What is the most common cause of anemia?

Iron deficiency. It is the most common cause of anemia worldwide and the most common nutritional deficiency. The blood count only hints at it through small red cells; ferritin, a separate test that reflects your iron stores, is what confirms it.

It depends on the cause and how fast the hemoglobin fell. A mild, slowly developing anemia from low iron is common and usually treatable once the source is found. But a hemoglobin drop paired with chest pain, breathlessness while sitting still, or fainting is a medical emergency, and an anemia that points to a gut cancer, kidney disease, or a marrow problem is serious because of what it reveals.

Can I fix anemia with diet or supplements alone?

Only when the cause is a nutritional shortage and any ongoing loss has been stopped, and that call belongs to a clinician who has found the cause. Taking iron or B12 on your own can mask the real problem, which is why the workup comes before any supplement. This page does not recommend doses.

Anemia: what a low hemoglobin means and how the cause is found

Q: How is anemia diagnosed?

Hemoglobin on a complete blood count (CBC) confirms it, the MCV on the same report sorts it into small, normal, or large red cells, and targeted tests pin the cause: ferritin and iron studies for small cells, vitamin B12 and folate for large cells, and a reticulocyte count to see whether the bone marrow is responding.

The distinctions

01
Anemia is a result, not a diagnosis
A low hemoglobin tells you that red cells, or the oxygen-carrying protein inside them, are in short supply; it does not say why. The fastest pointer to the cause is red-cell size (MCV), already printed on the same blood count, so the direction of the workup is often visible before ferritin, B12, or folate is ever ordered: small cells point to iron deficiency, normal cells to chronic or kidney disease or bleeding, and large cells to a B12 or folate shortage.
02
A normal ferritin does not clear iron deficiency
Ferritin is the blood test that confirms iron deficiency, but it also rises with inflammation, so a normal value in someone who is unwell can hide low iron. That is why guidelines raise the ferritin cutoff to 45 ng/mL in a person who is already anemic, rather than the usual 15 ng/mL.
03
New iron deficiency in a man or postmenopausal woman is a bleeding question
In a man of any age or a woman past menopause, iron deficiency has no menstrual or growth explanation, so guidelines call for a search for a bleeding source, usually in the gut, rather than an iron supplement alone.

A low hemoglobin on your blood count means anemia: not enough healthy red blood cells to carry the oxygen your body needs. It is a finding that starts a question rather than settles one. The number tells you oxygen delivery is down; it does not tell you why, and the why is what gets treated.

What is anemia?

Anemia is a shortage of healthy red blood cells, or of the hemoglobin inside them, so the blood carries less oxygen than the body needs. Hemoglobin is the iron-rich protein that binds oxygen in the lungs and releases it in the tissues, and it is the specific number that defines anemia.

Anaemia is a condition in which the number of red blood cells or the haemoglobin concentration within them is lower than normal. ⁶World Health Organization, Anaemia

By the WHO's 2024 guideline, adults have anemia when hemoglobin falls below 13.0 g/dL in men and below 12.0 g/dL in non-pregnant women. In pregnancy the cutoff is trimester-specific: below 11.0 g/dL in the first and third trimesters and below 10.5 g/dL in the second. The 2024 update kept the adult cutoffs for men and non-pregnant women, and revised thresholds for children 6 to 23 months, for the second trimester of pregnancy, and for people living at high altitude. ¹ A result a hair outside a lab's printed range is often not meaningful (see reference ranges explained), so the WHO cutoff, not the flag, is what marks true anemia.

Your report may flag hematocrit too, the share of blood volume taken up by red cells. It moves with hemoglobin at about three times its value, so a hemoglobin of 15 g/dL sits with a hematocrit near 45 percent; when the two drift apart, suspect hydration or a lab artifact before disease.

Where a hemoglobin sits (example, adult woman)g/dL

10.8 g/dLbelow the typical range · about 32 percent hematocrit by the rule of three

Anemia8–12Typical range12–15.5High15.5–18

A hemoglobin of 10.8 g/dL in a woman is below the WHO anemia cutoff of 12.0 g/dL, which confirms anemia but not its cause. The next number to read is the MCV. Bring this to your clinician.

Illustrative example for a non-pregnant adult woman. WHO anemia cutoff below 12.0 g/dL; typical US reference range about 12.0 to 15.5 g/dL. [[1]]

Severity	Men	Non-pregnant women	Pregnancy (1st and 3rd trimester)
Anemia below	13.0 g/dL	12.0 g/dL	11.0 g/dL
Mild	11.0 to 12.9	11.0 to 11.9	10.0 to 10.9
Moderate	8.0 to 10.9	8.0 to 10.9	7.0 to 9.9
Severe	below 8.0	below 8.0	below 7.0

Hemoglobin values in g/dL. Severity thresholds per the WHO 2024 guideline. In the second trimester the anemia cutoff is lower, below 10.5 g/dL rather than 11.0. ¹

1.9 billionpeople affected by anemia worldwideAbout a quarter of the global population, concentrated in young children and women of reproductive age (Global Burden of Disease Study 2021). [[7]]

What are the symptoms of anemia?

