Diagnostic Clinical Reasoning (DxR) is a strong focus within pre- and post-professional physical therapy (PT) training programs. However there is no consistency regarding what DxR is, how it is taught, or how PT measures progressive development of these skills. Further, there is little evidence correlating the development of DxR and expertise in practice. In particular, there is little distinction in the PT literature between diagnostic reasoning and therapeutic reasoning. The purposes of this srticle are two-fold: 1) to describe the construction of a novel Script Concordance Test (SCT) which proves capable of measuring the hypothetical clinical decisions of PT clinicians treating patients with lumbar spine disorders, and 2) to demonstrate the SCT’s validity and reliability as an indirect instrument for assessing clinical decision making.