The complete blood count (CBC) is the essential starting point for identifying, classifying, and initially assessing the etiology of anemia. Although it is a routine laboratory test, it offers a wealth of clinical information—provided it is interpreted in a structured way and contextualized within the patient’s clinical picture.
A proper reading of the CBC goes beyond evaluating the hemoglobin level alone and requires integrated analysis of multiple red cell indices, taking into account clinical context, speed of onset, and patient age. The CBC alone cannot determine the cause of anemia, but it plays a decisive role in guiding the diagnostic work-up and the choice of further tests, which are detailed in specific pages for each type of anemia.
Key parameters
Hemoglobin (Hb): measures the hemoglobin concentration in blood. Threshold values for anemia vary by age, sex, and pregnancy status. In adults, anemia is defined as Hb <13 g/dL in men and <12 g/dL in women.
Hematocrit (HCT): indicates the percentage of blood volume occupied by red blood cells. It is influenced by hydration status and may be unreliable in cases of hemoconcentration or hemodilution.
Red blood cell count (RBC): measures the absolute number of erythrocytes. While of limited value in isolation, it may help distinguish hypoproliferative from hyperproliferative forms.
MCV (mean corpuscular volume): a key parameter for the morphological classification of anemia. It allows the subdivision into microcytic (MCV <80 fL), normocytic (80–100 fL), and macrocytic (>100 fL) anemias.
MCH (mean corpuscular hemoglobin) and MCHC (mean corpuscular hemoglobin concentration): express the average hemoglobin content and concentration per erythrocyte, respectively. Useful to define whether anemia is hypochromic, normochromic, or (rarely) hyperchromic.
RDW (red cell distribution width): measures size variability among red blood cells. An elevated RDW indicates anisocytosis, helping distinguish nutritional deficiencies from chronic or genetic anemias.
When interpreted together, these parameters support an initial morphological classification and guide clinical interpretation. In subsequent pages, each type of anemia is discussed in detail based on these indices (microcytic anemias, macrocytic anemias, etc.).
Examples of integrated interpretation
Knowing threshold values is not enough: proper CBC interpretation requires evaluating combinations of key parameters. Typical examples include:
Microcytic hypochromic anemia with MCV <80 fL, low MCH, high RDW: highly suggestive of iron deficiency. If ferritin is also low, iron deficiency anemia is confirmed.
Normocytic normochromic anemia with low reticulocytes: indicates impaired marrow production, warranting erythropoietin level assessment or bone marrow examination. Common in anemia of chronic disease or aplastic anemia.
Macrocytic anemia with MCV >100 fL and elevated RDW: suggests vitamin B12 or folate deficiency, to be confirmed with vitamin assays. See: megaloblastic anemia.
More complex findings, such as dual erythrocyte populations, are discussed in specific forms like sideroblastic anemia or in post-transfusion phases.
Reticulocytes: a marker of erythropoietic activity
Reticulocyte count is the main indicator of bone marrow response to anemia. Reticulocytes are immature red cells released into peripheral blood in response to erythropoietin.
Both relative (percentage) and absolute (cells/μL) values should be assessed, along with the corrected reticulocyte index, especially in severe anemia.
Interpretation is critical to distinguish hypoproliferative from hyperproliferative anemias:
Low or inappropriately normal reticulocytes: indicate insufficient erythropoiesis. Typical of nutritional deficiencies, chronic disease anemia, or marrow aplasia.
High reticulocytes (>100,000/μL): reflect active erythropoiesis, often due to acute bleeding or hemolysis. See autoimmune hemolytic anemia.
Reticulocyte count should be ordered early when anemia is suspected, as it directly informs pathophysiologic interpretation and further work-up.
Peripheral smear: morphology as diagnostic clue
The peripheral blood smear enables direct microscopic observation of erythrocytes, providing irreplaceable qualitative data in the interpretation of anemia.
It allows evaluation of red cell morphology and identification of abnormal elements, including immature forms, inclusions, and membrane abnormalities.
Typical findings include:
Poikilocytosis, anisocytosis: common in nutritional deficiencies and anemia of chronic disease.
Elliptocytes, spherocytes, target cells: suggest hereditary membrane disorders, discussed in the section on congenital forms.
Macrocytes, Howell-Jolly bodies: indicative of abnormal erythroid maturation, typical of megaloblastic anemia.
In suggestive clinical contexts, the peripheral smear is indispensable for confirming or ruling out hypotheses raised by CBC findings.
Next steps: what to order after the CBC
Once anemia is classified based on MCV, reticulocytes, and red cell morphology, further testing can be rationally selected.
Suspicion of iron deficiency anemia warrants iron studies (ferritin, transferrin, transferrin saturation), while megaloblastic anemia requires measurement of vitamin B12 and folate. Suspected hemolysis calls for specific tests (haptoglobin, indirect bilirubin, LDH, direct Coombs test), as detailed in the pages on hemolytic anemias and megaloblastic syndromes.
If anemia presents as an isolated but persistent abnormality unexplained by first-line tests, a bone marrow examination (aspiration or biopsy) is indicated, discussed in the sections on aplastic anemia and myelodysplastic syndromes.
References
Tefferi A. Complete blood count: interpretation and clinical utility. Mayo Clinic Proceedings. 2005;80(7):923-933.