The symptoms come from tissues getting less oxygen, and they depend as much on how fast the hemoglobin fell as on how low it is. A slow drop over months can reach a strikingly low number with only mild tiredness, because the body adapts; a fast drop from bleeding causes symptoms sooner. ²

Why the speed of the fall matters more than the number

Slow decline (chronic low iron)Fast decline (acute gut bleed)

Illustrative. Both paths reach a similarly low hemoglobin, but a slow fall over months lets the body adapt, so symptoms stay mild, while a fast fall causes symptoms sooner at a higher number. Concept per Camaschella, N Engl J Med 2015. [[2]]

Fatigue and low energy that rest does not fix
Breathlessness, especially on exertion like climbing stairs
Pale skin, lips, or inner eyelids
Cold hands and feet
Dizziness or lightheadedness
A fast, pounding, or irregular heartbeat
Headaches or trouble concentrating

Why the cause matters more than the number

A hemoglobin of 10 g/dL from a slow gut bleed, from a vitamin B12 deficiency, and from kidney disease are three different problems with three different treatments. That is why anemia is never the endpoint. The single most useful next number is the MCV, or mean corpuscular volume, the average size of your red blood cells, and it is already printed on your blood count. Red-cell size forks the search into three.

Red-cell size (MCV) sorts anemia into three

Small cellsStart with iron studies

Microcytic, MCV under 80 fL

Average red-cell size (MCV)Below 80 fL

Points towardIron deficiency; thalassemia trait (an inherited difference in hemoglobin)

Next testFerritin and iron studies

Normal-sized cellsLook at reticulocytes (young red cells) and the kidneys

Normocytic, MCV 80 to 100 fL

Average red-cell size (MCV)80 to 100 fL

Points towardAnemia of chronic disease or inflammation, kidney disease, acute blood loss, early iron deficiency

Next testReticulocyte count, kidney function, inflammatory markers

Large cellsCheck B12 and folate

Macrocytic, MCV over 100 fL

Average red-cell size (MCV)Above 100 fL

Points towardVitamin B12 or folate deficiency, alcohol, an underactive thyroid, some medications

Next testVitamin B12 and folate levels

Illustrative sorting by red-cell size; overlap exists and mixed pictures occur. Cutoffs and causes per Camaschella, N Engl J Med 2015. [[2]]

Across all three groups, iron deficiency is the single most common cause of anemia worldwide and the most common nutritional deficiency, which is why small red cells are the first branch most workups follow. ² For how that cause is confirmed and treated, and why a new case prompts a hunt for bleeding, see the companion page on iron-deficiency anemia.

The AGA recommends a ferritin cutoff of 45 ng/mL, rather than 15 ng/mL, to diagnose iron deficiency in anemia; because ferritin rises with inflammation, a normal ferritin does not rule iron deficiency out.

AGA Clinical Practice Guidelines, Gastroenterology, 2020

Ferritin, the blood test that reflects how much iron you have in storage, is what confirms or excludes iron deficiency off a separate order; the blood count only hints at it. ³

What causes anemia?

Under the size buckets, the causes come down to three mechanisms: losing red cells faster than they are made, making too few, or destroying them early.

Blood loss: heavy periods, or slow bleeding in the gut from an ulcer, a polyp, or a cancer. This is why new iron-deficiency anemia without a menstrual explanation gets investigated rather than only supplemented.
Too little iron, B12, or folate to build red cells (small cells for iron, large cells for B12 or folate).
Chronic disease or inflammation, and kidney disease, which blunt red-cell production, usually leaving normal-sized cells.
Early destruction of red cells (hemolysis), which a high reticulocyte count (young red cells from the marrow) helps flag.

How is anemia diagnosed?

1Confirm it with hemoglobinA [complete blood count (CBC)](/topics/blood-count), a standard blood draw that needs no fasting, measures hemoglobin and confirms whether you are below the WHO cutoff; [what a CBC measures](/concepts/cbc-explained) explains the panel if this is your first one.
2Read the MCV to sort the typeThe same CBC reports the MCV, the average red-cell size, which places the anemia in the small, normal, or large bucket and points to the likely cause.
3For small cells, check ironFerritin and iron studies, usually a separate order, confirm or rule out iron deficiency; a very low ferritin is close to proof.
4For large cells, check B12 and folateBlood levels of vitamin B12 and folate sort out the common causes of large-cell anemia.
5Check whether the marrow is respondingA [reticulocyte count](/biomarkers/reticulocyte-percent), which measures young red cells fresh from the bone marrow, separates an anemia where production is failing from one where cells are being lost or destroyed.

~50likelihood ratio for iron-deficiency anemia when ferritin is below 15 ng/mLA ferritin that low is close to proof of iron deficiency (Guyatt 1992). [[5]]

Is anemia serious?

It depends on the cause and how fast it developed. A mild, slowly developing anemia from low iron is common and usually treatable once the source is found. A hemoglobin that drops fast, or one tied to chest symptoms, is urgent. And an anemia that is a clue to something else, a gut cancer, a kidney problem, a marrow disorder, is serious because of what it points to, which is the whole reason the cause is chased rather than the number alone.

Link · NHLBI (US National Heart, Lung, and Blood Institute)AnemiaPlain-language overview of anemia types, causes, symptoms, and treatment from the US National Institutes of Health.nhlbi.nih.gov

LifeFrom membershipGet your hemoglobin and MCV read in context, with every marker on your blood count explained. Start with a full LifeFrom panel.Get started

Markers to check

HEMHemoglobin HEMHematocrit MCVMean Corpuscular Volume

Citations

Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.