Feature Articles

Understanding Hematocrit Levels for Blood Donation

August 13, 2008

blood donation bagsEvery year, nearly 5 million people in the United States receive life-saving blood transfusions. Almost all of the blood used for these transfusions is donated by healthy volunteers, who are screened for health risks that might make the donation unsafe for either the donor or the recipient. It is common for people to be temporarily disqualified or “deferred” from donating blood because of anemia.1

If you choose to donate blood, you will be examined and asked specific questions about your medical history to make sure that donating blood isn't hazardous to your health. During your medical screening, your hematocrit value (or hemoglobin level) is tested to make sure that you do not have anemia and are not likely to become anemic after donation. A very small blood sample, usually taken from pricking your finger, is tested to find this level. In order to donate blood, your hematocrit value must be at least 38% (or your hemoglobin level must be 12.5 or higher).5 This level is established by the U.S. Food and Drug Administration (FDA) and individuals with hematocrit levels that are too low are temporarily not permitted to donate blood.

What If I Was Deferred from Donating Blood?
If you cannot donate blood because of a low hematocrit, it is possible that you have anemia. Most male donors with a hematocrit below the acceptable levels are likely to be anemic, but many women within the normal, healthy hematocrit range of 36-38% are not able to donate blood because the FDA requires levels of 38%. Approximately 10% of female donors will not have a high enough hematocrit value to donate blood.2 If you are deferred from donating blood, your doctor should determine if you have anemia, identify the cause and correct your low iron levels.

Donating Blood with Anemia

A low hematocrit level is one of the most common reason people are temporarily disqualified or “deferred” from donating blood.3 Yet, some donors can actually have anemia and still be eligible to donate. Men with a hematocrit level below 39% are considered anemic, but can still donate blood if they have a level at 38%.5

It is important to know that being deferred from donation due to a low hemoglobin or hematocrit does not always mean that you have anemia or a medical problem. If you narrowly miss the cutoff value, you may be eligible on your next attempt to donate. It is normal for these values to vary from day to day or even hour to hour.2 For example, during the summer, hemoglobin levels in healthy adults are approximately three-percent lower than hemoglobin levels in the winter.3 Changes in activity and diet, as well as changes in the fluid levels of the blood itself, are possible reasons for this difference.

Donating Blood Can Cause Iron Deficiency
Besides testing to see if you already have anemia, donating blood can actually cause iron deficiency anemia. When you donate blood, you will be giving 1 pint – about 10% of the blood in your body – which also contains 250mg of iron. Your body will compensate for your donation by replacing the blood you have lost, but this takes time and you may experience symptoms of anemia after donating.

It takes a couple of days to replace plasma, the watery substance of your blood. Making red blood cells take a little longer, but a healthy donor can replace those that were donated within two weeks. To produce these red blood cells, your body will need iron to make hemoglobin, the part of the red blood cell that carries oxygen.3 It is therefore very important for blood donors to have an adequate amount of iron in their diet. This is especially important for female donors, who have lower amounts of stored iron due to loss of iron in menstrual periods. Even one unit of blood donation in a susceptible woman and three or four donations by men may exhaust the iron their bodies have stored.

Considerations for Frequent Donors
Each year, less than 5% of the population donates blood, and almost half comes from frequent donors – only 1% of the population. Donors are eligible to donate up to five times per year or every 56 days. However, how frequently you can donate depends upon how rapidly your body replaces the donated blood. Some donors, especially women who have menstrual periods, will not be able to donate every 56 days because they do not have enough stored iron to make new red blood cells. Iron supplements may be necessary if you donate blood more than once per year.

Visit our Anemia & Nutrition feature article to learn more about the important role of iron in your blood.


  1. AABB. Blood Donation FAQs. Link.
  2. Canadian Blood Services. Hemoglobin: Information for Donors. Link.
  3. American Red Cross. Newsroom: Study Shows Eligible Blood Donors Decrease as Summer Temps Increase. August 14, 2007. Link.
  4. Sebok MA, Notari EP, Chambers LA, Benjamin RJ, Eder AF. Seasonal temperature variation and the rate of donor deferral for low hematocrit in the American Red Cross. Transfusion. 2007 May;47(5):890-4.
  5. Department of Health and Human Services, Food and Drug Administration. Requirements for Human Blood and Blood Components Intended for Transfusion or for Further Manufacturing Use. Federal Register: November 8, 2007 (Volume 72, Number 216). Link.