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Background: Several clinical tests used to identify patients with lumbar instability have reported diagnostic accuracy in separate studies with conflicting results. To augment the diagnostic process, tests that are better able to identify lumbar instability suitable for use in the clinical setting are required. The aim of this study was to identify the probability to diagnose patients with lumbar instability, using x-ray imaging as the reference standard., Methods: This study was a cross-sectional, diagnostic validity study. One hundred forty participants with chronic low back pain underwent an x-ray assessment and 14 clinical examinations. Data were analysed using multivariate regression methods to determine which clinical tests were most diagnostic for lumbar instability when they were applied together., Results: Eighteen (12.85%) participants had radiological lumbar instability. Three clinical tests i) interspinous gap change during flexion-extension, ii) passive accessory intervertebral movement tests, iii) posterior shear test demonstrated an ability to diagnose lumbar instability of 67% when they were all positive. At this probability threshold, sensitivity, specificity, positive likelihood ratio (+LR), and negative likelihood ratio (-LR) were 5.56, 99.18%, 6.78, and 0.95., Conclusions: These 3 clinical tests could be useful in identifying patients with lumbar instability in the general community. These three tests are simple to perform by physical therapists, reliable to use in a clinical setting, and safe for patients. We recommend physical therapists use these three tests to assess patients who are suspected of having lumbar instability, in the absence of an x-ray assessment, to receive appropriate targeted intervention or referral for further investigation., Trial Registration: Thai Clinial Trial Registry (TCTR 20180820001; 19th August 2018)., (© 2021. The Author(s).)