HEMATOLOGY LAB · CBC INTERPRETED · TOPIC: ANEMIA & IRON DEFICIENCY · REF: ASH / CDC / FDA
The Hemoglobin Report Hematology reference · U.S. edition
Hematology · Nutritional deficiency

Anemia is a symptom. The question is always: anemia from what?

Roughly 3 million Americans live with anemia, and iron deficiency is the most common cause worldwide. But a low hemoglobin by itself tells you very little — the diagnosis depends on what the red blood cells look like, what the iron stores are, and, in adults, what is bleeding that should not be.

14 min read
ASH-aligned guidance
CBC · iron studies · smear
3M+
Americans live with anemia — and many more have iron deficiency without anemia.
< 12 / 13
g/dL threshold for anemia — below 12 in women, below 13 in men.
< 30
Ferritin (ng/mL) generally indicates iron deficiency, though cutoffs vary.
1 in 5
Of premenopausal women in the U.S. have iron deficiency — most do not know it.
Section 01 · Reading the CBC

What each value on a blood count actually tells you.

A complete blood count is one of the most commonly ordered tests in medicine. Four values drive the initial interpretation of anemia — and they narrow the diagnosis faster than any single test.

Hemoglobin (Hgb) is the protein that carries oxygen. It is the number that defines anemia — below 13 g/dL in men and below 12 g/dL in non-pregnant women. Symptoms do not track it perfectly: some patients feel exhausted at 11 g/dL, and others function at 8 for months.

MCV (mean corpuscular volume) is the average size of the red blood cells. It is the single most useful value for classification. A low MCV (below 80) means the cells are microcytic — most often iron deficiency, thalassemia, or chronic disease. A high MCV (above 100) means the cells are macrocytic — most often B12 or folate deficiency, alcohol use, or hypothyroidism. A normal MCV points toward the normocytic anemias: acute blood loss, chronic kidney disease, or anemia of inflammation.

RDW (red cell distribution width) measures how varied the cell sizes are. Iron deficiency and B12 deficiency typically produce a high RDW because the marrow is struggling to make normal cells. Thalassemia traits often have a normal RDW despite a low MCV — a useful clue when the two patterns need to be distinguished.

Ferritin reflects iron stores. A ferritin below 30 ng/mL almost always indicates iron deficiency; below 15 is diagnostic. Ferritin is also an acute-phase reactant — it rises with inflammation, infection, and liver disease, so a "normal" ferritin does not exclude iron deficiency in a chronically ill patient.

Section 02 · Iron metabolism

The path from a bite of food to a working red blood cell.

Iron metabolism is a tight, conserved loop. The body has no active way to excrete excess iron — it only regulates how much is absorbed. That is why iron deficiency is common and iron overload is dangerous.

The iron pathway — six steps
Absorption → transport → storage → use

Dietary iron

Heme iron (meat) absorbs at 15–35%; non-heme iron (plants) at 2–20%, depending on what else is eaten.

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Duodenal absorption

The duodenum is the only part of the GI tract that takes up iron. Bypass surgery, celiac, and PPIs reduce this.

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Transferrin

Iron binds transferrin in the blood and is delivered to cells that express transferrin receptors — mainly the bone marrow.

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Storage in ferritin

Liver, spleen, and bone marrow store iron inside ferritin. Serum ferritin reflects how full the warehouse is.

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Heme synthesis

The bone marrow uses iron to build heme, which becomes hemoglobin inside new red blood cells.

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Recycling

Old red blood cells are broken down in the spleen. Their iron is recovered and returned to the pool — the body reuses iron very efficiently.

Where disease interrupts the loop: Bleeding removes iron from the body entirely — the only normal way iron leaves. Menstruation, GI bleeding, and repeated blood draws all cause iron deficiency. Malabsorption blocks the duodenal step. Chronic inflammation traps iron inside ferritin and prevents its release (hepcidin), which is why anemia of inflammation looks similar to iron deficiency on some tests but has normal or high ferritin.
Section 03 · The blood smear

What the red blood cells look like when iron runs out.

