(P-450) Incidence in Pharmacoepidemiology – New on the Drug versus New on the Class of Drugs

Background: It is essential to identify new users for a specific drug reliably when using new-user designs in pharmacoepidemiology. However, when only a single substance (ATC-code) is considered, patients may be identified as new users even though they have previously been treated with substances belonging to the same drug class.

Objectives: In this study, we investigated how much a cohort of new users for different statin medications changed when ignoring or considering other statins in the patient's treatment history.

Methods: Design & setting:
A retrospective study covering all individuals in Sweden 2019 dispensed at least one prescription of C10AA HMG CoA reductase inhibitors (statins) or C10AB01-C01AB09 statins in fixed combinations. Data from Swedish Prescribed Drug Register at the National Board of Health and Welfare for dispensations from 1st of July 2005.
For each individual's first dispensation of a statin in 2019 number of days since the previous dispensation of the same substance identified. In addition, the number of days and ATC-code since the last dispensation for another statin, sex and age for each individual.
Main outcome measures:
The incidence proportion 2019 with a run-in period of 12 months and positive predictive value, PPV, compared to 10 years run-in.
For each statin, three different definitions of incidence proportion:
I. New on specified substance whether or not earlier treated with another substance in the group (sum of II and III).
II. New on specified substance and new on group
III. New on specified substance and not new on group
For statins as a group:
IV. New on group regardless of substance

Results: For all ages, both sexes, all of Sweden, 2019.

I. New on specified statin whether or not earlier treated with another substance in the group
simvastatin 2.24 (2.21-2.27), n=22,921, PPV 50%
atorvastatin 10.69 (10.62-10.75), n=109,313, PPV 80%
rosuvastatin 1.65 (1.65-1.68), n=16,895, PPV 82%
II. New on specified statin and new on statins
simvastatin 2.14 (2.11-2.17), n=21,905, PPV 48%
atorvastatin 9.99 (9.93-10.05), n=102,161, PPV 68%
rosuvastatin 1.21 (1.19-1.23), n=12,365, PPV 44%
III. New on specified statin and not new on statins
simvastatin 0.10 (0.09-0.11), n=1,016, PPV 42%
atorvastatin 0.70 (0.68-0.72), n=7,152, PPV 83%
rosuvastatin 0.44 (0.43-0.46), n=4,530, PPV 92%
IV. New on statins regardless of substance 13.71 (13.64-13.79), n=140,288, PPV 62%

Conclusions: When defining new users of a drug, it is important not only to differentiate between first-ever use and recurrent treatment episode but also to consider the earlier use of other substances that can be considered substitutes within the same group of substances.