Aims Midregional pro atrial natriuretic peptide (MR-proANP) is useful for diagnosing acute heart failure (HF) in patients presenting to the Emergency Department with dyspnoea. Optimal interpretation of MR-proANP requires understanding of how various demographic variables influence its levels and performance as a diagnostic marker. We sought to determine how age, race, sex, and body mass index (BMI) affect the levels and interpretation of MR-proANP for the diagnosis of acute HF. Methods and results The Biomarkers in Acute Heart Failure (BACH) study was an international 15-centre study of 1641 patients presenting to the Emergency Department with acute dyspnoea. Of these, 1352 had complete information on age, race, sex, and BMI. MR-proANP levels increased with age and were higher in men and in patients with lower BMI. MR-proANP performed better as a diagnostic marker in younger individuals and in blacks compared with whites. Despite this, MR-proANP at the recommended cut-off point of 120 pmol/L was >90% sensitive in ruling out the diagnosis of acute HF in all subgroups of patients except white subjects