index Characteristics

A total of 130,305 patients were included in the study: 26,867 (21%) and 103,438 (79%) were enrolled in MAPD and PD-only plans, respectively, from January 1, 2011, to June 30, 2013. On average, MAPD plan enrollees were younger than PD-only plan enrollees, with a significantly lower proportion of patients being 80 years or older (30.0% vs 37.7%;

P <.001) (Table 2).

The largest difference between the 2 cohorts was the pharmacy benefit type in which they enrolled, with 66.5% of MAPD plan enrollees participating in an enhanced plan compared with 3.5% of PD-only plan enrollees (P <.001). A lower percentage of PD-only plan enrollees had LIS status than MAPD plan enrollees (12.7% vs 18.2%; P <.001), although a similar number had Medicare and Medicaid dual coverage (LIS dual: 17.5% vs 17.6%; P = .578).

There was a higher percentage of MAPD plan enrollees compared with PD-only plan enrollees, who lived in zip codes with residents having an annual household income less than $35,000 (59.7% vs 52.6%; P <.001). A lower percentage of MAPD plan enrollees lived in zip codes with residents having a college degree (P <.001).

A modestly lower percentage of MAPD plan enrollees were continuing users of COPD maintenance medications (as opposed to new users at the index fill) compared with PD-only plan enrollees (84.1% vs 85.6%; P <.001). MAPD plan enrollees were more likely than PD-only plan enrollees to receive a prescription for their index medication from a primary care physician (59.6% vs 54.1%; P <.001), while a higher percentage of PD-only plan enrollees obtained this prescription from a pulmonologist compared with MAPD plan enrollees (22.8% vs 19.0%; P <.001).

Furthermore, a significantly larger percentage of PD-only plan enrollees had an Rx-HCC score that was categorized as low compared with MAPD plan enrollees (78.8% vs 71.8%; P <.001). For both cohorts, over 90% of patients were in the coinsurance phase of their coverage.

Adherence. Mean PDC was higher for PD-only plan enrollees compared with MAPD plan enrollees (0.643 vs 0.626; P <.001). PD-only plan enrollees were significantly more adherent (PDC ≥80%) than MAPD plan enrollees (40.5% vs 38.7%; P <.001). Likewise, in the multivariable analysis, adherence was significantly higher for PD-only plan enrollees compared with MAPD plan enrollees (odds ratio [OR], 1.09; 95% CI, 1.03-1.15; P = .002) (Table 3).

Continuing users of COPD maintenance medications were more likely to be adherent than new users (OR, 3.82; 95% CI, 3.66-3.99; P <.001). For every 1 year of increased age, there was a 0.8% drop in the likelihood of being adherent when all other variables remained the same (P <.001).

In the assessment of pharmacy benefit factors, patients with an enhanced pharmacy benefit plan were shown to be less likely to be adherent than those with a standard plan (OR, 0.92; 95% CI, 0.86-0.98; P <.001). Patients with LIS dual coverage were less adherent compared with those who received only an LIS, but were more adherent compared with those who did not receive an LIS (OR, 1.44; 95% CI, 1.38-1.50; P <.001).

Patients who filled an index prescription in the coverage gap phase or catastrophic phase were more likely to be adherent than patients who were in the coinsurance phase (OR, 1.40; 95% CI, 1.26-1.56; and OR, 3.26; 95% CI, 2.72-3.91, respectively [P <.001]). In addition, although only 3% of the patients had gap coverage, those patients were more likely to be adherent than those who did not have gap coverage (P <.001). Gender, rural/urban residence, and median household income were not associated with adherence.

Persistence. In unadjusted analysis, PD-only plan enrollees had a longer mean DOT than MAPD plan enrollees (235.9 days vs 230.7 days; P <.001). However, the Cox proportional hazard model indicated a similar likelihood of discontinuation of COPD maintenance medication between MAPD and PD-only plan enrollees (HR, 1.00; 95% CI, 0.96-1.05; P = .877) (Table 4).

