Aims: This retrospective analysis sought to identify markers that might distinguish between acute heart failure (HF) and worsening HF in chronic outpatients.Methods and Results: The BIOSTAT-CHF index cohort included 2516 patients with new or worsening HF symptoms: 1694 enrolled as inpatients (acute HF) and 822 as outpatients (worsening HF in chronic outpatients). A validation cohort included 935 inpatients and 803 outpatients. Multivariable models were developed in the index cohort using clinical characteristics, routine laboratory values, and proteomics data to examine which factors predict adverse outcomes in both conditions and to determine which factors differ between acute HF and worsening HF in chronic outpatients, validated in the va...