Pyruvate kinase deficiency is an autosomal recessive trait, meaning that both parents must each pass on one mutated allele for the offspring to be affected.
Pyruvate kinase is the last enzyme in the glycolysis pathway. It removes a phosphate group from phosphoenolpyruvate to form pyruvate and convert ADP into ATP.
With a deficiency in pyruvate kinase, less ATP is created from glycolysis; recall that mature RBCs lack mitochondria, and thus are solely dependent on glycolysis for ATP production. This deficiency leads to a chronic lack of ATP within the RBCs, and interruption of energy-requiring processes.
These patients produce inadequate intracellular ATP necessary for plasma membrane Na+/K+ pump functioning, so cells lose water and electrolytes; the subsequent dehydration damages the cell membrane and forms misshapen RBCs, known as echinocytes. These abnormally appearing RBCs are hemolyzed outside of the blood vessels in the spleen and liver in a process known as extravascular hemolysis.
Pyruvate kinase deficiency may be in the differential diagnosis for newborns with hemolytic anemia. These patients may have elevated bilirubin from excessive heme breakdown.
An RBC enzyme assay is a panel that tests for various RBC enzyme deficiencies in patients with either acute or chronic hemolytic anemia, and can confirm a pyruvate kinase deficiency. It may also be useful to order a blood smear, CBC with differential, and reticulocyte count to support the diagnosis.
Echinocytes, also known as burr cells, may appear on a blood smear for a patient with pyruvate kinase deficiency. These cells have a serrated appearance with small uniform projections that result from poor membrane stability, due to chronic lack of ATP.
Burr cells are another name for echinocytes. These thorny, misshapen cells may appear on a blood smear for a patient with pyruvate kinase deficiency. Think of a burr that sticks to clothing to picture the shape of these cells.
Burr cells, or echinocytes, have a misshapen cell membrane that looks like it has many spines or thorns along the surface. In fact, "ekhinos" is the Greek word for "sea urchin."
In a normocytic anemia, patients are in an anemic state with a mean corpuscular volume (MCV) between 80 and 100 fL, and a corrected reticulocyte count greater than 3%.
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