Why Medication Adherence Matters: From Gaps to Gains in Value-Based Care 

Value-based care (VBC) depends on multiple factors working together to drive better outcomes while managing costs. Among these various levers, medication adherence plays an important role, particularly for chronic conditions where consistent treatment is essential for maintaining health and preventing complications. When patients don’t take medications as prescribed, don’t take them correctly, or experience gaps in therapy, even the most effective treatments can’t deliver their intended benefits. This post unpacks how adherence is measured, what poor adherence does to costs and utilization, why patients fall off track (including the role of social determinants of health), and which strategies actually work. We also show how Illustra Health helps organizations surface and address adherence risks across a wide range of conditions. 

How Medication Adherence is Measured (and Why it Matters in Value-Based Care Contracts) 

The World Health Organization (WHO) defines adherence as the extent to which a person’s behavior—taking medication, following a diet, or executing lifestyle changes—corresponds with agreed recommendations from a healthcare provider (World Health Organization, 2003). In practice, health plans, Pharmacy Benefit Managers (PBMs), and population health teams almost always measure adherence using pharmacy claims. 

Three Key Refill-Based Measures: 

Proportion of Days Covered (PDC): Proportion of Days Covered counts the share of days in a measurement period that a patient has medication on hand. It caps overlap (to avoid double-counting early refills) and is the industry standard for performance reporting. A PDC of 80% or greater is the common adherence threshold for many measures (Pharmacy Quality Alliance, 2020/2021; Oueini, 2022). 

Medication Possession Ratio (MPR): Medication Possession Ratio is calculated as the total days’ supply dispensed divided by days in the measurement period. A limitation of this metric is that it can overstate adherence if patients refill prescriptions early. 

Medication Gaps: Medication Gaps identify specific periods when patients have no medication available, measuring both the frequency and duration of treatment interruptions. Unlike PDC’s aggregate view, medication gaps provide granular insights into adherence patterns—revealing whether poor adherence stems from occasional long gaps or frequent short interruptions. Gaps are typically defined as periods of 7, 15, or 30+ days without medication coverage, depending on the therapeutic class and clinical urgency. This measure is particularly valuable for identifying patients at risk of clinical deterioration and for timing targeted interventions. 

The Impact of Adherence on Star Ratings 

Medicare Part D Star Ratings include three PQA PDC measures—Diabetes Medications, Hypertension (RAS antagonists), and Cholesterol (statins)—plus related measures like Statin Use in Persons with Diabetes and MTM CMR completion (Oueini, 2022). Plans with strong adherence performance more often achieve overall Star ratings of 4 or above, with financial and enrollment impacts that extend across benefit design and member growth (Borelli et al., 2025). While medication gaps aren’t directly included in Star ratings, they serve as early warning indicators that help prevent the PDC deterioration that impacts these critical performance measures. 

What Claims-Based Measures Do—and Don’t—Tell You 

PDC shows whether medication was available to the member, not whether it was taken. Medication gaps reveal when patients definitely couldn’t take their medication, making them valuable for identifying the most urgent adherence risks.  These measures function as validated intermediate outcomes that correlate with downstream clinical results and costs (Pharmacy Quality Alliance, 2020/2021). 

The Impact of Poor Adherence on Outcomes, Utilization, and Cost 

Roughly 1 in 5 new prescriptions are never filled (primary nonadherence), and approximately half of filled prescriptions are taken incorrectly with respect to dose, timing, or frequency (Neiman et al., 2017). Across the U.S., estimates attribute $100–$300 billion in annual healthcare costs to nonadherence when direct and indirect costs are counted (Neiman et al., 2017; IMS, 2013; Iuga & McGuire, 2014; Benjamin, 2012; Mykyta & Cohen, 2023). An IMS Institute analysis estimated that nonadherence accounted for $105 billion in avoidable healthcare costs in 2012, with hospitalizations responsible for 69% of that total (IMS, 2013). 