Continuing user status was one of the strongest predictors of therapy discontinuation, with continuing users of COPD therapy being less likely to discontinue than users who were new to therapy at index date (HR, 0.38; 95% CI, 0.37-0.39; P <.001). Patients who filled their index prescription in the coverage gap phase or catastrophic phase were also more likely to be persistent than patients who did so in the coinsurance phase (P <.001).

Rural residents were less likely to discontinue therapy than those who lived in urban settings (P = .001). Patients with enhanced benefits were less likely to persist with therapy compared with patients who had standard benefits (P <.001). LIS dual patients were less likely to be persistent than LIS subsidy patients, but were more likely to be persistent compared with those who did not receive LIS (P <.001). Age, gender, and median household income were not associated with discontinuation.

DISCUSSION

To our knowledge, this is the first study to assess adherence and persistence with maintenance medication in patients with COPD who were enrolled in either PD-only or MAPD plans. Counter to our expectations, a significantly higher percentage of patients enrolled in PD-only plans were adherent compared with MAPD plan enrollees, based on both descriptive and multivariable analyses. Regarding persistence, although there was a significantly greater mean DOT for PD-only compared with MAPD enrollees, a similar likelihood of discontinuation was seen for both cohorts in multivariable analysis controlling for patient characteristics.

Adherence rates were similar to those published by Erten and colleagues (2013) in their 2-year evaluation of MAPD and PD-only plan enrollees who were prescribed antidiabetic, antihyperlipidemic, or angiotensin-converting enzyme inhibitor/angiotensin receptor blocker medications.16 For all 3 classes of medications, adherence was significantly higher with PDC greater by 0.4 to 0.6 for PD-only plan enrollees compared with MAPD plan enrollees (P <.001).¹⁶

Adherence and persistence to COPD medications were found to be relatively high. The reason for this could be that a majority (85%) of the patients were continuing users. We adjusted for user status in multiple regression analysis. Status as a continuing user of COPD maintenance medication (as opposed to being new to therapy at index) was the strongest predictor for both adherence and persistence. We also stratified by user status, and the PD plan effect still prevailed in continuing users. Furthermore, similar to previous studies, MAPD plan enrollees were more likely to be nonwhite.^17,18

Patients living in areas with a high proportion of whites were more adherent and persistent. This finding is consistent with a previous study of patients with a range of medical conditions, reporting an OR of whites being adherent to COPD/asthma medications compared with nonwhites of as 1.76 (P <.001).¹⁹

We expected that MAPD plan enrollees would be more adherent and persistent than PD-only plan enrollees because MAPD health plans have greater incentives to actively improve medication adherence in order to reduce spending on medical services. One possible explanation for PD-only plan enrollees being more adherent in this current study may be related to differences in pre-index characteristics between the groups. However, in sensitivity analysis where confounding was controlled via propensity score matching, similar results were obtained (data not shown).

Another explanation for higher adherence among PD-only plan enrollees is the recent finding that PD-only beneficiaries have lower cost-sharing for prescriptions than do MAPD plan beneficiaries.²⁰ For example, the median cost sharing in MAPD plans for preferred and nonpreferred brands was found to be $45 and $95, respectively, compared with $40 and $85 in PD plans.²⁰

Limitations

Certain limitations in the present study should be noted when considering these findings. We avoided using inhaled corticosteroids in the patient identification algorithm because we did not want to include patients with only asthma. This may have led to exclusion of some COPD patients who use combination therapies but did not use medications we included in the algorithm.

One limitation specific to this study is that medications that are billed through the medical benefits, such as some nebulizer therapies, were not captured because medical claims for PD-only plan enrollees were not available. This is particularly relevant for patients who used long-acting beta-agonists in addition to other medications filled through the pharmacy benefit. Secondly, common to most analyses of claims data, this study is limited by the degree to which the data can accurately capture individuals’ medical history, since the data are subject to coding errors and claims-specific concerns.

