The SLR test is used to help distinguish the source of sciatic pain, whether it be a space-occupying lesion such as a disc protrusion in the path of the nerve root,  inflammation, tumor, adhesions, edema, etc. or dysfunction of the sacroiliac or lumbosacral joints. 

To perform the test, the will be patient lying supine on the exam table, with both legs extended out.  Stabilizing the knee with one hand, the doctor will place the other hand under the ipsilateral heel and begin to slowly and gently raise the leg, being sure to keep the leg in an extended position.  It is important for the examiner to note the angle (in degrees) at which pain is produced in the leg.  The leg can normally be raised 15 to 30 degrees before nerve root pain occurs from traction through the intervertebral foramen (IVF).  

For this parameter, record the angle in degrees at which pain is produced. A patient is able to pass the test with the highest score if there is no pain and full ROM at 90 degrees. Sciatic pain at: 0-30 degrees indicates nerve root compression and possible space-occupying mass ( for example: disc protrusion), 31-60 degrees indicates sacroiliac joint disease or IVD issue, 61-70 degrees indicates lumbosacral involvement. Any pain produced at 60-70 degrees most likely originates in the joint.

(Source: Illustrated Orthopedic Physical Assessment by Ronald C. Evans)