AI Planning and/or Scheduling techniques have been applied to problems that concern managing patient tests in hospital laboratories. In this paper, we propose the PTTPS model (Patient Test Timeline-based Planning and Scheduling) for planning and scheduling of patient tests in hospitals. PTTPS is an action-based model, which is based on the Constraint Network on Timelines (CNT) framework. The PTTPS knowledge representation schema enables the representation of various attributes of laboratories, equipment, hospital personnel, patients, tests, requests for tests, medical rules, as well as possible conflicts, constraints and appropriate sequence of tests. Patient tests are encoded as macro actions, each of which includes a set of subactions that represent steps of a patient test. We use CNT framework to produce appropriate constraints for the proposed PTTPS action-based model and we model an example of a patient test problem, the pyelography test. Keywords: AI planning and scheduling, constraint-based modeling, timelines, patient tests;