The data on mortality is not available in pharmacy claims; hence, we were not able to assess whether or not the mortality rate was similar in the 2 groups. An additional limitation is the potential for residual confounding—that is, not all patient characteristics that may influence both compliance and choice of a MAPD or PD-only plans were available in the data source (eg, severity of COPD). It should be noted that filled pharmacy claims did not indicate whether medications were taken as prescribed. In addition, as socioeconomic variables were not reported in the claims data, these were imputed based on census data using enrollees’ zip code information.

CONCLUSIONS

Overall, the finding that patients with MAPD plans have lower adherence merits the consideration of several possibilities. First, differences in adherence between MAPD and PD-only plans may be explained by adherence programs accessed by those enrolled in PD-only plans that are not captured by the data used in this analysis. Second, MAPD adherence programs may not be effective enough. Active monitoring and counseling have shown to be successful in COPD and other chronic diseases.^21,22

Third, there may be factors related to the system that may explain the finding. Finally, despite using best available techniques to address selection effects, there may be unmeasured patient factors associated with both plan choice and adherence that impact findings. Regardless of the underlying explanation, our analysis did not identify a strong positive effect of MAPD plan membership on adherence, despite the intent of these plans to manage patient care more effectively.

This may have implications for collaborative care models that are being evaluated by CMS. Policy researchers should further investigate both adherence metrics and adherence programs, as these metrics are being used to assess quality of care provided by plans.

Acknowledgments

GlaxoSmithKline funded medical writing assistance provided by Alan Saltzman, PhD, and Sarah Thornburg, MS, of Fishawack Indicia, Inc, Horsham, PA.

Author Affiliations: GlaxoSmithKline (RS), Research Triangle Park, NC; Optum (JM, AA), Eden Prairie, MN.

Funding Source: This study was funded by GlaxoSmithKline (GSK).

Author Disclosures: Dr Shenolikar was an employee and stockholder of GSK during the conduct of this study. Drs Mao and Altan are employees of Optum, a consulting firm hired by GSK to conduct this study.

Authorship Information: Concept and design (JM, AA, RS); acquisition of data (JM, AA); analysis and interpretation of data (JM, AA, RS); drafting of the manuscript (JM, AA); critical revision of the manuscript for important intellectual content (JM, AA, RS); statistical analysis (JM); provision of study materials or patients (JM); obtaining funding (RS); administrative, technical, or logistic support (AA); and supervision (RS).

Send correspondence to: Rahul Shenolikar, PhD, HEOR Director, Astrazeneca, US Medical Affairs, 101 Orchard Ridge Dr, Gaithersburg, MD 20878. E-mail: rahul.shenolikar@astrazeneca.com.

REFERENCES

1. National Institutes of Health: National Heart, Lung, and Blood Institute. Morbidity & Mortality: 2012 Chart Book on Cardiovascular, Lung, and Blood Diseases. Bethesda, MD: National Institutes of Health: National Heart, Lung, and Blood Institute; 2012.

2. Ford ES, Murphy LB, Khavjou O, Giles WH, Holt JB, Croft JB. Total and state-specific medical and absenteeism costs of COPD among adults aged ≥18 years in the United States for 2010 and projections through 2020. Chest. 2015;147(1):31-45. doi:10.1378/chest.14-0972.

3. Akinbami LJ, Liu X. Chronic obstructive pulmonary disease among adults aged 18 and over in the United States, 1998-2009. NCHS Data Brief. 2011;(63):1-8.

4. CMS. Your Guide to Medicare Prescription Drug Coverage. Baltimore, MD: CMS; June 2014.

5. Report to the Congress: Medicare payment policy. Medicare Payment Advisory Commission website. http://medpac.gov/documents/reports/mar14_entirereport.pdf. Published March 2014. Accessed January 2016.

6. Hoadley J, Summer L, Hargrave E, Cubanski J, Neuman T. Analysis of Medicare prescription drug plans in 2011 and key trends since 2006. Kaiser Family Foundation website. https://kaiserfamilyfoundation.files.wordpress.com/2013/01/8237.pdf. Published September 2011. Accessed January 2016.

7. Jung K, McBean AM, Kim JA. Comparison of statin adherence among beneficiaries in MA-PD plans versus PDPs. J Manag Care Pharm. 2012;18(2):106-115.

