Hemoglobin test: what it measures, normal levels for men and women, and what low or high hemoglobin means

Hemoglobin is the protein that gives red blood cells their color and their job. Each molecule has four iron-containing heme groups, each of which binds one oxygen molecule in the lungs and releases it where oxygen is low; hemoglobin also carries some carbon dioxide back and buffers acid. The test measures the concentration of hemoglobin in whole blood, in grams per deciliter in the United States and grams per liter elsewhere (multiply by 10).

Hemoglobin comes as part of the complete blood count, where it sits with the red cell count, hematocrit and the red cell indices. Hematocrit (the percentage of blood volume made up of red cells) tracks hemoglobin closely at roughly three times its value; MCV, MCH and MCHC describe the size and hemoglobin content of the cells and point to the cause when hemoglobin is low. The analyzer measures hemoglobin directly by lysing the cells and reading the color.

Anemia is defined by the hemoglobin level, not by symptoms or by the red cell count: the World Health Organization uses below 13 grams per deciliter for men and below 12 for non-pregnant women, with 11 in pregnancy, and laboratory ranges are set slightly higher. The number says how much oxygen-carrying capacity is present; the indices and the iron, B12 and folate studies say why.

Hemoglobin is on every complete blood count, so it is checked at most annual physicals and before surgery, and it is the first test for anyone with unexplained fatigue, breathlessness on exertion, palpitations, dizziness, pallor, or a racing heart. Women with heavy periods, pregnant women (at the first visit and again in the third trimester), people with gastrointestinal bleeding, ulcers or inflammatory bowel disease, vegans and vegetarians, frequent blood donors, endurance athletes and older adults are tested more deliberately.

People with chronic kidney disease, cancer, rheumatoid arthritis and other inflammatory conditions, and those on chemotherapy or medications that suppress the marrow, have hemoglobin followed routinely. At the other end, smokers, people living at altitude, those with sleep apnea or chronic lung disease, and men on testosterone therapy are checked for a high hemoglobin, which thickens the blood.

In children, hemoglobin is screened around the first birthday because iron deficiency is common then, and again in adolescence, particularly in menstruating girls.

Hemoglobin comes on the complete blood count, with or without a differential, which a doctor can order or you can order online through TestWell for a draw at a Quest or Labcorp patient service center; a licensed physician reviews the order and results post to your portal within one to three business days against sex-specific ranges. Pairing the CBC with ferritin, iron and TIBC, or with B12 and folate, answers the "why" in the same draw when anemia is suspected.

No fasting is needed. Hydration matters more than usual: dehydration concentrates the blood and raises hemoglobin by half a gram or more, while a large volume of fluid shortly before the draw dilutes it slightly, so drink normally. Hard exercise in the hours before the draw and a recent blood donation (which lowers hemoglobin for several weeks) are worth avoiding or mentioning. Smoking raises the value; high altitude raises it over days to weeks.

Hemoglobin falls by about 1 to 1.5 grams per deciliter in the second trimester of pregnancy as plasma volume expands faster than red cell mass, which is normal and is read against pregnancy-specific thresholds.

Most laboratories report about 13.5 to 17.5 grams per deciliter for adult men and 12 to 15.5 for adult women; some use 13.2 to 16.6 and 11.6 to 15. The sex difference comes from testosterone, which stimulates red cell production, and from menstrual iron loss. In pregnancy the lower limit falls to about 11 in the first and third trimesters and 10.5 in the second. Newborns run high (14 to 24) and fall through infancy, with children typically 11 to 14.

Values decline slightly with age, and a hemoglobin at the lower edge of the range in someone over 70 is common; whether it is anemia or aging is debated, and most clinicians look for a cause when it is below 12 in women or 13 in men at any age. People of African ancestry average about half a gram lower than people of European ancestry, which some laboratories account for.

Clinicians grade anemia by severity — mild (roughly 10 to the lower limit), moderate (8 to 10) and severe (below 8) — and by speed: a hemoglobin of 8 reached over a year from iron deficiency is tolerated far better than the same value reached in a day from bleeding. The MCV decides the first branch of the work-up: small cells point to iron deficiency or thalassemia trait, large cells to B12 or folate deficiency, alcohol or liver disease, and normal-sized cells to blood loss, chronic disease, kidney disease or a marrow problem.

Low hemoglobin has three broad mechanisms. The body may not make enough red cells: iron deficiency (the commonest cause worldwide, from menstrual or gastrointestinal blood loss or poor intake), B12 or folate deficiency, chronic kidney disease (too little erythropoietin), the anemia of chronic inflammation or disease, an underactive thyroid, and marrow disorders including the effects of chemotherapy. It may lose them: acute or slow bleeding from the gut, heavy periods, surgery or trauma. Or it may destroy them early: hemolytic anemias, sickle cell disease and thalassemias, some medications and infections.

The symptoms depend on how low and how fast: fatigue and weakness, pale skin and inner eyelids, shortness of breath and a racing heart on exertion, headaches, dizziness or light-headedness, cold hands and feet, trouble concentrating, and in iron deficiency brittle nails, hair shedding, ice cravings and restless legs. Mild anemia that develops slowly is often found on a routine count in someone who feels well.

The work-up follows the red cell indices and a few add-on tests: ferritin, iron and TIBC for small cells; B12, folate and sometimes methylmalonic acid for large cells; a reticulocyte count to see whether the marrow is responding; kidney function, thyroid, and inflammatory markers for normal-sized cells. Finding and treating the cause — not just the number — is the point, because iron deficiency in an adult man or a post-menopausal woman is a reason to look for bleeding in the gut.

The commonest cause of a high hemoglobin is a concentrated sample: dehydration, from illness, heat, diuretics or simply not drinking, raises hemoglobin and hematocrit together and settles with fluids. Among genuine increases, the body makes extra red cells in response to low oxygen — smoking (carbon monoxide), living at altitude, chronic lung disease, sleep apnea, heart disease with right-to-left shunting, and obesity hypoventilation — and in response to testosterone or anabolic steroid use and, rarely, erythropoietin-producing tumors.

Polycythemia vera, a bone-marrow disorder in which red cells are overproduced independently of oxygen, is the diagnosis to exclude when hemoglobin is persistently above about 16.5 in men or 16 in women without an obvious secondary cause; a JAK2 mutation test and erythropoietin level are the usual next steps.

High hemoglobin thickens the blood and can cause headaches, dizziness, a flushed complexion, itching after a warm shower, blurred vision and fatigue, and it raises the risk of clots. Men on testosterone therapy are monitored because a hematocrit above about 54 percent is a reason to adjust the dose.

Hemoglobin is never read alone. Hematocrit confirms it (about three times the hemoglobin); the red cell count, MCV, MCH and MCHC describe the cells; RDW shows how varied they are in size, rising early in iron and B12 deficiency; and the reticulocyte count shows whether the marrow is producing. Ferritin is the most useful single follow-up for a low hemoglobin with small cells, and B12 and folate for large cells.

Hemoglobin is also different from hemoglobin A1c, which measures the fraction of hemoglobin bound to glucose as a three-month average of blood sugar — and a low or abnormal hemoglobin can make A1c unreliable, which is one more reason the two are often read together. Hemoglobin electrophoresis identifies sickle cell trait, thalassemia and other inherited variants when the indices suggest them.