Anemia is diagnosed with a simple blood test called a complete blood count (CBC), which measures your hemoglobin, hematocrit, and red blood cell levels. For adult men, hemoglobin below 13.6 g/dL signals anemia. For adult women, the cutoff is below 12 g/dL. But the CBC is just the starting point. Pinpointing the cause of anemia often requires additional blood work, and the cause determines the treatment.
Physical Signs That Prompt Testing
Before any blood is drawn, certain physical signs can raise suspicion. Pale skin on its own isn’t always meaningful, but pallor inside the lower eyelids (the conjunctiva), in the nail beds, or in the creases of your palms suggests hemoglobin has dropped below roughly 9 g/dL. That’s a level where anemia is already moderate to severe.
Other visible clues point toward specific types. Spoon-shaped nails that curve upward at the edges are a classic sign of iron deficiency. A swollen, smooth tongue or cracked corners of the mouth suggest iron, folate, or B12 deficiency. Yellowing of the skin and eyes alongside fatigue can indicate red blood cells are being destroyed too quickly, which is a different category of anemia entirely. These findings tell a clinician which follow-up tests to order.
The Complete Blood Count
The CBC is the first and most important test. It reports three key values:
- Hemoglobin: the oxygen-carrying protein inside red blood cells. Anemia is defined as hemoglobin below 13.6 g/dL in adult men and below 12 g/dL in adult women.
- Hematocrit: the percentage of your blood that’s made up of red blood cells. Below 40% in men or 37% in women qualifies as anemia.
- Red blood cell count: the total number of red cells per unit of blood. Below 4.5 million per microliter in men or 4 million in women is low.
For infants and children, normal values change with age, so pediatricians use age-specific reference charts. The American Academy of Pediatrics recommends anemia screening at intervals during childhood, with risk assessment guiding when testing is appropriate.
How Red Blood Cell Size Narrows the Cause
The CBC also reports a value called mean corpuscular volume (MCV), which measures the average size of your red blood cells. This single number is one of the most useful clues for figuring out why you’re anemic, because different causes produce differently sized cells. Normal MCV falls between 80 and 100 fL.
When MCV is below 80, your red blood cells are smaller than normal. This pattern, called microcytic anemia, is most commonly caused by iron deficiency. It also shows up in thalassemia (an inherited blood disorder) and a few rarer conditions. Iron deficiency and thalassemia can look similar on a CBC, so additional tests are needed to tell them apart.
When MCV is in the normal range but hemoglobin is still low, the anemia is normocytic. This happens with chronic kidney disease, many chronic inflammatory conditions, and acute blood loss. The red blood cells are a normal size; there just aren’t enough of them, or they aren’t carrying enough hemoglobin.
When MCV exceeds 100, red blood cells are larger than normal. This macrocytic pattern is a hallmark of vitamin B12 or folate deficiency. It also appears with chronic alcohol use and certain liver conditions. The distinction matters because B12 deficiency left untreated can cause irreversible nerve damage, so identifying it early has real consequences.
Iron Studies
If the CBC points toward iron deficiency (small red blood cells, low hemoglobin), the next step is a set of iron-specific blood tests. The most important is ferritin, a protein that reflects how much iron your body has stored.
The World Health Organization defines low ferritin as below 15 μg/L in adults, but in everyday clinical practice, levels below 30 μg/L are generally considered diagnostic of iron deficiency. Here’s the complication: ferritin rises during inflammation, infection, and chronic disease, even when iron stores are actually depleted. In someone with an ongoing inflammatory condition, ferritin can read between 100 and 300 μg/L and still mask a true iron shortage.
That’s where transferrin saturation comes in. This measures how much of your blood’s iron-transport protein is actually loaded with iron. A transferrin saturation below 20% confirms iron deficiency regardless of what ferritin says, making it especially valuable when inflammation is present. If your doctor suspects both iron deficiency and chronic inflammation are at play, they’ll typically order both ferritin and transferrin saturation together.
Vitamin B12 and Folate Levels
When the CBC shows large red blood cells, B12 and folate blood tests come next. A B12 level above 300 pg/mL is normal. Below 200 pg/mL confirms deficiency. The range between 200 and 300 pg/mL is a gray zone where additional testing may be needed to clarify the picture.
For folate, levels below 2 ng/mL confirm deficiency. Readings between 2 and 4 ng/mL are borderline. Both deficiencies produce the same type of anemia (large, immature red blood cells flooding the bloodstream), but the treatments are different, and giving folate alone to someone who is actually B12-deficient can mask the deficiency while nerve damage progresses silently. That’s why both vitamins are typically tested together.
The Blood Smear
Sometimes the numbers on a CBC don’t tell the full story, and a lab technician or pathologist examines a drop of blood under a microscope. This is called a peripheral blood smear, and it reveals the actual shape of your red blood cells.
Healthy red blood cells are round and slightly concave, like a disc with a dimple. In iron deficiency anemia, you may see elongated oval cells (elliptocytes) and fragmented cells. Sickle cell disease produces the distinctive crescent-shaped cells it’s named after. Tiny fragments of shredded red blood cells (schistocytes) suggest the cells are being physically torn apart in the bloodstream, pointing toward conditions like a blood clot disorder or a malfunctioning heart valve. Teardrop-shaped cells can indicate a bone marrow problem.
Each abnormal shape acts like a fingerprint, pointing toward a specific diagnosis that numbers alone might miss.
Testing for Inherited Blood Disorders
When someone has unexplained anemia, especially with small red blood cells that don’t improve with iron supplements, or anemia alongside jaundice, testing for inherited hemoglobin disorders becomes important. The primary tool is hemoglobin electrophoresis, a test that separates the different types of hemoglobin in your blood.
Normal adult hemoglobin is mostly type A. The test can detect hemoglobin S (the variant responsible for sickle cell disease), elevated hemoglobin F (fetal hemoglobin, which in adults suggests certain types of thalassemia), and elevated hemoglobin A2, which is the hallmark of beta-thalassemia trait. In beta-thalassemia trait, HbA2 levels rise above 3.7%, while normal values stay below 3.3%.
Fetal hemoglobin in healthy adults stays below 1%. Levels between 1% and 3% appear in about a third to half of people carrying beta-thalassemia trait. Levels above 5%, combined with small, pale red blood cells, suggest a different thalassemia subtype and typically trigger further genetic testing.
Putting the Diagnosis Together
Anemia diagnosis works like a decision tree. The CBC confirms anemia exists and reveals red blood cell size. Cell size points the investigation in a direction: small cells lead to iron studies and possibly hemoglobin electrophoresis, large cells lead to B12 and folate testing, and normal-sized cells prompt a search for chronic disease, kidney problems, or blood loss. A blood smear can be ordered at any point to look for abnormal cell shapes that clarify an uncertain picture.
Most anemia workups require only two or three blood draws total. For straightforward iron deficiency or vitamin deficiency, results are typically available within a day or two. Hemoglobin electrophoresis and more specialized tests may take a week. In the majority of cases, anemia has a identifiable and treatable cause, and the diagnostic process is designed to find it efficiently rather than requiring every possible test upfront.