Doctors can diagnose Guillain-Barré syndrome based on physical examination that shows weakness that is equal on both sides of the body and loss of reflexes, particularly at the ankles and knees. Specific tests for Guillain-Barré syndrome, such as a spinal tap or lumbar puncture, nerve conductions, and antibody tests, can help exclude other diagnoses and confirm the diagnosis of Guillain-Barré syndrome.
A spinal tap or lumbar puncture is a procedure where a needle is inserted in the lower back to sample the spinal fluid that surrounds the brain, spinal cord, and the nerves that come out of the spinal cord. In Guillain-Barré syndrome, the spinal fluid protein concentration is high, but there is no evidence of inflammation. The test is most likely to be abnormal a week or more after symptoms start.
Nerve conduction studies can tell whether the insulation of the nerves or the nerves themselves are damaged. This test is more likely to be abnormal two weeks after symptoms start. Nerve conduction studies can also help determine prognosis, particularly if they are repeated during the first few weeks after symptoms start.
There are antibody tests on blood that can help confirm that a patient has the Miller Fisher variant of Guillain-Barré syndrome, but routine antibody tests for the more common form of GBS are not available.