Electronic Letters to Improve Patient Activation in IHD: The NUDGE-IHD Trial

The goal of this clinical trial is to learn whether simple electronic information letters can increase patient activation and improve risk-factor monitoring in adults in Denmark with ischemic heart disease (IHD) who have LDL cholesterol above the recommended treatment target. A subgroup of participants with elevated lipoprotein(a) \[Lp(a)\] will also be randomized to receive an additional information letter. The main questions the study aims to answer are: * Does sending an electronic letter about elevated LDL cholesterol increase the proportion of patients who have at least one LDL-cholesterol test within 6 months? * Does sending an electronic letter about elevated LDL cholesterol reduce major adverse cardiovascular events (cardiovascular death, myocardial infarction, or stroke) over 5 years? * Among patients with ischemic heart disease and elevated Lp(a), does receiving an information letter about Lp(a) increase the proportion who have at least one LDL-cholesterol test within 6 months, and does it reduce major adverse cardiovascular events over 5 years? Because this is a randomized trial, researchers will compare people who receive the electronic letter(s) with people who do not receive any letter to determine whether the letters encourage patients to take action, such as obtaining laboratory tests or contacting their doctor. Participants will: * Receive an electronic letter through Denmark's national digital mailbox system (Digital Post) or receive no letter, depending on random assignment. * Continue their usual health care, with no additional visits, treatments, or procedures required for the study. * Have all study information collected from existing Danish nationwide health registries.

Inclusion criteria (primary study population) * Alive and resident in Denmark on the identification date * Diagnosis of IHD defined as an A (action) diagnosis code of I20-I25 in the Danish National Patient Register, first registered \> 6 months prior to the identification date but after January 1, 2000 * Latest available LDL-C is over 1.4 mmol/l measured up to 3 years prior to the identification date, defined using NPU codes NPU01568, NPU10171 or DNK35308 from the Danish National Laboratory Register * Age \>= 18 years and \= 105 nmol/L or \>= 50 mg/dL. Only participants in this subgroup undergo the second randomization to the Lp(a) letter.