NT-proBNP is a biomarker used in the diagnosis of heart failure, but its values are often elevated in atrial fibrillation, which complicates the diagnosis. The study evaluated 155,347 people, of whom 17,403 had atrial fibrillation. Heart failure was diagnosed in 23.9% of people with atrial fibrillation (median NT-proBNP 1,852 pg/ml) and 7.6% without atrial fibrillation (median 1,110 pg/ml). NT-proBNP discriminated heart failure better in people without atrial fibrillation (AUC 0.877) than in people with atrial fibrillation (AUC 0.743). In atrial fibrillation, the specificity of NT-proBNP was 98.8% at 125 pg/ml and 93.2% at 400 pg/ml, the sensitivity was 13.2% and 35.3%. In older age (≥ 65 years), the ability of NT-proBNP to distinguish heart failure was worse. The study recommends considering a higher NT-proBNP reference threshold in atrial fibrillation due to higher median levels.