We describe a simple and quantitative approach to the measurement of cell surface antigens using CD20 as an example. Though it is believed that effective treatment with rituximab will mask CD20 expression, quantitation of levels of CD20 may help in determining efficacy of therapy and give insight on the optimal dosage and scheduling of therapy. Patients without complete masking may not have received adequate dosing. This method can be modified to detect surface antigens in plasma and can be expanded to other antibody-based therapies such as alemtuzumab (anti-CD52; CamPath®, Berlex, Montville, NJ) (14) and gem-tuzumab ozogamicin (anti-CD33; Mylotarg®, Wyeth Ayerst, Madison, NJ) (15).
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