Methods and apparatus for assaying the level of analytes in a sample, related to VLA-4, are disclosed. A method of decreasing the level of an anti-integrin antibody in a subject is described including a) contacting a biological sample from a subject with a detectable capture agent associated with a substrate, wherein the capture agent can bind an anti-integrin antibody in the sample; b) detecting binding of the capture agent with the level of the anti-integrin antibody; and c) treating the subject with plasma exchange until the level of the anti-integrin antibody in the sample reaches a predetermined level.