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 claims data. Study design and setting We conducted a cohort study using 2002–2007 US Medicaid data. Initiators of eight drug groups were indentified: statins, metformin, atypical antipsychotics, warfarin, proton pump inhibitors (PPIs), angiotensin converting enzyme (ACE)-inhibitors, non-steroidal 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 Daily Defined Doses (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 (70days vs. 75days), PPIs (44days vs. 45days) and coxibs (44days vs. 45days). 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 0.2–0.8. Conclusion 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.

Comparative evaluation of methods approximating drug prescription durations in claims data: modeling, simulation, and application to real data

“…Because dose requirements depend on individual clearance of the drug and are often influenced by age, estimations using individual parameters for dose calculations appear more promising than strategies assuming standard doses for all patients (e.g., OTAD or DDD). Thus, it may be speculated that drug consumption expressed as DDD may lead to considerable miscalculations in such situations …”

“…Thus, it may be speculated that drug consumption expressed as DDD may lead to considerable miscalculations in such situations. 33,34 In this study, we compared different approximations of drug prescription durations in claims data with respect to accuracy and impact on clinical endpoints. M&S revealed that longitudinally averaging drug utilization was the method yielding the most accurate estimations regarding several measures of accuracy.…”

Can Disease-Modifying Anti-Rheumatic Drugs Reduce the Risk of Developing Dementia in Patients with Rheumatoid Arthritis?

“…As patients may discontinue or restart drug therapy, the analyses assumed that a patient's exposure to DMARDs contributed both cumulatively and continuously to their long-term risk of dementia. Defined daily dose (DDD) is the assumed average maintenance dose per day for a drug used for its primary usage in adults [22]. To investigate the effect of dose, the cumulative use of DMARDs was calculated as the total prescribed DDD (i.e., the same as the total dispensed DDD).…”

The international generalisability of evidence for health policy: A cross country comparison of medication adherence following policy change

“…[31] The method of calculating a days' supply variable using the DDD and its concordance with the conventional days' supply variable is outlined in our previous work. [32] Monthly PDCs were calculated for each patient, running consecutively from cohort entry to the end of follow up for each individual. If a dispensing occurred before the previous dispensing ran out, the new dispensing was assumed to begin the day after the end of the prior dispensing and the diary was adjusted accordingly.…”