The Trendelenburg test is a test to show the efficiency of the hip abductors, specifically gluteus medius.
Anything making gluteus medius work extra hard, compromises its function and results in a positive Trendelenburg. For example, CDH, osteoarthritis, hip fracture. For this reason the Trendelenburg test is not differential. It will not tell you what condition is causing the test to be positive, however, when negative, it can be useful as it helps the practitioner know there is no fracture for example.