Cerebrospinal fluid is a clear fluid surrounding the brain and the spinal cord. It is collected via a lumbar puncture under sterile conditions. CSF is analyzed to confirm certain diseases. Common tests performed on CSF include protein and glucose levels, cell counts and differential count, microscopic examination, stains, and cultures. CSF may also be tested for glutamine, lactate dehydrogenase, lactic acid, tumor markers and C-reactive proteins. Normally, only a small amount of proteins are present in the CSF as they are large molecules and cannot cross the blood-brain barrier easily. A decrease in CSF protein level is not significant, however, increased CSF protein levels reveal undetected neurologic illness. High protein levels in demyelinating polyneuropathies, often known as post-infectious conditions, can be useful. Protein levels might also be high in diseases such as meningitis, brain abscess, tumors of the brain or spinal cord, multiple sclerosis, Guillain-Barre syndrome, and syphilis. Protein can enter the CSF as a result of a traumatic tap. A repeat tap is preferred over an estimate of 1 mg of protein for every 750 RBCs.