Objective: To analyze factors associated with Thai heart failure with reduced ejection fraction patients who received sub-target doses of Angiotensin converting enzyme inhibitors (ACEIs), angiotensin receptor-neprilysin inhibitors (ARNIs), and angiotensin receptor blockers (ARBs).

Methods: The retrospective cohort study was designed to collect data on Thai heart failure with reduced ejection fraction patients at Out-patient Clinic, Trat Hospital and Phrapokklao Hospital from January 2016 to December 2020. Thirteen factors were analyzed to determine their association with the sub-target group.

Inclusion Criteria:

• Patients who were first diagnosed be heart failure with left ventricular ejection fraction less than 40%.
• Received one of ACEIs,ARBs,ARNI drugs.

Exclusion Criteria:

• Electronic medical records was not complete enough to analyse.
• Patients who had history of heart transplantation or had heart transplantation plan in last 12 months.

Adult out - patients who were first diagnosed be heart failure with left vetricular ejection fraction less than 40% and recieved one of ACEIs,ARBs,ARNI drugs.