Accurate ability to diagnose lumbar spine clinical instability is controversial for numerous reasons, including inaccuracy and limitations in capabilities of radiographic findings, poor reliability and validity of clinical special tests, and poor correlation between spinal motion and severity of symptoms. It has been suggested that common subjective and objective identifiers are specific to lumbar spine clinical instability. The purpose of this study was to determine if consensual, specific identifiers for subjective and objective lumbar spine clinical instability exist as determined by a Delphi survey instrument. One hundred and sixty eight physical therapists identified as Orthopaedic Clinical Specialists (OCS) or Fellows of the American Academy of Orthopaedic Manual Physical Therapists participated in three Delphi rounds designed to select specific identifiers for lumbar spine clinical instability. Round I consisted of open-ended questions designed to provide any relevant issues. Round II allowed the participants to rank the organized findings of Round I. Round III provided an opportunity to rescore the ranked variables after viewing other participant's results. The results suggest that those identifiers selected by the Delphi experts are synonymous with those represented in related spine instability literature and may be beneficial for use during clinical differential diagnosis.