When a donor arrives at the donation site, it is important to ensure that donating blood will be a safe experience. This is done by testing the donor's hemoglobin and/or hematocrit levels before the donation begins to detect existing anemia or prevent the onset of symptoms of anemia. Blood centers may choose to measure either hemoglobin or hematocrit, with some choosing to measure both. The FDA-established guidelines are listed in the table below.
| Hemoglobin | Acceptable Levels |
| Males | 13.0 g/dL or higher |
| Females | 12.5 g/dL or higher |
| Hematocrit | Acceptable Levels |
| Males | 39% or higher |
| Females | 38% or higher |
Lower hemoglobin and hematocrit levels may be normal for female donors. The FDA also accepts female donors with a hemoglobin level between 12.0–12.5 g/dL or a hematocrit level between 36–38%, provided the donation center has taken adequate steps to assure that the donor will not suffer any adverse health effects as a result of the donation.
Low hemoglobin or hematocrit levels are the most common reason for donor deferral.
