Hematocrit (HCT) blood test: what high or low hematocrit means

What hematocrit measures

Your blood is made of cells floating in a fluid called plasma. Hematocrit is the share of that volume taken up by red blood cells, which carry oxygen. A hematocrit of 42% means red cells make up 42% of your blood.

Hematocrit moves closely with hemoglobin, the oxygen-carrying protein inside red cells, and the two are read together. The red cell indices (MCV, MCH, MCHC and RDW) then help explain why a hematocrit is high or low.

Because it is a percentage, hematocrit also shifts with how much fluid is in your blood. Dehydration pushes it up; extra fluid, as in pregnancy, pulls it down.

What a low hematocrit can mean

A low hematocrit usually means anemia: too few red cells to carry oxygen well. When hematocrit and hemoglobin are both low, the cause is usually one of three things: blood loss, the bone marrow making too few red cells, or red cells breaking down too early. Common causes:

• Iron deficiency, often from heavy periods or bleeding in the gut. See ferritin levels.
• Low vitamin B12, folate or other nutrients needed to make red cells.
• Long-term illness, including chronic kidney disease, thyroid disease and inflammatory conditions.
• Pregnancy, because plasma volume rises faster than red cell volume.
• Less common: red cells breaking down early (hemolysis), bone marrow conditions, leukemia and medication effects.

What a high hematocrit can mean

A high hematocrit is called erythrocytosis. Often the body is making extra red cells because it needs more oxygen, or the plasma is low. Common causes:

• Dehydration, for example from not drinking enough before the test, or from vomiting or diarrhea.
• Smoking and carbon monoxide exposure.
• Sleep apnea and lung or heart conditions that lower blood oxygen.
• Living at high altitude.
• Testosterone therapy or anabolic steroids. A rising hematocrit is one of the main safety checks on testosterone therapy, so share results with the prescriber.
• Less common: kidney conditions, rare hormone-making tumors, and polycythemia vera, a bone marrow condition. It is usually considered when hematocrit is above about 49% in men or 48% in women without a clear outside cause.

What to do with an abnormal result

1. Read it with hemoglobin and the rest of the CBC. The red cell indices point toward the type of anemia; white cell and platelet counts help when a marrow cause is possible.
2. For a low hematocrit, ask about ferritin, an iron panel, B12 and folate, plus kidney and thyroid tests if they fit your history.
3. For a high hematocrit, rule out the simple causes first. Retest well hydrated, and tell your clinician about smoking, snoring or daytime sleepiness, altitude and any testosterone use. If it stays high, ask whether an erythropoietin (EPO) level or a JAK2 gene test makes sense.
4. Retest after a change. Red cells live about 120 days, so a recheck after about three months shows whether treatment is working.

See a clinician promptly if an abnormal hematocrit comes with chest pain, shortness of breath, fainting, black or bloody stools, a severe headache or vision changes, a swollen and painful leg, or sudden weakness or trouble speaking. Call 911 for chest pain or signs of a stroke.