Fast spreading of antimicrobial resistance is now considered a major global health threat. New technologies are required, enabling rapid diagnostics of bacterial infection combined with fast antimicrobial susceptibility testing (AST) for evaluating the efficiency and dosage of antimicrobial compounds in vitro. This work presents an integrated chip-based isothermal nanocalorimetry platform for direct microbial metabolic heat measurements and evaluates its potential for fast AST. Direct detection of the bacteria-generated heat allows monitoring of metabolic activity and antimicrobial action at subinhibitory concentrations in real-time. The high heat sensitivity of the platform enables bacterial growth detection within only a few hours of incubation, whereas growth inhibition upon administration of antibiotics is revealed by a decrease or the absence of the heat signal. Antimicrobial stress results in lag phase extension and metabolic energy spilling. Oxygen consumption and optical density measurements provide a more holistic insight of the metabolic state and the evolution of bacterial biomass. As a proof-of-concept, a metabolic heat-based AST study on Escherichia coli as model organism with 3 clinically relevant antibiotics is performed and the minimum inhibitory concentrations are determined.