The peripheral blood smear is one of the oldest and most informative tests in hematology. It shows, visually, what the MCV and MCH numbers describe.

◆ Normal smear MCV ~90 · uniform size · normal central pallor

Normal red cells

Uniform size, round shape, and a small central zone of pallor that takes up about one third of the cell's diameter.

◆ Iron-deficiency smear MCV < 80 · variable size · enlarged pallor

Microcytic, hypochromic cells

Smaller than normal (microcytic), paler than normal because they carry less hemoglobin (hypochromic), and uneven in size. This is the classic iron-deficiency pattern.

Section 04 · Classification

Three patterns — and what each one usually means.

Anemia is classified by the size of the red blood cells, which is determined by the MCV. The pattern directs the next set of tests.

Pattern MCV Most common causes
Microcytic < 80 fL Iron deficiency (most common worldwide), thalassemia traits, anemia of chronic inflammation, lead poisoning, sideroblastic anemia. Order ferritin and iron studies first.
Normocytic 80–100 fL Acute blood loss, chronic kidney disease (low erythropoietin), anemia of inflammation, hypothyroidism, marrow suppression, hemolysis. Order reticulocyte count, creatinine, and inflammatory markers.
Macrocytic > 100 fL Vitamin B12 deficiency, folate deficiency, alcohol use, hypothyroidism, certain medications (methotrexate, hydroxyurea, zidovudine), myelodysplastic syndrome. Order B12, folate, and TSH.

Two patterns deserve special attention. Anemia of chronic inflammation is common in patients with autoimmune disease, chronic infection, cancer, or kidney disease. It is driven by hepcidin, a hormone that traps iron inside storage cells and prevents its release — so the iron is present but unavailable. Ferritin is typically normal or high, transferrin saturation is low, and treatment is directed at the underlying condition rather than at iron supplementation.

Thalassemia trait is a genetic condition that produces small red cells but does not deplete iron. It is often mistaken for iron deficiency, and treating it with iron can cause overload. The distinguishing features are a low MCV with a normal RDW, a normal or mildly low hemoglobin, normal iron studies, and a family history from a region where thalassemia is common (Mediterranean, Southeast Asian, Middle Eastern, African).

Section 05 · Iron therapy

Oral vs. IV iron — and when each is the right choice.

Oral iron — first-line for most patients
Ferrous sulfate · gluconate · fumarate

Ferrous sulfate 325 mg (65 mg elemental iron) is the standard starting point. Take every other day rather than daily. Recent evidence shows that alternate-day dosing produces better absorption, because a daily dose triggers a hepcidin spike that blocks the next day's absorption.

Take on an empty stomach with vitamin C (orange juice or a 250 mg ascorbic acid tablet). Vitamin C roughly doubles non-heme iron absorption. If GI side effects are intolerable, take with a small amount of food — absorption drops, but so does the nausea. Continue for at least 3 months after hemoglobin normalizes to rebuild ferritin.

Examplesferrous sulfate · ferrous gluconate · ferrous fumarate · polysaccharide-iron complex
IV iron — when oral is not enough
Infusion or injection · clinic-based

Modern IV iron formulations correct iron deficiency in one to two visits rather than three to six months. Serious allergic reactions are rare with current-generation products. Indicated when oral iron is not tolerated, not absorbed (celiac, gastric bypass, IBD), or when rapid correction is needed (late pregnancy, severe symptomatic anemia, chronic kidney disease on dialysis).

Requires a clinic visit and monitoring for 30 minutes after infusion. Very effective; hemoglobin rises within 2 to 4 weeks. Some newer formulations (ferric carboxymaltose) can cause transient low phosphate — usually asymptomatic but occasionally symptomatic.

