New Prescription Medication Gaps: A Comprehensive Measure of Adherence to New Prescriptions

Medication nonadherence has well-established clinical consequences. In diabetes, poorer medication adherence has been associated with poorer glycemic, blood pressure, and lipid control (Morris et al. 1997; Ho et al. 2006a;), higher rates of acute metabolic events (Morris et al. 1997), and increased risk of hospitalization and mortality (Ho et al. 2006a,b; Simpson et al. 2006; Jackevicius, Li, and Tu 2008;). There are well-established and validated methods to quantify secondary adherence for ongoing, long-term (prevalent) medication use (Steiner et al. 1988; Steiner and Prochazka 1997;). However, these secondary adherence measures give biased estimates for newly prescribed therapies because the methodologies exclude significant periods of nonadherence by (1) excluding patients who never initiated therapy, (2) excluding patients who never obtained a refill (because at least two dispensings are required to determine a secondary adherence rate), and (3) censoring follow-up time at the last fill, thus ignoring nonadherence after discontinuation. Conventional adherence measures therefore systematically underestimate the public health burden of poor medication adherence for newly prescribed medications (DiMatteo et al. 2002; Sabate 2003;). The recent introduction of electronic medical records with electronic recording of physician orders for new prescriptions (as well as dosage changes for prevalent prescriptions, discontinuation orders, and notation of drug-related side effects) now facilitates a more detailed and comprehensive assessment of adherence. We can identify cohorts of patients who have been prescribed new medications (new prescription cohorts) and determine whether medication was ever dispensed (primary adherence), was refilled at least once (early persistence), used as prescribed (secondary adherence), or subsequently discontinued without clinical advice or recognition. Adherence during the initial stages following a new prescription, particularly primary adherence, has been studied in only a few populations, despite the obvious public health implications of not initiating newly prescribed therapy for a chronic condition. In this paper, we describe the stages of adherence for newly prescribed medications and present and validate a novel, comprehensive adherence metric, New Prescription Medication Gap (NPMG), which summarizes gaps in medication supplies during a defined period following a new prescription. This new measure more comprehensively summarizes adherence than existing secondary (ongoing) adherence measures because it captures additional, critical periods of nonadherence. We will compare NPMG to a widely used conventional measure of secondary adherence (continuous, multiple-interval measure of gaps [CMG]; Steiner and Prochazka 1997) in a study of medication adherence observed over the 2 years following a new prescription for any cardiometabolic (antihypertensive, cholesterol lowering, or antihyperglycemic) therapy in a cohort of diabetic patients. NPMG is not meant to replace existing, validated measures of secondary adherence (e.g., CMG), but rather to provide an additional and more comprehensive assessment of adherence in patients who are prescribed a new medication for a chronic condition.