Subject of this validation was the quantitative determination of Tysabri ® (Natalizumab) in serum samples. This assay employed the quantitative antibody sandwich enzyme immunoassay technique (ELISA). A microtiter plate was coated with a specific anti-Natalizumab antibody.After blocking, calibration samples, quality control samples (QCs) or study samples were pipetted into the wells. After washing, the HRP-conjugated secondary anti-Natalizumab antibody