Other tests for identifying demyelinating diseases are used less commonly than oligoclonal banding and the CSF IgG index, Dr. Willrich said. Total protein in CSF can be used as a rule-out assay: An elevated total protein of greater than 100 mg/dL suggests diseases other than MS. Albumin index, an older test that is a ratio of albumin CSF (mg/dL) to albumin serum (g/dL) × 1,000, is generally used to study the blood-brain barrier integrity, and it is part of the CSF IgG index calculation. The newer tests are the kappa-free light chain index, which is a calculation similar to the IgG index, but replacing the total IgG with the immunoglobulin κ-free light chain measurements in serum and CSF, coupled to measures of albumin on the same samples (n = four assays), and the simpler κ-free light chains assay, a single measurement of κ-free light chains in CSF alone.
The 2017 McDonald diagnostic criteria for the diagnosis of multiple sclerosis require imaging studies, Dr. Willrich said, and they should show dissemination in space or dissemination in time for the brain lesions that are characteristic of demyelination.
“The 2017 criteria say that if a patient has presented with two clinical attacks and has any number of lesions on imaging studies, they can already be diagnosed with MS,” Dr. Willrich said. If there has been a single clinical attack and there are two or more lesions, the criteria still required dissemination in time with a second MRI. “But if they have only one lesion, they need to show dissemination in space and time.”
In patient presentations where there is one clinical attack, the 2017 McDonald diagnostic criteria allowed positive CSF oligoclonal band tests to replace the criteria for dissemination in time or dissemination in space. “So potentially if the patient has positive oligoclonal bands—and the criteria are very specific in saying they should be assessed by isoelectric focusing with two or more unique CSF bands as positive—that can be considered a replacement for that dissemination in time or in space,” Dr. Willrich said, “allowing the patient to have an earlier diagnosis and access to therapy sooner.”
CSF electrophoresis and isoelectric focusing
CSF electrophoresis and isoelectric focusing are the two main methods used for oligoclonal banding in the United States (Fig. 2). Among laboratories that participate in CAP proficiency testing, 25 percent still use CSF agarose gel electrophoresis to detect oligoclonal banding. Seventy-five percent of participating laboratories use isoelectric focusing.
CSF agarose gel electrophoresis is similar to serum protein electrophoresis, with the five zones of proteins and staining for total protein. “Then you look for unique bands in the gamma region that would correspond to immunoglobulins, and a positive sample would have two or more bands,” Dr. Willrich said.
Isoelectric focusing, in contrast, separates the immunoglobulins by their isoelectric point, so all the different glycosylated forms can be seen. “And you’re staining specifically for IgG, so you get much better separation, and many more bands are visible in contrast to the agarose gel electrophoresis.” This method’s sensitivity is superior to that of CSF electrophoresis, she added.