A 52-year-old man presents to his physician for a wellness examination. He reports no specific symptoms other than mild, generalized fatigue and occasional numbness in his fingers. He has a long-standing history of severe alcohol use disorder and reports drinking about six beers per day. He does not take any medications or supplements. Vital signs are within normal limits. On physical examination, there is mild hepatomegaly but no jaundice or splenomegaly.
Laboratory evaluation reveals the following:
Hemoglobin: 11.0 g/dL
Mean corpuscular volume (MCV): 106 fL
Platelet count: 135,000/μL
Reticulocyte count: 1.0%
Peripheral blood smear: round macrocytes without hypersegmented neutrophils.
Which of the following is the most likely primary mechanism for the macrocytosis observed in this patient?
The correct answer is:
C) Direct toxic effect of ethanol on erythropoiesis
Alcohol is one of the most common causes of non-megaloblastic macrocytosis. Ethanol and its metabolite, acetaldehyde, exert a direct toxic effect on the bone marrow, interfering with the development of red blood cell membranes. This results in the production of round macrocytes. It is crucial to distinguish this from megaloblastic anemia caused by B12 or folate deficiency, where the macrocytes are typically oval (macro-ovalocytes) and are accompanied by hypersegmented neutrophils. In this patient, the absence of hypersegmented neutrophils and the presence of round macrocytes point toward a direct alcohol effect or associated liver disease rather than a vitamin deficiency.
Answer choice A: Deficiency of vitamin B12, is incorrect. While individuals who abuse alcohol can have nutritional deficiencies, vitamin B12 deficiency is less common than folate deficiency in this population. Furthermore, B12 deficiency causes megaloblastic changes (oval macrocytes and hypersegmented neutrophils) and is often associated with more profound neurologic deficits than the mild paresthesias described.
Answer choice B: Deficiency of folate, is incorrect. Folate deficiency is a common cause of macrocytic anemia in those who abuse alcohol chronically due to poor intake and impaired absorption. However, like B12 deficiency, it presents with a megaloblastic smear (hypersegmented neutrophils and oval macrocytes). The specific finding of round macrocytes without hypersegmented neutrophils on this patient's smear makes direct ethanol toxicity a more likely primary cause for the macrocytosis itself.
Answer choice D: Ineffective erythropoiesis due to liver cirrhosis, is incorrect. While liver disease can cause macrocytosis, often due to increased cholesterol deposition in the RBC membrane leading to target cells, the primary and most common cause of macrocytosis in the setting of chronic alcohol use, even before the onset of cirrhosis, is the direct effect of alcohol on the bone marrow.
Answer choice E: Myelodysplastic syndrome, is incorrect. Although myelodysplastic syndrome (MDS) can cause macrocytic anemia in older adults, it usually presents with more severe cytopenias and characteristic dysplastic features on the peripheral smear (such as pseudo-Pelger-Huët cells), which are not described here.
Key Learning Point
Alcohol-induced macrocytosis is often non-megaloblastic. The distinguishing feature on a peripheral blood smear is the presence of round macrocytes and the absence of hypersegmented neutrophils. While chronic alcohol abuse increases the risk for folate deficiency, which causes megaloblastic macrocytosis, the direct toxic effect of ethanol on the bone marrow remains a frequent and independent cause of an elevated MCV.
