When dealing with pain and numbness in #leftarmnumbness, you should immediately go to a doctor. The doctor will ask for your complete medical history. When you go to a doctor, you should tell all the symptoms you are feeling, even if you feel like they are not related to your left arm numbness. Depending on what the doctor finds in the physical examination, they can order additional tests. These tests may include electrolyte level testing, blood tests, vitamin level testing, etc. the doctor can also suggest a lumbar puncture or spinal tap.