An application program interface (API) for use in monitoring a computer application executed on a computer system. The API includes software code added to the computer application for assigning a single general reference to characteristic transactional information associated with a transaction to be executed by the computer application. The API further includes an agent for marking the time at which the software code is executed and tagging that time with the characteristic transactional information as the characteristic transactional information is being currently processed by the computer application. The API and its method of operation provide precise measurement of the latency of information flowing through the computer system while being uncomplicated in design and implementation, minimally invasive, and highly scalable.