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?
• Among patients with ischemic heart disease and elevated Lp(a), does receiving an information letter about Lp(a) increase patient activation, reflected by cardiometabolic risk-factor monitoring?

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.

This study is a nationwide, pragmatic, registry-based, partially factorial randomized trial conducted in Denmark. The trial evaluates whether simple electronic information letters ("nudges") can increase patient activation and support guideline-directed risk-factor monitoring among persons with ischemic heart disease (IHD) who have LDL cholesterol levels above guideline-recommended treatment targets in routine clinical care.

The primary study population consists of adults with documented ischemic heart disease and LDL cholesterol above the recommended target level. Eligible participants are identified using Danish nationwide health registries and randomized 1:1 to receive either an electronic information letter addressing elevated LDL cholesterol (the "LDL letter") or no letter. The LDL letter provides general, non-technical information about LDL cholesterol as a cardiovascular risk factor in ischemic heart disease and encourages recipients to consult their physician if relevant.

A secondary study population is defined as a subset of the primary population with elevated lipoprotein(a) [Lp(a)] levels identified from laboratory registry data. Participants in this subgroup are independently randomized 1:1 to receive or not receive an additional electronic information letter addressing elevated Lp(a) (the "Lp(a) letter"). The Lp(a) letter provides information on Lp(a) as a genetically determined and independent cardiovascular risk factor and emphasizes the importance of optimizing other modifiable risk factors. The two randomizations are conducted independently, resulting in a partially factorial design in which participants with elevated Lp(a) may receive no letter, the LDL letter only, the Lp(a) letter only, or both letters.

All intervention letters are delivered through Denmark's mandatory national digital mailbox system (Digital Post). The interventions are informational and non-invasive. The study does not involve any in-person visits, clinical procedures, or changes to usual medical care.

The primary outcome is disease-related patient activation, defined as the proportion of participants with at least one LDL cholesterol measurement recorded within 6 months after randomization. Secondary outcomes include general patient activation, assessed by the number of healthcare contacts in general practice or outpatient hospital settings, and cardiometabolic risk-factor monitoring, defined as the proportion of participants with at least one cardiometabolic blood test (lipid profile, hemoglobin A1c, or creatinine) within 6 months after randomization. Additional exploratory outcomes related to laboratory testing, healthcare utilization, medication use, and clinical events are assessed during longer-term follow-up.

All baseline characteristics, outcomes, and follow-up data are obtained from Danish nationwide health and administrative registries. Due to the registry-based and informational nature of the interventions, informed consent is not required. The primary analysis is conducted 6 months after randomization, with additional exploratory follow-up extending up to 5 years.

Inclusion criteria

• Alive on identification date
• Diagnosis of IHD defined as either an A or B diagnosis code of I20-I25 in the Danish National Patient Register first registered > 6 months prior to identification date
• 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 <= 85 on identification date

Exclusion criteria

• Nursing home residents
• Exemption from and thus not access to Digital Post

Data will be collected from Danish administrative health registries, which are subject to Danish legislation and can only be made available to a third party under certain conditions. Please contact the sponsor-investigator in case of any inquiries.