Look for Possible Connected Biomarkers:  Low hematocrit + low RBC count + low hemoglobin indicates anemia. 

Anemia is usually defined as a decrease in 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 of low hematocrit include:

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

- Chronic inflammatory diseases or conditions.

- 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).

- 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 spread to the marrow.

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

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

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

Look for Possible Connected Biomarkers: A high hematocrit alongside 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 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.

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

- Smoking

- Lung (pulmonary) disease—if someone cannot 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.

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

- Kidney tumor that produces excess erythropoietin

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