Utilization and Mortality 

Disease-specific studies consistently link better medication adherence with fewer hospitalizations and better survival. In patients with cardiovascular disease, for example, eliminating medication copays following myocardial infarction led to a 4–6 percentage-point increase in medication adherence and no increase in total spending (Choudhry et. al., 2011). For patients with heart failure, a meta-analysis found that medication adherence interventions reduced readmissions and mortality (Ruppar et al., 2015; Ruppar et al., 2016;  Khazanie & Allen 2016). Similar patterns appear across other conditions including diabetes, hyperlipidemia, hypertension, and HIV. 

Condition-level slices of an IMS analysis showed large avoidable costs associated with nonadherence in diabetes, hyperlipidemia, and hypertension, with additional billions tied to HIV and heart failure (IMS, 2013). Beyond disease-specific estimates, CDC reports also tie cost-related nonadherence to worse control of diabetes and cardiovascular disease, driving ED visits, admissions, and mortality (Iuga & McGuire, 2014; Benjamin, 2012). 

Why Patients Skip Their Medications—and the Role of Social Context 

Medication adherence is not a single discrete behavior; rather, it is a sequence of interdependent steps—diagnosis → prescription → fill → initiation → persistence → self-management of side effects → refill. Disruption at any point in this chain can negatively affect clinical outcomes. Factors contributing to suboptimal medication adherence can be grouped into four broad domains: 

1. Patient-level factors 

• Understanding and health literacy: Challenges in interpreting instructions and understanding therapeutic rationales reduce persistence (Wilder et al., 2021). 

• Beliefs and side effects: Patients’ perceptions of medication necessity versus concerns (e.g., statin-associated myalgias, antidepressant-related activation) influence adherence decisions. 

• Mental health and cognitive load: Conditions such as depression, bipolar disorder, schizophrenia, and Parkinson’s disease complicate patients’ ability to maintain consistent medication routines. 

2. Treatment-level factors 

• Complex regimens: Multiple daily doses, polypharmacy, and injectable routes can reduce adherence. 

• Financial considerations: Up-front and ongoing costs—including copayments, deductibles, and non-formulary status—pose barriers to consistent medication use. 

3. Health system & access 

• Pharmacy access (“pharmacy deserts”): Research in Chicago and subsequent multi-city analyses demonstrate that predominantly Black and Hispanic neighborhoods have fewer pharmacies and more closures, contributing to poorer access and adherence (Qato et al., 2014; Wittenauer et al., 2024; Hooper, 2022; Anderson & Mattingly, 2025; Chen, 2021). 

• Care fragmentation: Inadequate coordination and poor handoffs at hospital discharge further undermine adherence. 

4. Social determinants of health (SDoH) 

• Structural and socioeconomic barriers: Transportation limitations, housing instability, food insecurity, and financial strain are all associated with lower adherence, as demonstrated in systematic reviews and analyses of older adult and emergency department populations (Wilder et al., 2021; Adeoye-Olatunde et al., 2025; Yang et al., 2025; Farley & Pradeep, 2024). 

• Out-of-pocket medication costs: Retail drug out-of-pocket spending reached $63 billion in 2021, placing adherence at risk for many patients, particularly those who are uninsured, disabled, or have lower incomes (Mykyta & Cohen, 2023; Moreno, 2023). 

What works: Evidence-based Strategies to Improve Adherence 

While no single intervention universally addresses adherence challenges across all conditions, a coordinated bundle of pragmatic strategies consistently improves Proportion of Days Covered (PDC), reduces medication gaps, and enhances clinical outcomes. 

1. Reduce Financial Barriers 

• Lowering or eliminating copays for high-value medications has demonstrated measurable impact. In the MI FREEE trial, full coverage of medication costs after a myocardial infarction increased medication adherence by 4–6 percentage points and appeared cost-effective without raising overall spending (Choudhry et al., 2011). Value-based insurance design applies this principle to chronic disease classes with robust evidence. 

2. Simplify Regimens 

• Providing 90-day supplies and automatic refills mitigates gaps in therapy. 

• Medication synchronization—aligning refill dates—has been shown to increase PDC in real-world programs. 

• Fixed-dose combinations and once-daily dosing reduce cognitive burden. Multiple observational studies and policy guidance endorse regimen simplification as a best practice. 

3. Enhance Access 

• Mail and home delivery services facilitate adherence for patients in pharmacy deserts or with transportation limitations (Qato et al., 2014; Wittenauer et al., 2024; Hooper, 2022; Anderson & Mattingly, 2025; Chen, 2021). 

