Study type: Blood collected in yellow-top [acid-citrate-dextrose (ACD)] tube. Specimens collected in a lavender-top [EDTA] or green-top [heparin] tube also may be acceptable in some laboratories; related body system: Circulatory/Hematopoietic system.

Pyruvate kinase (PK) is an enzyme that forms pyruvate and adenosine diphosphate during glycolysis. Isoenzymes of PK are present in specific tissue: M₁ type PK is mainly found in the heart, skeletal muscle, and brain; L type is found in the liver, kidney cortex, and intestine; M₂ is found in leukocytes and most other tissues; type R is specifically found in red blood cells (RBCs). Deficiency of this enzyme can be acquired by ingestion of a drug or as an effect of liver disease. There is also a hereditary form of pyruvate kinase deficiency that can be transmitted as an autosomal recessive trait, meaning both parents must have the affected gene for the enzyme deficiency to be passed on to their offspring. RBCs lacking this enzyme have a membrane defect resulting from low levels of adenosine triphosphate and are more susceptible to hemolysis.