8. Priest J, Buikema A, Engel-Nitz NM, Cook CL, Cantrell CR. Quality of care, health care costs, and utilization among Medicare Part D enrollees with and without low-income subsidy. Popul Health Manag. 2012;15(2):101-112. doi:10.1089/pop.2011.0008.

9. Simoni-Wastila L, Wei YJ, Qian J, et al. Association of chronic obstructive pulmonary disease maintenance medication adherence with all-cause hospitalization and spending in a Medicare population. Am J Geriatr Pharmacother. 2012;10(3):201-210. doi:10.1016/j.amjopharm.2012.04.002.

10. Toy EL, Beaulieu NU, McHale JM, et al. Treatment of COPD: relationships between daily dosing frequency, adherence, resource use, and costs. Respir Med. 2011;105(3):435-441. doi:10.1016/j.rmed.2010.09.006.

11. Mapel DW, Petersen H, Roberts MH, Hurley JS, Frost FJ, Marton JP. Can outpatient pharmacy data identify persons with undiagnosed COPD? Am J Manag Care. 2010;16(7):505-512.

12. Star ratings. Medicare website. https://www.medicare.gov/find-a-plan/staticpages/rating/planrating-help.aspx? Accessed January 2016.

13. Choudhry NK, Shrank WH, Levin RL, et al. Measuring concurrent adherence to multiple related medications. Am J Manag Care. 2009;15(7):457-464.

14. Medicare prescription drug benefit manual: chapter 13 - premium and cost-sharing subsidies for low-income individuals. CMS website. http://www.cms.gov/Regulations-and-Guidance/Guidance/Transmittals/Downloads/Chapter13.pdf. Updated July 29, 2011. Accessed January 2016.

15. Medicare managed care manual: chapter 7 - risk adjustment. CMS website. http://www.cms.gov/Regulations-and-Guidance/Guidance/Manuals/downloads/mc86c07.pdf. Updated September 19, 2014. Accessed January 2016.

16. Erten MZ, Stuart B, Davidoff AJ, Shoemaker JS, Bryant-Comstock L, Shenolikar R. How does drug treatment for diabetes compare between Medicare Advantage prescription drug plans (MAPDs) and stand-alone prescription drug plans (PDPs)? Health Serv Res. 2013;48(3):1057-1075. doi:10.1111/1475-6773.12016.

17. Neuman P, Strollo MK, Guterman S, et al. Medicare prescription drug benefit progress report: findings from a 2006 national survey of seniors. Health Aff (Millwood). 2007;26(5):w630-w643.

18. Cline RR, Worley MM, Schondelmeyer SW, et al. PDP or MA-PD? Medicare part D enrollment decisions in CMS Region 25. Res Social Adm Pharm. 2010;6(2):130-142. doi:10.1016/j.sapharm.2010.04.002.

19. Rolnick SJ, Pawloski PA, Hedblom BD, Asche SE, Bruzek RJ. Patient characteristics associated with medication adherence. Clin Med Res. 2013;11(2):54-65. doi:10.3121/cmr.2013.1113.

20. Hoadley J, Summer L, Hargrave E, Cubanski J, Neuman T. Medicare Part D in its ninth year: the 2014 marketplace and key trends, 2006-2014. Henry J. Kaiser Family Foundation website. http://kff.org/medicare/report/medicare-part-d-in-its-ninth-year-the-2014-marketplace-and-key-trends-2006-2014/. Published August 18, 2014. Accessed November 18, 2014.

21. Tommelein E, Mehuys E, Van Hees T, et al. Effectiveness of pharmaceutical care for patients with chronic obstructive pulmonary disease (PHARMACOP): a randomized controlled trial. Br J Clin Pharmacol. 2014;77(5):756-766. doi:10.1111/bcp.12242.

22. van Boven JF, Stuurman-Bieze AG, Hiddink EG, Postma MJ, Vegter S. Medication monitoring and optimization: a targeted pharmacist program for effective and cost-effective improvement of chronic therapy adherence. J Manag Care Spec Pharm. 2014;20(8):786-792