• Bedside medication delivery at discharge reduces primary nonadherence during transitions of care. 

4. Implement Team-Based, Pharmacist-Enabled Care 

• Comprehensive medication reviews (CMRs) and pharmacist outreach address side effects, therapy duplication, and adherence barriers; CMR completion is itself a Star measure (Oueini, 2022). 

• Pharmacist-led adherence interventions have reduced both readmissions and mortality among patients with heart failure (Ruppar et al, 2015; Ruppar et al, 2016; Khazanie & Allen, 2016). 

5. Leverage Behavioral Supports and Digital Tools 

• Text (SMS) reminders, interactive voice calls, mobile app prompts, and smart packaging support timely dosing. A 2008 Cochrane review highlights that multi-component interventions—combining education, regimen simplification, and structured follow-up—outperform single-component strategies (Haynes et al., 2008). 

6. Align Formulation with Clinical Context 

• Long-acting injectable antipsychotics improve persistence and reduce relapse-related hospitalizations in serious mental illness (Kishimoto et al., 2014; Boyer et al. 2023; Barnett & Pappa, 2025) 

• Single-tablet regimens and long-acting injectables for HIV enhance adherence and viral suppression in appropriately selected patients (Cutrell & Bedimo, 2016; Nachega et al., 2023; Liu et al., 2025) 

7. Address Social Determinants of Health (SDoH) 

• Transportation services, copay assistance, food and housing support, and benefits navigation are integral to adherence strategies. Recent analyses indicate that linking patients to social supports can reduce total healthcare expenditures by approximately 11%, with improved adherence serving as a key mechanism (Farley & Pradeep, 2024). 

How Illustra Health Helps You Identify and Address Adherence Problems 

Value-based organizations need two core capabilities: (1) the ability to proactively identify patients with medication adherence risks before adverse outcomes occur, and (2) the ability to effectively reduce barriers and improve adherence. 

Turning Data into Action for Care Teams 

Illustra Health integrates claims, EHR (electronic health records), pharmacy, and social determinants of health (SDoH) data to create a whole-person view and translate it into prioritized actions for clinicians, care managers, and outreach teams. Illustra provides multiple medication adherence metrics—including proportion of days covered (PDC), medication-related problems (MRPs), and treatment gap analyses—across a wide range of conditions. These metrics are surfaced alongside risk stratification and SDoH insights to give care teams actionable opportunities at both the population and patient level. 

Illustra delivers adherence data and insights for: 

• Diabetes 

• Disorders of Lipid Metabolism 

• Heart Failure (CHF) 

• HIV 

• Hypertension 

• Hypothyroidism 

• Immunosuppression (post-transplant) 

• Ischemic Heart Disease 

• Osteoporosis 

• Parkinson’s Disease 

• Persistent Asthma 

• Seizure Disorders 

• Depression 

• Bipolar Disorder 

• Schizophrenia 

Integrating SDoH for Adherence Improvement 

Beyond medication metrics, Illustra incorporates both documented social needs (e.g. transportation, food insecurity, housing instability) and geographically based risk indicators to provide a comprehensive picture of adherence risk. This allows care teams to align clinical strategies with social supports. 

Why this Matters for Value-Based Performance 

Because CMS Star Ratings and many value-based contracts explicitly reward adherence in therapeutic classes such as diabetes, RAS antagonists, and statins—and because adherence strongly influences outcomes in HIV, transplant, heart failure, asthma, osteoporosis, and other conditions—Illustra’s prioritized, patient-level action lists help teams close gaps that improve both clinical outcomes and financial performance. 

Conclusion 

Medication adherence is one of the clearest “do well by doing good” levers in value-based care:  

Better adherence → better outcomes → fewer avoidable admissions → lower total cost of care  

Measuring adherence (PDC, medication gaps), understanding the social and structural barriers, and deploying a multi-component playbook are the foundations for improving medication adherence. Platforms like Illustra Health operationalize this work by unifying clinical, claims, and social data, flagging patients with medication adherence issues, and guiding teams to interventions that work—for diabetes, hypertension, lipids, heart failure, HIV, transplant, mental health conditions, and beyond. 

Citations 

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