I have a rotator cuff injury and drop arm test shows supraspinatus tear. do i need to have surgery?

Depends: If you have a complete rotator cuff tear with weakness and night pain in your dominant shoulder then yes, at age 45 you may need surgery. You don't mention having an mri. If you do get an MRI arthrogram as it provides more information. Have you tried any physical therapy yet ?

Yes: A full thickness rotator cuff tear will not heal on its own. Small tears will eventually become large tears and cause arthritis. Arthroscopic rotator cuff repair is a very successful procedure with low risk of complication. See a board certified orthopaedic surgeon for evaluation and treatment.

Depends: http://www.theshouldercenter.com/rotator-cuff-tear.htm an acute injury can result in significant weakness but not mean that there is a large tear. We need more information and a clearer extent of pathology and diagnosis. A drop arm test by itself is not sufficient to make the decision for surgery. Consider a more detailed evaluation with a shoulder specialist to review your options.