Dec 16 (Reuters) - Biogen Inc (BIIB.O) said on Thursday it was expecting to submit a final design for a post-marketing confirmatory study of its newly approved Alzheimer"s drug, Aduhelm, to the U.S. Food and Drug Administration in March, and begin screening of patients in May. Aduhelm, the first new treatment for the memory-robbing disease in nearly 20 years, is battling slow uptake as experts have questioned the FDA"s rationale for clearing the drug without more definitive proof of benefit. The drug was approved in June using the FDA"s accelerated approval pathway, under which Biogen is required to conduct a confirmatory clinical trial to demonstrate the drug works as intended, but has several years to do so. Biogen said it anticipates primary completion date to be about four years after the study begins, ahead of the FDA"s nine-year requirement. The study will be global trial with more than 1,300 early Alzheimer"s disease patients.