A complete blood count (CBC) in pernicious anemia may show low hemoglobin, fewer red blood cells, and red blood cells that are larger than usual. The test may also reveal changes in white blood cells and platelets when vitamin B12 deficiency affects blood cell production. However, a CBC cannot confirm pernicious anemia by itself. Additional blood tests help identify vitamin B12 deficiency and determine its cause.
Which CBC findings are associated with pernicious anemia?
Pernicious anemia is a type of vitamin B12 deficiency anemia. It occurs when an autoimmune problem interferes with the body's ability to absorb vitamin B12, often because intrinsic factor, a protein needed for absorption, is missing or affected. Vitamin B12 is essential for normal red blood cell development. When it is lacking, blood cells may develop abnormally and become larger than usual.
According to MedlinePlus, a healthcare professional may use a CBC alongside vitamin B12 testing, other blood tests, and tests for antibodies related to intrinsic factor to investigate pernicious anemia.
Hemoglobin and hematocrit may be lower than expected
Hemoglobin is the protein in red blood cells that carries oxygen. Hematocrit measures how much of the blood's volume consists of red blood cells. When vitamin B12 deficiency interferes with the production of healthy red blood cells, hemoglobin and hematocrit may fall, indicating anemia. This can be associated with fatigue, weakness, lightheadedness, or shortness of breath during activity, although some people have few noticeable symptoms.
The red blood cell count may be low
A CBC measures the number of red blood cells in a given volume of blood. In pernicious anemia, the count may be lower than expected because vitamin B12 deficiency disrupts normal red blood cell production. However, the red blood cell count does not always tell the full story. Clinicians also consider hemoglobin, hematocrit, and red blood cell size when assessing the overall pattern.
Mean corpuscular volume (MCV) may be high
Mean corpuscular volume, or MCV, estimates the average size of red blood cells. Vitamin B12 deficiency commonly causes macrocytosis, meaning the cells are larger than usual. When enlarged red blood cells occur with anemia due to impaired cell development, the pattern is often called macrocytic or megaloblastic anemia. An increased MCV can support suspicion of vitamin B12 deficiency, but it is not specific to pernicious anemia.
The Merck Manual Professional Edition describes macrocytic red blood cells and changes in blood cell development as common findings in megaloblastic anemia, which can result from vitamin B12 or folate deficiency.
Red blood cell distribution width (RDW) may increase
RDW describes how much red blood cell size varies. In vitamin B12 deficiency, some red blood cells may be much larger than others, so the RDW may be higher than usual. This finding can add context to an increased MCV, but RDW alone cannot identify the reason for the variation. Other nutritional deficiencies or blood conditions can also affect red blood cell size.
Can white blood cells and platelets also change?
Yes. Vitamin B12 deficiency can affect the bone marrow's production of more than one type of blood cell. Alongside anemia, a CBC may show a lower white blood cell count, a lower platelet count, or changes in both. These findings are not present in every case, and their significance depends on the full blood count and clinical context.
- White blood cells: The total count may be lower than expected in some cases.
- Platelets: The count may also fall when vitamin B12 deficiency significantly disrupts blood cell production.
- More than one affected cell type: When red blood cells, white blood cells, and platelets are all reduced, clinicians may investigate whether the bone marrow is producing blood cells normally.
These changes are not unique to pernicious anemia. They can occur in other conditions that affect blood cell production, so further evaluation may be needed to understand their cause.
What might a blood smear show?
A CBC may be accompanied by a peripheral blood smear, in which a laboratory professional examines blood cells under a microscope. In vitamin B12-related megaloblastic anemia, the smear may show large, oval-shaped red blood cells and neutrophils with more nuclear segments than usual. These are called macro-ovalocytes and hypersegmented neutrophils, respectively. They provide additional clues about abnormal blood cell development.
The Merck Manual's review of megaloblastic anemia describes these microscopic findings as part of the pattern that can accompany vitamin B12 or folate deficiency. A smear can help characterize the anemia, but it does not establish pernicious anemia as the underlying cause.
Can pernicious anemia occur with a normal MCV or CBC?
Yes. Although a high MCV and anemia are common in established vitamin B12 deficiency, the pattern is not universal. A person may have vitamin B12 deficiency before anemia develops, and some people experience nerve-related symptoms even when their blood count does not show the typical changes. A normal CBC therefore does not always rule out vitamin B12 deficiency when the medical history or symptoms raise concern.
Other conditions can also change the expected pattern. For example, iron deficiency occurring at the same time as vitamin B12 deficiency may make the average red blood cell size appear less elevated than expected. This is one reason clinicians interpret the CBC alongside other laboratory findings rather than relying on MCV alone.
What tests help confirm pernicious anemia after a CBC?
The CBC helps identify a pattern of anemia, but it does not measure vitamin B12 directly or explain why a deficiency has occurred. If the results or clinical history suggest vitamin B12 deficiency, a healthcare professional may order additional tests to clarify the diagnosis.
- Vitamin B12 blood test: Measures the amount of vitamin B12 in the blood.
- Folate test: Helps assess another nutritional deficiency that can cause a similar pattern of large red blood cells.
- Methylmalonic acid (MMA): May help clarify possible vitamin B12 deficiency when the initial B12 result is uncertain.
- Intrinsic factor antibody test: Looks for antibodies associated with pernicious anemia and can help identify an autoimmune cause.
- Additional tests when needed: A clinician may consider other antibody tests, a reticulocyte count, or further investigations based on the initial findings.
The NHS guidance on diagnosis explains that blood tests can assess hemoglobin, red blood cell size, vitamin B12, and folate, with further testing used to investigate suspected pernicious anemia.
An intrinsic factor antibody result must also be interpreted in context. A positive result can support the diagnosis, but a negative result does not always exclude pernicious anemia. The clinician considers the complete blood count, vitamin levels, symptoms, medical history, and other test results together.
How should you interpret CBC results if pernicious anemia is suspected?
Look at the overall pattern rather than any single result. Lower-than-expected hemoglobin combined with a higher MCV may prompt investigation for macrocytic anemia. Changes in RDW, white blood cells, or platelets may add useful information. However, these findings can have several causes, including folate deficiency, certain medicines, liver disease, and other blood disorders.
Reference ranges also vary by laboratory, and blood count results can differ according to age and other individual factors. If a CBC shows unusual findings, a healthcare professional can determine whether vitamin B12 testing or other investigations are appropriate. Persistent numbness, tingling, balance problems, or changes in memory deserve prompt medical assessment because vitamin B12 deficiency can affect the nervous system even without obvious anemia.