Is there a lab test for multiple sclerosis?

No. Not really. Multiple sclerosis is diagnosed by clinical exam, history and MRI of the cns. We do perform a lumbar puncture that can confirm oligoclonal bands as lab tests. The combination of all these factors play a role in the diagnosis of ms.
No blood test. Cannot diagnose via blood or urine testing. Spinal fluid may correlate or confirm. MRI the best current test.

Related Questions

I have multiple sclerosis, and I am being treated for breast cancer, my 2nd occurrence. Recent labs show "lymphocytes absolute 0.64". Possible causes?

Treatment. Although I'm not sure which treatment you are receiving, the drop in lymphocytes is almost certainly from this. These cells are particularly susceptible to chemotherapy and their levels are being monitored by your treating physician.