Anemia is diagnosed primarily through a blood test called a complete blood count, or CBC. This single test measures your hemoglobin level, which is the protein in red blood cells that carries oxygen. If your hemoglobin falls below the threshold for your age and sex, you have anemia. From there, additional blood tests help pinpoint the specific type and underlying cause, which determines what treatment you need.
What Happens During the Initial Evaluation
Before ordering labs, your doctor will likely ask about your symptoms and do a physical exam. Common signs they look for include pale skin, pale mucous membranes inside your mouth and under your eyelids, a rapid heart rate, low blood pressure (especially when you stand up), and a heart murmur. Some types of anemia cause additional findings like a sore or inflamed tongue, spoon-shaped fingernails, or easy bruising. None of these signs alone confirm anemia, but they help guide the diagnostic workup.
You may also be asked about your diet, menstrual history, family history of blood disorders, any recent blood loss, and chronic conditions like kidney disease or autoimmune disorders. These details matter because anemia has dozens of possible causes, and your history often narrows the list before a single tube of blood is drawn.
The Complete Blood Count: Your First Test
The CBC is the cornerstone of anemia diagnosis. It reports your hemoglobin concentration, your hematocrit (the percentage of blood volume occupied by red blood cells), and several red blood cell indices that describe the size and hemoglobin content of your individual cells. The most important of these indices is the mean corpuscular volume, or MCV, which tells your doctor whether your red blood cells are smaller than normal, larger than normal, or the right size.
This size classification is one of the fastest ways to narrow down the cause:
- Microcytic (small cells): Most commonly caused by iron deficiency or chronic disease. Cells don’t have enough hemoglobin to fill them out.
- Macrocytic (large cells): Often linked to vitamin B12 or folate deficiency, which disrupts how red blood cells divide and mature.
- Normocytic (normal-sized cells): Seen in anemia from chronic kidney disease, sudden blood loss, or bone marrow problems. The cells are made correctly but there aren’t enough of them.
A CBC doesn’t require fasting in most cases, though your doctor may ask you to fast if they’re ordering additional tests at the same time, like an iron panel. Let your provider know about any vitamins or supplements you take, since iron or B12 supplements can skew results.
How Doctors Identify the Cause
Iron Studies
If your CBC suggests microcytic anemia, your doctor will typically order an iron panel. The most useful marker is ferritin, a protein that reflects your body’s iron stores. Ferritin below 30 micrograms per liter indicates iron deficiency in most adults. The World Health Organization uses a stricter cutoff of 15 micrograms per liter, but in clinical practice, levels under 30 are enough to make the diagnosis.
There’s a catch, though. Ferritin rises during inflammation, infection, and chronic illness, so a “normal” ferritin level can mask true iron deficiency in someone with an ongoing inflammatory condition. In those situations, the threshold is raised to 100 micrograms per liter, and doctors rely on a second marker called transferrin saturation. When transferrin saturation drops below 16%, it strongly suggests the body isn’t getting enough iron to its red blood cells, regardless of what ferritin shows.
Vitamin B12 and Folate Levels
When the CBC shows large red blood cells, B12 and folate deficiency become prime suspects. A serum B12 level below 200 picograms per milliliter is considered deficient, while a folate level below 2 nanograms per milliliter confirms folate deficiency. Both vitamins are essential for normal cell division, and when either is missing, the bone marrow produces oversized, poorly functioning red blood cells.
B12 deficiency is particularly common in older adults, vegetarians, vegans, and people with digestive conditions that impair absorption. Folate deficiency is less common now that many grain products are fortified, but it still occurs with poor dietary intake or increased demand during pregnancy.
Reticulocyte Count
This test measures how many young, newly made red blood cells your bone marrow is releasing into your bloodstream. It’s a powerful diagnostic tool because it answers a fundamental question: is your bone marrow responding appropriately to the anemia?
A reticulocyte index above 3% in someone with anemia means the bone marrow is working overtime to replace lost red blood cells. This points toward blood loss or red blood cell destruction (hemolysis) as the cause. A reticulocyte index below 2% in someone with anemia means the bone marrow isn’t keeping up, suggesting a production problem like nutrient deficiency, bone marrow disease, or suppression from chronic illness.
Telling Iron Deficiency Apart From Chronic Disease
This distinction trips up a lot of diagnoses because both conditions can look similar on a basic CBC. In iron deficiency, ferritin is low, transferrin saturation is low, and the body ramps up its iron transport protein (transferrin) in an attempt to move whatever iron is available. In anemia of chronic disease, the body deliberately locks iron away in storage as part of the immune response. Ferritin may be normal or even elevated above 200 nanograms per milliliter, while transferrin saturation remains low. This pattern of high ferritin with low transferrin saturation is a hallmark of inflammation-driven iron sequestration, and recognizing it prevents unnecessary iron supplementation that wouldn’t address the real problem.
Diagnosis During Pregnancy
Pregnancy naturally increases blood volume, which dilutes red blood cells and lowers hemoglobin. Because of this, the diagnostic thresholds for anemia shift by trimester. In the first trimester, hemoglobin below 11 grams per deciliter qualifies as anemia. In the second trimester, the cutoff drops slightly to 10.5 grams per deciliter as blood volume peaks. By the third trimester, the threshold returns to 11 grams per deciliter.
Iron deficiency is the most common cause of anemia in pregnancy, driven by the substantial iron demands of a growing fetus and expanding blood supply. Routine screening with a CBC is standard at the first prenatal visit and again in the late second or early third trimester. If hemoglobin is low, your provider will check ferritin and iron studies just as they would outside of pregnancy.
What Additional Testing Looks Like
Most anemia is diagnosed and classified with the tests above. Occasionally, when the cause remains unclear or the anemia is severe, your doctor may order more specialized tests. A peripheral blood smear involves examining your blood cells under a microscope to look for abnormal shapes, which can indicate conditions like sickle cell disease or certain types of hemolytic anemia. Rarely, a bone marrow biopsy is needed to evaluate how well the marrow is producing cells.
The diagnostic process is usually straightforward: a CBC identifies the anemia and classifies cell size, targeted blood tests reveal the underlying cause, and treatment follows from there. Most people get answers within a day or two of their initial blood draw, since these are all routine lab tests with fast turnaround times.