The effects of mandatory prescribing of thiazides for newly treated, uncomplicated hypertension: interrupted time-series analysis

Abstract

Background: The purpose of our study was to evaluate the effects of a new reimbursement rule for antihypertensive medication that made thiazides mandatory first-line drugs for newly treated, uncomplicated hypertension. The objective of the new regulation was to reduce drug expenditures.

Methods and findings: We conducted an interrupted time-series analysis on prescribing data before and after the new reimbursement rule for antihypertensive medication was put into effect. All patients started on antihypertensive medication in 61 general practices in Norway were included in the analysis. The new rule was put forward by the Ministry of Health and was approved by parliament. Adherence to the rule was monitored only minimally, and there were no penalties for non-adherence. Our primary outcome was the proportion of thiazide prescriptions among all prescriptions made for persons started on antihypertensive medication. Secondary outcomes included the proportion of patients who, within 4 mo, reached recommended blood-pressure goals and the proportion of patients who, within 4 mo, were not started on a second antihypertensive drug. We also compared drug costs before and after the intervention. During the baseline period, 10% of patients started on antihypertensive medication were given a thiazide prescription. This proportion rose steadily during the transition period, after which it remained stable at 25%. For other outcomes, no statistically significant differences were demonstrated. Achievement of treatment goals was slightly higher (56.6% versus 58.4%) after the new rule was introduced, and the prescribing of a second drug was slightly lower (24.0% versus 21.8%). Drug costs were reduced by an estimated Norwegian kroner 4.8 million (0.58 million Euros, US$0.72 million) in the first year, which is equivalent to Norwegian kroner 1.06 per inhabitant (0.13 Euros, US$0.16).

Conclusions: Prescribing of thiazides in Norway for uncomplicated hypertension more than doubled after a reimbursement rule requiring the use of thiazides as the first-choice therapy was put into effect. However, the resulting savings on drug expenditures were modest. There were no significant changes in the achievement of treatment goals or in the prescribing of a second antihypertensive drug.

Figure 1. An ITS is a Quasi-Experimental Design that Can Be Used to Evaluate an Intervention when a True Randomised Controlled Experiment Is Not Feasible

In this study, the intervention was a policy change that was being delivered to all physicians in Norway and, because of this, no control was available. An ITS design strengthens a before-and-after design by taking repeated measurements of the outcome over time, both pre- and post-intervention. Using appropriate statistical methods, it is then possible to estimate the effect of an intervention given the underlying trend of the data, and taking into account effects such as seasonality or serial correlation. There are a variety of possible intervention effects in an ITS experiment, for example where the intervention has had an effect on the slope and level of the outcome.

Figure 2. Proportion of Thiazide Prescriptions among All Prescriptions for Patients Started on Treatment for Uncomplicated Hypertension

Figure 3. Proportion of Patients Achieving Treatment Goals (≤140/90 mm Hg) within 4 mo, among All Started on Treatment for Uncomplicated Hypertension

Figure 4. Proportion of Patients Receiving a Second Antihypertensive Drug within 4 mo of Starting Treatment for Uncomplicated Hypertension

Figure 5. Proportion of Thiazide Prescriptions among Practices from Intervention Group in Earlier Trial (Group 1) and Other Practices (Group 2)