PK deficiency (pyruvate kinase deficiency) is a rare inherited blood disorder where red blood cells break down faster than normal because they lack enough of an enzyme called pyruvate kinase. It affects an estimated 3 to 9 people per million in Western populations, though the true number is likely higher because mild cases often go undiagnosed. The condition ranges from barely noticeable anemia to severe, transfusion-dependent disease that begins at birth.
How PK Deficiency Destroys Red Blood Cells
Pyruvate kinase is an enzyme that helps red blood cells produce energy in the form of ATP. Red blood cells are unusual compared to other cells in the body: they have no nucleus and no mitochondria, so they rely almost entirely on a single energy-producing pathway. Pyruvate kinase catalyzes one of the final steps in that pathway.
When pyruvate kinase is deficient, red blood cells can’t make enough ATP to power the tiny pumps in their membranes that regulate sodium, potassium, and water balance. Without those pumps working properly, the cells lose potassium and water, becoming rigid and misshapen. These stiff, damaged cells get trapped and destroyed in the spleen, a process called hemolysis. The body tries to compensate by producing red blood cells faster, but in moderate to severe cases, it can’t keep up with the destruction.
Genetics and Inheritance
PK deficiency follows an autosomal recessive inheritance pattern, meaning a child must inherit a defective copy of the responsible gene from each parent. The gene involved is called PKLR, located on chromosome 1, and more than 200 different mutations have been identified in it so far. Parents who each carry one mutated copy typically have no symptoms themselves. When both parents are carriers, each pregnancy carries a 25% chance of producing a child with the condition.
Because so many different mutations exist, the severity of the disease varies widely. Some mutations nearly eliminate pyruvate kinase activity, while others only reduce it modestly. A person’s specific combination of two mutations largely determines where they fall on the spectrum from mild to severe.
Symptoms and Severity
The hallmark of PK deficiency is chronic hemolytic anemia, meaning ongoing destruction of red blood cells. The most common signs include fatigue, pale skin, yellowing of the skin and eyes (jaundice), and an enlarged spleen. Jaundice occurs because the breakdown of red blood cells releases a yellow pigment called bilirubin faster than the liver can process it.
Severity varies enormously. The most severe cases show up in newborns, who may need blood transfusions in the first days of life. About 50% of children under age 5 with the condition require regular transfusions. That number drops to roughly 25% in children aged 5 to 12, and fewer than 10% of adults need ongoing transfusions. This improvement happens partly because childhood infections, which worsen both red blood cell destruction and bone marrow suppression, become less frequent with age.
One quirk of PK deficiency is that people often tolerate their anemia better than their hemoglobin numbers would suggest. The same metabolic bottleneck that causes the disease also leads to a buildup of a molecule that helps red blood cells release oxygen more efficiently to tissues. So someone with PK deficiency may feel relatively well at hemoglobin levels that would leave a person with other types of anemia feeling terrible.
How It’s Diagnosed
Diagnosis typically involves two approaches that complement each other. The first is an enzyme activity test, which measures how much pyruvate kinase is working in a blood sample. People with PK deficiency show enzyme activity averaging about 39% of normal, while carriers average around 57%. The second approach is genetic testing of the PKLR gene, which can identify the specific mutations responsible.
Many centers use one method to screen and the other to confirm. One important caveat: if you’ve had a recent blood transfusion, the donated red blood cells will have normal pyruvate kinase levels and can mask the deficiency. A minimum of 50 days after a transfusion is considered a safe window for accurate enzyme testing.
Long-Term Complications
PK deficiency is not just anemia. Data from a large natural history study of 254 patients revealed complication rates that affect quality of life well beyond fatigue: iron overload occurred in 48% of patients, gallstones in 45%, and bone fractures in 17%.
Iron overload deserves special attention because it happens even in people who have never received a transfusion. Among never-transfused patients in the study, 26% still had liver iron overload. Chronic hemolysis itself drives iron absorption from the gut into overdrive, so the body accumulates excess iron regardless of whether transfusions add to the problem. Iron deposits in the liver were found in 62% of all patients studied. If left unchecked, iron overload can progress to liver cirrhosis (reported at nearly 6% in PK deficiency patients, compared to 0.4% in the general population) and pulmonary hypertension (4.6% versus 0.3%).
Bone health is another concern. Osteoporosis rates were significantly higher in patients with PK deficiency than in the general population (15.6% versus essentially 0% in age-matched controls), and about 30% of patients experienced bone fractures across all severity groups. The bone problems likely stem from the bone marrow expanding to produce more red blood cells, combined with the effects of iron on bone density.
Treatment Options
Blood Transfusions
Transfusions remain the backbone of treatment for moderate to severe cases, but they’re guided by symptoms and growth rather than hitting a fixed hemoglobin target. Because people with PK deficiency compensate for anemia better than those with other blood disorders, the threshold for transfusing is typically more individualized. In children, transfusions are triggered primarily by growth delays or symptoms of anemia rather than by lab numbers alone.
Iron Management
Because iron overload is so common, regular monitoring is essential. Patients who receive more than six transfusions per year should have their ferritin levels (a blood marker of iron stores) checked twice yearly, and iron removal therapy is typically started after 10 to 14 transfusions. Even patients who rarely or never receive transfusions need ferritin checks every one to two years and at least one MRI scan to assess liver iron levels. For non-transfused patients with adequate hemoglobin, therapeutic blood removal (phlebotomy) can serve as an alternative to iron-removing medications.
Splenectomy
Removing the spleen reduces the rate of red blood cell destruction and can decrease or eliminate transfusion needs. However, splenectomy is less effective in PK deficiency than in some other blood disorders, and it carries ongoing risks of serious infections from certain bacteria and an increased tendency for blood clots.
Mitapivat
The FDA has approved a medication called mitapivat (brand name Pyrukynd) specifically for PK deficiency. It works by binding directly to the defective pyruvate kinase enzyme and boosting its activity, essentially coaxing the mutated enzyme to work harder. Treatment starts at a low dose taken twice daily and is gradually increased over several weeks to a maximum dose, with the goal of raising hemoglobin levels and reducing red blood cell destruction. This is the first treatment to target the underlying enzyme problem rather than managing symptoms.
Pregnancy Considerations
Women with severe PK deficiency face additional risks during pregnancy, including worsening anemia, blood clots, high blood pressure, and restricted fetal growth. Pre-pregnancy planning ideally includes cardiac imaging to check for iron-related heart damage, screening for infections, and testing the partner’s carrier status. High-dose folic acid is standard during pregnancy to support the increased demand on red blood cell production. Iron removal medications are stopped during pregnancy due to potential risks to the fetus, though this decision involves weighing the dangers of worsening iron overload against fetal safety. Fetal growth monitoring through ultrasound is important because chronic anemia and possible cardiac effects from iron overload may reduce blood flow to the placenta.