A complete blood count, commonly called a CBC, is the primary test used to detect anemia. It measures your hemoglobin level, which is the protein in red blood cells that carries oxygen. Anemia is diagnosed when hemoglobin drops below 13.5 g/dL in adult men or below 12 g/dL in adult women. Depending on what the CBC reveals, your doctor may order follow-up tests to pinpoint the cause.
What a CBC Tells You
A CBC is a routine blood draw that gives a snapshot of all three major blood cell types, but for anemia, the red blood cell measurements matter most. The test reports three core values: hemoglobin (the oxygen-carrying protein), hematocrit (the percentage of your blood made up of red blood cells), and your total red blood cell count. If any of these fall below normal thresholds, you have anemia.
For adult men, the cutoffs are hemoglobin below 13.6 g/dL, hematocrit below 40%, or a red blood cell count under 4.5 million per microliter. For adult women, the thresholds are hemoglobin below 12 g/dL, hematocrit below 37%, or red blood cells under 4 million per microliter. Pregnant individuals have naturally lower hemoglobin due to increased blood volume, with thresholds around 11 g/dL in the first trimester and about 10.6 g/dL in the second trimester. For children under five, the World Health Organization uses a general cutoff of 11 g/dL.
How Red Blood Cell Size Narrows the Cause
The CBC also reports a value called mean corpuscular volume, or MCV, which measures the average size of your red blood cells. This single number helps categorize anemia into three groups, each pointing toward different causes.
- Microcytic (MCV below 80 fL): Small red blood cells typically signal iron deficiency or inherited conditions like thalassemia.
- Normocytic (MCV 80 to 100 fL): Normal-sized cells with low hemoglobin suggest anemia from chronic disease, kidney problems, or acute blood loss.
- Macrocytic (MCV above 100 fL): Larger-than-normal cells often point to vitamin B12 or folate deficiency, or sometimes liver disease.
Another useful value on the CBC is the red cell distribution width, or RDW, which measures how much your red blood cells vary in size. A high RDW can reveal that two problems are happening at once, like a combined iron and B12 deficiency, which might otherwise be masked because the average cell size looks normal.
The Peripheral Blood Smear
When CBC numbers look abnormal, a doctor may order a peripheral blood smear. A technician spreads a drop of your blood on a glass slide and examines the red blood cells under a microscope. The shape of the cells can be remarkably telling. Sickle-shaped cells confirm sickle cell disease. Tiny spherical cells suggest hereditary spherocytosis. Fragmented cells, called schistocytes, point toward a condition where red blood cells are being physically torn apart inside blood vessels. Teardrop-shaped or oval-shaped cells can appear in iron deficiency or bone marrow disorders. This visual inspection adds diagnostic detail that numbers alone can miss.
Iron Studies
If your CBC suggests iron deficiency (small, pale red blood cells), a set of blood tests called an iron panel helps confirm it. The most important marker is ferritin, a protein that reflects your body’s iron stores. A ferritin level below 30 ng/mL is a clear sign of iron deficiency, whether or not you’re anemic yet.
There’s a catch, though. Ferritin rises when your body is fighting inflammation or infection, so someone with an inflammatory condition could have ferritin up to 100 ng/mL and still be iron deficient. In those cases, another measurement called transferrin saturation becomes more helpful. When transferrin saturation drops below 20%, it indicates your body doesn’t have enough iron available to make red blood cells, even if ferritin looks deceptively normal.
Vitamin B12 and Folate Levels
When the CBC shows large red blood cells, B12 and folate blood tests are the next step. Vitamin B12 levels below 150 ng/L are considered clear evidence of deficiency, while levels between 150 and 400 ng/L fall into a gray zone that may need further evaluation. B12 deficiency can cause a specific type of anemia where red blood cells grow abnormally large because they can’t divide properly. It can also cause nerve damage, including numbness and tingling, even before anemia becomes obvious.
Folate deficiency produces a similar picture of oversized red blood cells. A serum folate level below 4 mcg/L suggests deficiency. Since B12 and folate deficiencies look nearly identical on a CBC, testing both at the same time helps your doctor distinguish between them and choose the right treatment.
The Reticulocyte Count
One of the most underappreciated tests in anemia workups is the reticulocyte count. Reticulocytes are young, freshly made red blood cells that your bone marrow releases into the bloodstream. Counting them reveals whether your bone marrow is responding appropriately to anemia or whether it’s part of the problem.
A high reticulocyte count in someone with anemia means the bone marrow is working overtime to replace lost cells. This pattern is typical when red blood cells are being destroyed faster than normal (hemolytic anemia) or when you’re actively losing blood. A low or normal reticulocyte count in someone who’s anemic, on the other hand, signals that the bone marrow isn’t keeping up. This can happen with nutritional deficiencies, bone marrow failure, or aplastic anemia, a rare condition where the marrow simply stops producing enough cells.
Preparing for Your Blood Test
A standard CBC doesn’t require fasting. You can eat, drink, and take your usual medications beforehand. Iron and vitamin tests, however, can sometimes be affected by recent meals or supplements. If your doctor is ordering an iron panel or checking B12 and folate levels, let them know about any vitamins or supplements you’re taking, as these can skew results. Some providers will ask you to fast or hold iron supplements for 24 hours before the draw, but don’t stop any prescription medication unless specifically told to do so.
Most anemia blood work is drawn from a vein in your arm and results are typically available within a day or two. If the initial CBC points to anemia, the follow-up tests described above usually happen in the same visit or shortly after, so the full picture often comes together quickly.