right shift

The terms 'left shift' and 'right shift' originate from the historical Arneth count, which graphed neutrophil maturity based on nuclear lobe number. A right shift indicates a population of neutrophils skewed toward hypersegmentation (≥5 nuclear lobes), reflecting impaired DNA synthesis during neutrophil maturation in the bone marrow. This occurs because vitamin B12 and folate are essential cofactors for DNA synthesis; their deficiency leads to ineffective hematopoiesis and delayed nuclear maturation while cytoplasmic maturation continues normally, producing macrocytic red cells and hypersegmented neutrophils. Clinically, right shift is a key peripheral smear finding in megaloblastic anemia, alongside macro-ovalocytes and anisopoikilocytosis. It can also be seen in myelodysplastic syndromes and chronic renal failure. This is diagnostically useful to differentiate megaloblastic anemia from other causes of macrocytic anemia (e.g., liver disease, hypothyroidism) where hypersegmented neutrophils are typically absent. In contrast, a 'left shift' (increased bands and immature granulocytes) reflects a bone marrow response to acute inflammation or infection, often accompanied by toxic granulations and Döhle bodies. Understanding right shift is important for USMLE questions differentiating causes of macrocytic anemia and recognizing peripheral smear findings associated with B12/folate deficiency versus bacterial infections.