title: Measuring drug exposure: concordance between defined daily dose and days' supply depended on drug class.
abstract: OBJECTIVE
  To determine the concordance between two methods to measure drug exposure duration from pharmacy claim data.
  
  
  METHODS
  We conducted a cohort study using 2002-2007 US Medicaid data. Initiators of eight drug groups were identified: statins, metformin, atypical antipsychotics, warfarin, proton pump inhibitors (PPIs), angiotensin-converting enzyme (ACE) inhibitors, nonsteroidal anti-inflammatory drugs (ns-NSAIDs), and coxibs. For each patient, we calculated two measures of exposure duration using (1) observed days' supply available in US pharmacy claims and (2) the World Health Organisation's Defined Daily Dose (DDD) methodology. We used Wilcoxon signed rank tests to compare medians and Spearman correlations to assess correlation between the two measures.
  
  
  RESULTS
  Cohort sizes ranged from 143,885 warfarin users to >3,000,000 ns-NSAID users. Similar median exposure durations were observed for ACE inhibitors (70 days vs.75 days), PPIs (44 days vs. 45 days), and coxibs (44 days vs. 45 days). The DDD method overestimated exposure duration for ns-NSAIDs and underestimated for the remaining drug groups, relative to days' supply. Spearman correlation coefficients ranged from 0.2 to 0.8.
  
  
  CONCLUSIONS
  Using DDDs to estimate drug exposure duration can result in misclassification. The magnitude of this misclassification might depend on doses used which can vary according to factors such as local prescribing practices, renal function, and age.
authors.affiliation: Department of Epidemiology and Public Health, University College Cork, 4th Floor Western Gateway Building, Cork, Ireland. Electronic address: sarah-jo.sinnott@esri.ie.
authors.forename: Sarah-Jo
authors.initials: SJ
authors.lastname: Sinnott
authors.affiliation: Division of Pharmacoepidemiology and Pharmacoeconomics, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, 1620 Tremont Street, Suite 3030, Boston, MA, 02120, USA.
authors.forename: Jennifer M
authors.initials: JM
authors.lastname: Polinski
authors.affiliation: Pharmaceutical Care Research Group, School of Pharmacy, University College Cork, College Rd, Cork, Ireland.
authors.forename: Stephen
authors.initials: S
authors.lastname: Byrne
authors.affiliation: Division of Pharmacoepidemiology and Pharmacoeconomics, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, 1620 Tremont Street, Suite 3030, Boston, MA, 02120, USA.
authors.forename: Joshua J
authors.initials: JJ
authors.lastname: Gagne
mesh_terms: D000328:Adult; D015331:Cohort Studies; D016208:Databases, Factual; D004334:Drug Administration Schedule; D011307:Drug Prescriptions / Q000706:statistics & numerical data*; D004358:Drug Therapy / Q000706:statistics & numerical data*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D013997:Time Factors
keywords: Bias (epidemiology); Daily defined dose; Drug exposure; Medicaid; Pharmacoepidemiology; Prescription drugs/supply