The hematocrit (Hct) is a quick and easy screening exam. While not a sole determining factor, this test indirectly measures blood cell mass and may indicate further exploration. When bundled as part of a complete blood count (CBC) this test may aid in the diagnosis of anemia and dehydration, -or hint to situations involving blood loss, like uterine fibroids and cancers.
The Idea
Human blood is a complex soup that includes hormones, proteins, nutrients, and metabolic waste. The hematocrit is a simple procedure in which this broth is seperated into two general groups which are then proportionally compared. These two groups are the formed elements and plasma.
Formed Elements - The Cellular Component of Blood
Formed elements are created in the bone marrow and then migrate into the blood stream. The major contributor of the formed elements is the erythrocyte, or red blood cell (RBC). RBCs deliver oxygen to the body's tissues and facilitate the removal of waste carbon dioxide. Though dependant on many factors, RBCs generally take up about 45% of blood volume.
Other formed elements are leukocytes or white blood cells (WBCs) and thrombocytes or platelets (Plts). While crucial to life, these formed elements are in much lower quantities compared to the RBCs. WBCs (lymphocytes, monocytes, neutrophils, eosinophils, basophils, and macrophages) are important components of the immune system that modulate cellular defence and inflammation. Plts are small “crumbly” cellular fragments that govern coagulation -the ability of the blood to clot.
Plasma - A Mysterious Liquid
Plasma is the fluid portion of blood. This fluid has a yellowish “straw” tint and contains millions of enzymes, antibodies, hormones, carrier proteins, clotting factors, nutrients, gasses, wastes, and other components yet to be discovered. This fluid is generally about 55% of blood.
The Procedure
In the Hct procedure blood is obtained either through venopuncture or small cut and then spun on a centrifuge for about ten minutes. The centrifugal force separates the heavier formed elements from the plasma, packing them at one end. The majority of this consists of RBCs with a thin “buffy coat” of leukocytes and platelets on top. The remaining straw tinted liquid is the plasma. When obtaining results a comparison is made between the packed RBCs and the plasma liquid. This is often performed by machine but is an easy procedure to by hand. Results vary depending on age, sex, and conditions. In addition, levels may vary between laboratory systems.
Expected Hct values:
- Newborns: 55-68%
- One (1) week of age: 47-65%
- One (1) month of age: 37-49%
- Three (3) months of age: 30-36%
- One (1) year of age: 29-41%
- Ten (10) years of age: 36-40%
- Adult males: 42-54%
- Adult women: 38-46%
Lower than expected Hct values may be an indication of:
- Anemia (B12/Folate or iron deficiency, of chronic disease, malnutrition, RBC hemolysis)
- Acute and occult blood loss (common with uterine fibroids)
- Adrenal insufficiency
- Pregnancy
Higher than expected Hct levels may indicate:
- Dehydration
- High Altitude Living (expect higher levels if living in Denver)
- Polycythemia Vera
- Shock
- Sports Drug Enhancement -Erythropoietin (EPO)
- Type II Diabetes Mellitus
Further Helpful Resources:
McPherson RA and Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory
Methods. 21st ed. Philadelphia, Pa: WB Saunders; 2007:459-60.
Hoffman R, Benz Jr. EJ, Shattil SJ, et al., eds. Hematology: Basic Principles and
Practice. 4th ed. Philadelphia, Pa: Churchill Livingston; 2005:2674.
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