Construct validity and factor structure of survey-based assessment of cost-related medication burden

Millions of Americans are burdened by out-of-pocket prescription costs. Although many survey measures have been developed to assess this burden, the construct validity and the factor structure of these instruments have not been rigorously assessed.

To characterize the factor structure and the construct validity of items assessing cost-related medication burden.

We applied exploratory factor and confirmatory factor analyses to the 2009 Medicare Current Beneficiary Survey, focusing on 10 items assessing cost-related mediation burden among a nationally representative sample of community-dwelling Medicare beneficiaries. The fit of competing models was compared using several indices.

The study population (N=8777) was predominantly aged over 65 years (83.3%), female (54.4%), and white (84.3%). Two distinct factors were present for the medication cost-reduction strategies: (1) cost-related medication nonadherence and (2) drug-shopping behaviors, not directly impacting medication compliance. The two factors were moderately correlated (r=0.55), highlighting the presence of a 2 distinct but related constructs for cost-related medication burden. An item assessing the use of mail or internet pharmacies did not load well on either factor and may not necessarily measure medication-related cost burden. An item assessing reduced spending on basic needs loaded strongly on the same factor with the cost-related medication nonadherence items, suggesting they together may represent extreme compensatory behaviors that may adversely affect health outcomes.

Two distinct constructs were derived from these items examining cost-related medication burden. Although cost-related medication burden is often associated with nonadherence, drug-shopping behaviors that do not directly impact adherence are also important measure of this burden.