Impact of over-the-counter medication use on patients' health-related quality of life: development and psychometric validation of Over-the-Counter Medication Impact Scale.

BACKGROUND AND
OBJECTIVE: Over-the-counter (OTC) medication use can impact an individual's health-related quality of life (HRQoL); however, there is no instrument specifically developed to measure the impact of OTC medication use on patients' HRQoL. The aim of the study was to develop an OTC Medication Impact Scale (OTC-MIS), a generic patient-reported outcome measure to assess the impact of OTC medication use on patients' HRQoL.
METHODS: Items were generated based on the principles of the HRQoL theory, and were comparable to the domains of the Short-Form version 12 scale (SF-12). The OTC-MIS was tested for construct and criterion validity, internal consistency reliability, and floor and ceiling effects. The psychometric properties of the scale were evaluated in a general elderly population (n = 153), as well as in a disease-specific HIV population (n = 215), to assess generalizability of the scale in different populations.
RESULTS: All the items had response rates >95 %; there were no floor or ceiling effects in the scale. Reliability in terms of Cronbach's alpha was >0.9, and exploratory factor analysis indicated unidimensionality of the scale. The instrument showed known-group validity (p < 0.01) among patients with and without OTC medication-associated adverse events, and adequate criterion validity when compared with the SF-12v2 scale (p < 0.05).
CONCLUSION: The OTC-MIS is a validated and reliable measure to assess the impact of OTC medication use on patient HRQoL and may prove beneficial in evaluating the role OTC medications play in healthcare processes.