Introduction: Vastus mediallis muscle is the dynamic stabilizer of patella and current physical examination maneuvers for patellar instability are performed without quadriceps contraction. We coined a new patellar apprehension test. The inter observer agreement of this active patellar apprehension test was compared to that of current diagnostic maneuvers of patellar instability.Methods: The traditional apprehension test, moving apprehension test, and active patellar apprehension test were performed in the same patients with patella instability. The three tester groups of doctors performed each maneuver in the office without anesthesia in 42 patients. The active patella apprehension test was performed with the following technique. The examination begins with the knee held in full extension and the patella is manually translated laterally with the thumb. The knee is flexed to 90° and then brought back to full extension actively while the lateral force on the patella is maintained (i.e. subluxation maneuver). After that, knee is started in full extension, flex actively to 90° of flexion, and then back to full extension while the index finger is used to translate the patella medially (i.e reduction maneuver). For a positive test in exam, the patient cannot extend the knee (extension lag sign) or expresses apprehension, and the patient expresses no apprehension and free flexion and extension of the knee with the reduction.Results: The overall agreement of active apprehension test was 100%(κ=1.0). The overall agreement of moving apprehension test was 92%(κ=0.85) and traditional apprehension test was 89%(κ=0.78) respectively. The sensitivity and specificity of the apprehension test was 82% and the sensitivity of active apprehension test was 100%.Conclusions: The active patellar apprehension test is an accurate and reproducible physical examination technique that showed more interobserver agreement when compared to the traditional apprehension test.