Differential Diagnosis for Physical Therapists: Screening for Referral by Goodman and Helgeson serves as the primary text for identifying "red flags" and distinguishing between musculoskeletal and systemic pain. The screening process requires a systematic review of systems and the identification of critical symptoms requiring immediate medical referral, such as cauda equina syndrome, to ensure patient safety. Detailed guidelines are available through resources like Elsevier and Amazon.