The invention pertains to a method of measuring Aβ protofibrils in a sample. The method includes the steps of (a) providing a labeled monoclonal antibody or fragment thereof that binds wildtype Aβ 42/40 protofibril and Aβ42/40 Arc protofibril and has no or little cross-reactivity to Aβ 42/40 monomers with an agent that generates a measurable signal, (b) contacting said labeled antibody or fragment thereof with the fluid or tissue having or suspected of comprising Aβ protofibrils, and (c) measuring the amount of protofibrils bound to the antibody by measuring the signal generated by the agent.