Pyruvate Kinase Screen RBC
The red blood cell is dependent on glycolysis to produce energy. Pyruvate kinase is 1 of the 3 rate-limiting enzymes in the glycolytic pathway. It catalyzes the conversion of phosphoenolpyruvate, to pyruvate with the phosphorylation of ADP to ATP. One mole of glucose produces 2 moles of ATP.
Pyruvate kinase deficiency (PKD) is a relatively rare disease with few thousand known cases. It affects all ethnic groups around the world, but is more common in certain communities such as the Pennsylvania Amish and the Romani populations.
The pyruvate kinase gene is located on chromosome 1q21. More than 200 mutations have been identified. PKD is inherited as an autosomal recessive disorder. Clinical symptoms are most common in patients who are compound heterozygotes, but also occur in homozygotes.
PKD causes nonspherocytic chronic hemolytic anemia. Symptoms are variable and vary with age. Some infants may die in utero, some may require exchange transfusion, but others may have milder disease with only hyperbilirubinemia and jaundice. Children and aduts have variable clinical manifestations that range from mild to severe chronic hemolytic anemia. Gallstones and splenomegaly are common features. Anemia may worsen at times of infection or other stress.
PKD should be considered in patients after more common causes of hemolysis are ruled out. The peripheral blood smear may show a minor population of echinocytes, which are small, dense red cells with spicules. Direct antiglobulin test, G6PD enzyme activity, and hemoglobin electrophoresis should all be normal. The incubated osmotic fragility test may be abnormal, but the patient will not have a family history of hereditary spherocytosis.
Quantitative pyruvate kinase enzyme activity is the preferred test for diagnosis of PKD. A red cell hemolysate is incubated with adenosine diphosphate and phosphoenolpyruvate. The amount of pyruvate formed is quantitated by measuring the rate of decrease in absorbance at 340 nm after adding lactic dehydrogenase and NADH.
Contamination of the red blood cell hemolysate with white blood cells can produce falsely normal results because white blood cells contain a high concentration of enzyme. The enzyme assay may also produce spurious results in patients who who are transfusion dependent because transfused red blood cells have normal pyruvate kinase activity.
Reference range for children and adults varies between reference laboratories but usually ranges between 5 and 14 U/g Hb. Most patients with PKD have enzyme levels that are between 5% and 25% of normal. Specimen requirement is one 5 mL lavender top (EDTA) or yellow top (ACD) tube of blood. Specimen should be immediately refrigerated.
DNA sequencing of the PKLR gene may be helpful in patients who are transfusion dependent or in individuals who have enzyme levels at the lower end of the normal range.
