A Complete Blood Count (CBC) is a routine laboratory test that offers a detailed snapshot of the cells circulating in your bloodstream. Among the many values reported, the Mean Corpuscular Volume (MCV) and Mean Corpuscular Hemoglobin (MCH) provide specific information about your red blood cells (RBCs). When both measurements are lower than the standard range, it is not a diagnosis on its own. These findings indicate that the characteristics of your oxygen-carrying cells have changed, signaling a need for further investigation to determine the underlying reason.
Defining Low MCV and Low MCH
The MCV, or Mean Corpuscular Volume, reflects the average physical size of red blood cells. A low MCV indicates that the red blood cells are smaller than normal, a condition known as microcytosis. This measurement is typically reported in femtoliters (fL).
The MCH, or Mean Corpuscular Hemoglobin, measures the average amount of hemoglobin contained within each red blood cell. Hemoglobin is the protein responsible for transporting oxygen throughout the body. A low MCH value means the cells contain less hemoglobin, resulting in hypochromia, which literally means “less color.”
When both the MCV and MCH are low, it reveals a pattern of microcytic, hypochromic red blood cells. This pattern suggests an underlying issue with the body’s ability to produce sufficient hemoglobin. Impaired hemoglobin production causes the body to make smaller, less filled cells.
Iron Deficiency Anemia: The Primary Cause
Iron deficiency anemia (IDA) is the most common cause of simultaneously low MCV and MCH results. Iron is a fundamental component of the heme group within the hemoglobin molecule. Without adequate iron, the bone marrow cannot synthesize enough hemoglobin to properly fill developing red blood cells.
This lack of synthesis leads to the production of smaller (microcytic) and paler (hypochromic) red blood cells. The resulting reduction in the blood’s oxygen-carrying capacity leads to the symptoms associated with IDA.
Symptoms often include fatigue, weakness, and pallor (paleness of the skin). Some individuals may also experience pica, a craving for non-food substances like ice or clay.
Risk factors for IDA often involve chronic blood loss, such as heavy menstrual periods or gastrointestinal bleeding. Insufficient intake or absorption of iron from the diet is also a factor. Increased physiological demand, such as during pregnancy, can deplete iron stores.
Other Medical Conditions That Cause Low Readings
While iron deficiency is the most frequent cause, other conditions also cause the microcytic, hypochromic pattern.
Thalassemias
The most notable alternative causes are the thalassemias, inherited genetic disorders affecting hemoglobin production. Thalassemia involves defects in the synthesis of the globin chains, the protein structures surrounding the iron-containing heme. These disorders lead to the production of small, pale cells, often with a very low MCV, even if iron stores are normal or elevated. Differentiating thalassemia from iron deficiency is important because treatments are entirely different, and iron supplementation can be harmful if iron levels are adequate.
Anemia of Chronic Disease (ACD)
ACD is associated with long-term inflammation from autoimmune conditions or chronic infections. In ACD, inflammatory signals interfere with iron metabolism, trapping iron in storage cells and preventing its efficient use in the bone marrow. This can sometimes result in microcytic, hypochromic cells, although ACD often presents with cells of normal size.
Sideroblastic Anemia
Sideroblastic anemia is a less common disorder where the body has difficulty incorporating iron into the hemoglobin molecule, leading to iron accumulating in the red blood cell precursors.
Physician Consultation and Follow-Up Testing
Low MCV and MCH values on a CBC require consultation with a healthcare provider, as this finding is an indicator, not a final diagnosis. The next step involves ordering confirmatory tests to differentiate potential causes, since the treatment plan depends entirely on establishing the correct diagnosis.
A suite of iron studies is typically requested to confirm or rule out iron deficiency anemia. This includes measuring serum ferritin (which indicates the level of iron stored in the body) and total iron-binding capacity (TIBC). A low ferritin level is considered the most specific marker for iron deficiency.
To investigate genetic causes like thalassemia, a test called hemoglobin electrophoresis may be performed. This test separates the different types of hemoglobin present in the blood, identifying abnormal variants characteristic of thalassemia. Treatment will then be tailored based on the final diagnosis.