Iron deficiency anemia is diagnosed with a small set of blood tests: a complete blood count (CBC) showing low hemoglobin with small, pale red cells, and iron studies showing low ferritin, low serum iron, low transferrin saturation, and a high total iron-binding capacity. Low ferritin on its own is the most reliable single marker. Finding the anemia, though, is only half the job. The other half is finding out why iron ran low. Below I go through each test, what the numbers mean, and what happens next.
What Is Iron Deficiency Anemia?
Anemia means too little hemoglobin, the protein in healthy red blood cells that carries oxygen to the tissues. Each hemoglobin molecule needs iron. When the body’s iron stores run out, the marrow makes fewer red cells, and the ones it does make are smaller and paler than normal.
Iron deficiency is the most common cause of anemia worldwide and one of the most common hematological conditions. It develops in stages. Stores fall first, then iron supply to the marrow drops, and only then does hemoglobin fall. That is why lab tests can pick up iron deficiency before anemia appears.
Causes and Risk Factors
Iron deficiency happens when losses or needs outstrip intake and absorption. The main causes are:
- Blood loss: heavy menstrual periods, bleeding from the stomach or bowel (ulcers, polyps, cancer), or frequent blood donation.
- Increased demand: pregnancy, breastfeeding, and growth spurts in infancy and adolescence.
- Poor absorption: celiac disease, H. pylori infection, bariatric surgery, or long-term use of acid-suppressing medicines.
- Low dietary intake: diets low in iron-rich foods, especially if poorly planned.
In men and in women past menopause, iron deficiency should be treated as a sign of possible gut blood loss until proven otherwise. These patients usually need investigation of the stomach and bowel.
Symptoms That Prompt Testing
Symptoms often come on slowly, so people adapt and may not notice them. Common features include:
- Fatigue, weakness, and poor concentration
- Pale skin and pale inner eyelids
- Shortness of breath or a racing heart on exertion
- Brittle or spoon-shaped nails, hair thinning, and a sore tongue
- Pica, particularly craving ice, and restless legs
Diagnosing Iron Deficiency Anemia: Lab Tests and More
These are the core tests and the patterns that point to iron deficiency. Exact reference ranges differ a little between laboratories, so always read your results against the range printed on your own report.
| Test | What it measures | Typical finding in iron deficiency |
|---|---|---|
| Hemoglobin | Oxygen-carrying protein | Low (anemia is usually defined as below 13 g/dL in men and 12 g/dL in non-pregnant women) |
| MCV (mean corpuscular volume) | Average red cell size | Low, below about 80 fL (microcytic) |
| MCH | Hemoglobin per cell | Low (hypochromic) |
| RDW | Variation in red cell size | High |
| Serum ferritin | Stored iron | Low; below about 30 ng/mL strongly suggests deficiency, below 15 ng/mL confirms it |
| Serum iron | Iron circulating in blood | Low |
| TIBC / transferrin | Capacity to carry iron | High |
| Transferrin saturation | How full the carrier is | Low, often below 16 to 20% |
| Reticulocyte count | New red cells from the marrow | Low or inappropriately normal |
Complete Blood Count
The CBC is usually the first clue. It shows low hemoglobin and hematocrit, a low MCV, and a raised RDW. Platelets are often mildly raised in iron deficiency, especially with ongoing bleeding. The blood film shows small, pale cells of varied shapes, sometimes including elongated “pencil” cells.
Ferritin and Its Limits
Ferritin reflects the body’s iron stores, and a low value is very specific for iron deficiency. The catch is that ferritin also rises with inflammation, infection, liver disease, and cancer. A person with rheumatoid arthritis or kidney disease can be iron deficient with a “normal” ferritin. In those settings, clinicians use a higher ferritin threshold and lean more on transferrin saturation.
Telling It Apart from Other Microcytic Anemias
Not every small-cell anemia is due to iron deficiency. The main alternatives are thalassemia trait, where iron studies are normal and the red cell count is often high, and anemia of chronic disease, where ferritin is normal or high and TIBC is low. Hemoglobin electrophoresis can identify thalassemia. In unclear cases, a trial of iron or, rarely, a bone marrow iron stain settles the question.
Finding the Cause and Treating It
Once iron deficiency is confirmed, the next step is finding the source. This may include celiac antibody testing, stool tests, and endoscopy or colonoscopy, depending on age, sex, and symptoms. In women with heavy periods, gynecological assessment is often the key step.
Oral iron, such as ferrous sulfate, is first-line for most people. Taking it once daily or on alternate days can improve absorption and reduce stomach upset, and vitamin C or a glass of orange juice may help. Tea, coffee, calcium, and antacids reduce absorption, so keep them away from the dose.
Intravenous iron is used when oral iron is not tolerated or not absorbed, when blood loss is ongoing, in later pregnancy, or when a faster recovery is needed. A rising reticulocyte count within about a week and higher hemoglobin within a few weeks show that treatment is working. Iron is usually continued for around three months after hemoglobin normalizes to refill stores. More on this in our anemia guide and our overview of hematology.
When to See a Doctor
See a doctor if you have ongoing tiredness, breathlessness, or paleness, or if you notice black or bloody stools, very heavy periods, or unexplained weight loss. Seek urgent care for chest pain, fainting, or severe breathlessness. Do not start iron supplements long-term without testing, because they can mask a bleeding source and iron overload is harmful.
Frequently Asked Questions
Can I have iron deficiency with normal hemoglobin?
Yes. Iron stores fall before hemoglobin does, so a low ferritin with normal hemoglobin is common. It can still cause fatigue and restless legs, and it deserves the same search for a cause.
Why is my ferritin normal if my doctor suspects iron deficiency?
Inflammation, infection, and some chronic illnesses push ferritin up and can hide low stores. Your doctor may look at transferrin saturation, a CRP level, or repeat the test once you have recovered from an illness.
How quickly will iron tablets work?
Many people feel better within a couple of weeks, and hemoglobin usually improves over the following weeks. Full recovery of stores takes several months, which is why treatment continues after the blood count is normal.
Do I need to fast before iron studies?
Serum iron varies through the day and rises after meals or supplements, so many labs prefer a morning sample. Stop iron tablets for a day or so before testing if your doctor advises. Ferritin is less affected by timing.
