Anemia is diagnosed primarily through a blood test called a complete blood count, or CBC, which measures your hemoglobin level and the size and shape of your red blood cells. If your hemoglobin falls below established thresholds, you’re considered anemic. But the CBC is usually just the starting point. Depending on the results, your doctor may order additional blood tests to pinpoint the specific type and underlying cause.
The Physical Exam Comes First
Before any blood is drawn, a physical exam can reveal telltale signs. Pallor in the inner eyelids (the conjunctiva), nail beds, and skin is one of the most common findings. Iron deficiency specifically causes brittle or spoon-shaped fingernails, an unusual craving to chew ice, and calf cramps when climbing stairs. Vitamin B12 deficiency often shows up as early graying of the hair, a burning tongue, and trouble sensing where your limbs are in space. Folate deficiency can cause a sore tongue and cracked corners of the mouth.
None of these signs alone confirm anemia, but they help guide which lab tests to order and can point toward a cause before any results come back.
The Complete Blood Count
The CBC is the single most important test. It reports your hemoglobin concentration, your hematocrit (the percentage of blood volume occupied by red blood cells), and several measurements describing the red blood cells themselves. The two most useful of these are mean corpuscular volume (MCV), which tells how large your red blood cells are, and mean corpuscular hemoglobin concentration (MCHC), which tells how much hemoglobin is packed inside each cell.
Red blood cells are classified as microcytic (small) when MCV is below 80 fL and macrocytic (large) when MCV is above 100 fL. Anything in between is normocytic. Cells with low hemoglobin concentration (MCHC below 30%) are called hypochromic, meaning they look pale under a microscope. This sizing system matters because it narrows the list of possible causes immediately:
- Small, pale cells point toward iron deficiency or problems with hemoglobin production, like thalassemia.
- Large cells suggest vitamin B12 or folate deficiency.
- Normal-sized cells raise suspicion for chronic disease, kidney problems, or blood loss.
Red blood cells prioritize keeping their hemoglobin content stable even at the expense of their size. That’s why iron deficiency makes cells shrink: the body produces smaller cells to maintain hemoglobin concentration for as long as possible.
Hemoglobin Thresholds That Define Anemia
The World Health Organization sets specific hemoglobin cut-offs that vary by age and sex. For adult men, anemia is generally defined as hemoglobin below 13 g/dL. For non-pregnant women, the threshold is 12 g/dL. For children aged 6 to 23 months, the WHO’s updated guidelines place the cut-off at 10.5 g/dL. These thresholds account for normal biological variation; men naturally carry more hemoglobin than women, and young children have different baseline levels than adults.
Iron Studies
When the CBC suggests iron deficiency, a panel of iron-related blood tests helps confirm it. The most informative single marker is serum ferritin, a protein that reflects your body’s iron stores. Current recommendations flag levels below 30 ng/mL as consistent with iron deficiency, and some guidelines use a threshold of 50 ng/mL. In practice, some clinicians use a lower cut-off of 20 ng/mL, though this risks missing earlier stages of depletion.
Iron studies also help distinguish iron deficiency from anemia of chronic disease, a condition where the body has adequate iron stores but can’t use them properly. In chronic disease, a liver-produced protein called hepcidin rises in response to inflammation. Hepcidin blocks iron from being absorbed in the gut and traps it inside cells, starving the bone marrow of the iron it needs to make red blood cells. In this situation, ferritin levels are often normal or high (because iron is stored but locked away), which helps separate it from true iron deficiency where ferritin is low.
Vitamin B12 and Folate Levels
If your red blood cells are abnormally large on the CBC, testing for vitamin B12 and folate deficiency is the next step. A serum B12 level below 200 pg/mL (148 pmol/L) indicates low B12 status. For folate, a serum level below 3 ng/mL (7 nmol/L) signals negative balance, while a red blood cell folate level below 140 ng/mL (305 nmol/L) confirms deficiency.
Both deficiencies produce the same type of large red blood cells, so blood levels of both vitamins are typically checked together. The distinction matters because B12 deficiency can cause irreversible nerve damage if left untreated, while folate deficiency does not.
The Reticulocyte Count
Reticulocytes are young, newly released red blood cells. Counting them tells your doctor whether your bone marrow is responding to anemia the way it should. In a healthy person, reticulocytes make up 0.5% to 2.5% of circulating red blood cells.
When you’re anemic, the bone marrow should ramp up production, pushing that percentage higher. If the reticulocyte count is elevated, it means the marrow is working hard, and the problem likely lies in blood loss or destruction of red blood cells (hemolytic anemia). If the reticulocyte count stays low despite anemia, the bone marrow itself isn’t functioning properly, pointing toward nutritional deficiencies, bone marrow failure, or suppression by chronic disease.
A raw reticulocyte percentage can be misleading in anemia, though. When your total red blood cell count drops, the reticulocyte percentage automatically looks higher even if the marrow hasn’t actually increased production. To correct for this, labs calculate a reticulocyte production index that adjusts for the degree of anemia, giving a more accurate picture of bone marrow activity.
The Blood Smear
A peripheral blood smear involves spreading a thin layer of your blood on a glass slide and examining the cells under a microscope. This test reveals abnormalities in red blood cell shape that a CBC’s automated counting machines can miss. Sickle-shaped cells confirm sickle cell disease. Spherocytes (small, round cells without the normal disc shape) suggest hereditary spherocytosis or autoimmune destruction. Elliptocytes (oval-shaped cells) can indicate hereditary elliptocytosis or iron deficiency. Target cells, which have a bull’s-eye appearance, show up in thalassemia and liver disease. Schistocytes, which are fragmented pieces of red blood cells, signal that cells are being physically sheared apart inside blood vessels.
Each of these shapes tells a different story about what’s happening in your body, making the blood smear one of the most informative and inexpensive diagnostic tools available.
Bone Marrow Biopsy
Most anemia is diagnosed entirely through blood tests. A bone marrow biopsy is reserved for cases where the cause remains unclear after standard testing, or when a blood cancer like leukemia or lymphoma is suspected. The procedure involves inserting a needle into the back of the hip bone to withdraw a small sample of marrow tissue.
The sample allows direct examination of the cells where red blood cells are produced. In aplastic anemia, the marrow appears empty or replaced by fat. In leukemia, it’s packed with abnormal white blood cells. Results from a marrow aspiration (the liquid portion) are typically available within hours, while the solid tissue biopsy takes 10 to 14 days to process. For suspected leukemia, the aspiration alone is usually sufficient to start treatment. When lymphoma or multiple myeloma is suspected, both the liquid aspiration and the solid tissue biopsy are performed together because these cancers can affect the marrow in patchy areas that a liquid sample might miss.
How the Pieces Fit Together
Anemia diagnosis works like a decision tree. The CBC identifies that anemia exists and categorizes the red blood cells by size. The reticulocyte count determines whether the bone marrow is responding or failing. Iron studies, vitamin levels, and inflammatory markers then narrow down the specific cause. A blood smear adds visual evidence when the numbers alone aren’t conclusive. A bone marrow biopsy is the final step when everything else comes up short.
In straightforward cases like iron deficiency from heavy periods or a diet low in B12, diagnosis takes a single round of blood tests. In complex cases involving multiple overlapping causes or chronic illness, the workup may take several rounds of testing over weeks. The goal is always the same: not just confirming that anemia is present, but identifying exactly why it’s happening so treatment targets the root cause.