- Subjects who fail screening may be rescreened as soon as the investigator feels they may have become eligible. However, an individual subject may not rescreen more than twice. There is no predetermined amount of time that the investigator needs to wait to rescreen a previously ineligible subject, except those excluded for Hgb or folate who may only rescreen in 2 and 4 weeks, respectively, and those excluded for Vitamin B12 who may rescreen in 8 weeks