Examplesferric carboxymaltose · ferumoxytol · iron sucrose · ferric derisomaltose · low-molecular-weight iron dextran
Adjuvant therapy — for specific causes
EPO · B12 · folate

Erythropoiesis-stimulating agents (ESAs) — epoetin alfa, darbepoetin — are used in chronic kidney disease and some chemotherapy-related anemias. They require iron repletion first, otherwise they do not work. They carry a boxed warning for cardiovascular events and tumor progression when overused.

Vitamin B12 and folate are used specifically for their deficiencies. B12 replacement can be oral (1,000 mcg daily) or intramuscular. Folate replacement alone can mask B12 deficiency and allow neurological damage to progress — so both are usually checked together.

Examplesepoetin alfa · darbepoetin · cyanocobalamin (B12) · folic acid
What not to do — common mistakes
Avoid these patterns

Do not give iron without checking ferritin in a stable patient. Iron overload is a real risk in hemochromatosis, thalassemia, and chronic transfusion patients. Symptoms include fatigue, joint pain, and liver damage — the same symptoms iron therapy is intended to fix.

Do not stop oral iron as soon as hemoglobin normalizes. Hemoglobin is the last thing to fall and the first to recover. Ferritin takes months longer. Stopping at the hemoglobin milestone is the most common cause of recurrent iron deficiency.

Avoidiron with antacids · iron with calcium · iron with tea/coffee · iron without confirming deficiency
Section 06 · Absorption

What helps iron absorption — and what blocks it.

Vitamin C — helps

250 mg of vitamin C with a non-heme iron dose roughly doubles absorption. A glass of orange juice or a chewable vitamin C tablet is enough.

Heme iron — helps

Iron from meat, poultry, and fish is absorbed 15–35% and is not affected by inhibitors in the meal. Even small amounts of meat improve absorption of plant iron in the same meal.

Alternate-day dosing — helps

Oral iron induces a hepcidin surge that blocks absorption for roughly 24 hours. Every-other-day dosing produces higher cumulative absorption than daily dosing.

Calcium — blocks

Calcium (from dairy, supplements, or antacids) blocks both heme and non-heme iron. Separate calcium supplements and iron by at least 2 hours.

Tea, coffee, and tannins — block

Polyphenols in tea and coffee reduce non-heme iron absorption by up to 60%. Do not take iron with tea or coffee — water with vitamin C is best.

PPIs and antacids — block

Proton pump inhibitors and H2 blockers reduce stomach acid, which is needed to release iron from food and supplements. PPI use is a common unrecognized cause of refractory iron deficiency.

Section 07 · What to watch for

When iron deficiency is a sign of something that must be found.

Rule out these first

In adults, iron deficiency is a symptom, not a diagnosis. These situations demand investigation.

Iron deficiency in a man or a postmenopausal woman. The source is assumed to be gastrointestinal blood loss until proven otherwise. This requires evaluation for ulcers, colon polyps, and colorectal cancer — usually with upper endoscopy and colonoscopy.
Heavy menstrual bleeding (soaking through protection every 1–2 hours, clots larger than a quarter, periods over 7 days). Common and treatable, but the cause should be identified — fibroids, adenomyosis, bleeding disorders, thyroid disease.
Black, tarry stools or visible blood in stool. Never attribute to hemorrhoids without evaluation. Even a small amount of chronic bleeding can cause severe iron deficiency over months.
Unexplained weight loss, night sweats, or a new change in bowel habits. Any of these with iron deficiency makes the search for GI malignancy urgent, not elective.
Family history of colorectal cancer or of hereditary hemochromatosis. The first changes when and how to screen; the second means iron supplementation may be harmful.
Iron deficiency that does not respond to adequate oral therapy. Consider celiac disease, H. pylori infection, autoimmune gastritis, or ongoing unrecognized bleeding. Refractory deficiency is a diagnosis in itself, not a reason to simply increase the dose.
Section 08 · Access & affordability

Iron is inexpensive — and the price still varies.

