Hematocrit (HCT) / Packed Cell Volume (PCV)

A high hematocrit with a high RBC count and high hemoglobin indicates polycythemia. Polycythemia (also known as polycythaemia or polyglobulia) is a disease state in which the proportion of blood volume that is occupied by red blood cells increases. Some causes of a high hematocrit include:

- Dehydration—this is the most common cause of a high hematocrit. As the volume of fluid in the blood drops, the RBCs per volume of fluid artificially rises; with adequate fluid intake, the hematocrit returns to normal.

- Lung (pulmonary) disease—if someone is unable to breathe in and absorb sufficient oxygen, the body tries to compensate by producing more red blood cells.

- Congenital heart disease—in some forms, there is an abnormal connection between the two sides of the heart, leading to reduced oxygen levels in the blood. The body tries to compensate by producing more red blood cells.

- Kidney tumor that produces excess erythropoietin

- Smoking

- Living at high altitudes (a compensation for decreased oxygen in the air)

- Genetic causes (altered oxygen sensing, abnormality in hemoglobin oxygen release)

- Polycythemia vera—a rare disease in which the body produces excess RBCs inappropriately.

A low hematocrit with low RBC count and low hemoglobin indicates anemia. Anemia is usually defined as a decrease in the amount of red blood cells (RBCs) or hemoglobin in the blood. It can also be defined as a lowered ability of the blood to carry oxygen. 

Some causes include:

- Excessive loss of blood from, for example, severe trauma, or chronic bleeding from sites such as the digestive tract (e.g., ulcers, polyps, colon cancer), the bladder or uterus (in women, heavy menstrual bleeding, for example).

- Nutritional deficiencies such as iron, folate or B12 deficiency.

- Damage to the bone marrow from, for example, a toxin, radiation or chemotherapy, infection or drugs.

- Bone marrow disorders such as aplastic anemia, myelodysplastic syndrome, or cancers such as leukemia, lymphoma, multiple myeloma, or other cancers that spread to the marrow.

- Kidney failure—severe and chronic kidney diseases lead to decreased production of erythropoietin, a hormone produced by the kidneys that stimulates RBC production by the bone marrow.

- Chronic inflammatory diseases or conditions

- Decreased production of hemoglobin (e.g., thalassemia)

- Excessive destruction of red blood cells, for example, hemolytic anemia caused by autoimmunity or defects in the red blood cell itself.