Oral iron supplements are among the least expensive medications in the pharmacy. Ferrous sulfate 325 mg costs just a few dollars per month and is available both over the counter and as a prescription. Vitamin C is similarly cheap. This is one of the few areas of medicine where the effective treatment is genuinely affordable for nearly every patient.

IV iron is more expensive and typically given in an infusion center, but most insurance plans cover it when medically indicated. ESAs (erythropoiesis-stimulating agents) are considerably more expensive and covered primarily for dialysis patients and specific cancer-related anemias.

Where the price still varies at the pharmacy counter: generic ferrous sulfate tablets and iron-containing combination products can cost several times more at one licensed seller than another. A live prescription price comparison shows the current range in about two minutes, and the same logic applies to every generic on a patient's list. If you are filling an online refill for the first time, the process is documented step by step in this how to order prescriptions online guide — prescription first, licensed seller second, small first order third.

Before filling from any seller — online or local — verify them. A working VIPPS seal that resolves to a live NABP record, a real U.S. street address, and a prescription requirement that cannot be waived. A transparency page like this one about Metformin4You is a reasonable example of the standard to expect.

Section 09 · Questions

Five asked most often at the hematology clinic.

Q·01 What hemoglobin level is considered anemic? +
In the United States, anemia is generally defined as hemoglobin below 13.0 g/dL in men and below 12.0 g/dL in non-pregnant women. Levels below 8 g/dL are considered moderate to severe and often require prompt evaluation. Symptoms vary widely — some people feel significant fatigue at 11 g/dL, while others function without obvious symptoms until much lower.
Q·02 What is the difference between iron deficiency and iron deficiency anemia? +
Iron deficiency is a state in which the body's iron stores (measured by ferritin) are depleted. If this progresses far enough to lower hemoglobin, it becomes iron deficiency anemia. Iron deficiency without anemia still causes symptoms — fatigue, poor concentration, restless legs, hair thinning — and should be treated when identified, because it will otherwise progress.
Q·03 How long does it take for oral iron to work? +
Symptoms typically begin improving within 1 to 2 weeks, but hemoglobin usually takes 4 to 8 weeks to fully correct. Ferritin — the storage protein — takes much longer, often 3 to 6 months of continued therapy after hemoglobin normalizes. Stopping iron as soon as hemoglobin improves is one of the most common reasons for recurrence.
Q·04 What causes iron deficiency in adults? +
In premenopausal women, the most common cause is menstrual blood loss. In men and postmenopausal women, iron deficiency should be assumed to reflect gastrointestinal blood loss until proven otherwise — which means evaluating for ulcers, colon polyps, and colorectal cancer. Other causes include celiac disease, H. pylori infection, and medications such as NSAIDs or aspirin.
Q·05 Are IV iron infusions safer than oral iron? +
Both are safe when used appropriately. Modern IV iron formulations (ferric carboxymaltose, ferumoxytol, iron sucrose, ferric derisomaltose) have very low rates of serious reactions. IV iron is preferred when oral iron is not tolerated, not absorbed, or when rapid correction is needed — such as in late pregnancy or severe symptomatic anemia.
◆ Sources & Verification
This reference is aligned with public clinical guidance from the American Society of Hematology, the CDC micronutrient and anemia program, the National Heart, Lung, and Blood Institute (NIH), and the U.S. Food and Drug Administration's BeSafeRx program. Drug-label information draws on the National Library of Medicine's DailyMed database and MedlinePlus. Pharmacy verification follows the NABP VIPPS program.
◆ Before the next refill

Iron is cheap. Overpaying for it is not necessary.

Oral iron and most other anemia medications are available as low-cost generics, and the same tablet can cost several times more at one licensed pharmacy than another. A two-minute comparison across licensed U.S. sellers is a useful habit to build. Begin with the current Metformin price comparison as a live demonstration of how wide the spread runs.

Open the price comparison