These conditions are sometimes called congenital nonspherocytic anemias. The RBC contains many enzymes involved in various metabolic activities. Theoretically any of these may be affected by congenital or possibly by acquired dysfunction. The most frequent congenital abnormalities are associated with enzymes that participate in metabolism of glucose. After glucose is phosphorylated to glucose-6-phosphate by hexokinase, about 10% of the original molecules follow the oxidative hexose monophosphate (pentose) pathway and about 90% traverse the anaerobic Embden-Meyerhof route. The only common defect associated with clinical disease is produced by G-6-PD deficiency, which is a part of the hexose monophosphate shunt. The primary importance of this sequence is its involvement with metabolism of reduced glutathione (GSH), which is important in protecting the RBC from damage by oxidizing agents. The next most frequent abnormality, pyruvate kinase defect, a part of the Embden-Meyerhof pathway, is very uncommon, and other enzyme defects are rare. The various RBC enzyme defects (plus the unstable hemoglobins) are sometimes collectively referred to as the congenital nonspherocytic anemias.

Red blood cell enzyme defects of the hexose monophosphate shunt and others involved in glutathione metabolism are sometimes called Heinz body anemias. The term also includes many of the unstable hemoglobins and a few idiopathic cases. Heinz bodies are small, round, dark-staining, intraerythrocytic inclusions of varying size that are visualized only when stained by supravital stains (not ordinary Wright’s or Giemsa). In most cases, the Heinz bodies must be induced by oxidizing agents before staining.

Glucose-6-phosphate dehydrogenase defect

G-6-PD defect is a sex-linked genetic abnormality carried on the female (X) chromosome. To obtain full expression of its bad effects, the gene must not be opposed by a normal X chromosome. Therefore, the defect is most severe in males (XY) and in the much smaller number of females in whom both X chromosomes have the abnormal gene. Those females with only one abnormal gene (carrier females) have varying expressions of bad effect ranging from completely asymptomatic to only moderate abnormality even under a degree of stimulation that is greater than that needed to bring out the defect clinically in affected males or homozygous females.

The G-6-PD defect is found mainly in sub-Saharan Africans (10%-14% of African Americans) and to a lesser extent in persons whose ancestors came from Mediterranean countries such as Italy, Greece, or Turkey; from some areas of India; and some Jewish population groups. The defect centers in the key role of G-6-PD in the pentose phosphate glucose metabolic cycle of RBCs. As RBCs age, they normally are less able to use the pentose phosphate (oxidative) cycle, which is an important pathway for use of glucose, although secondary to the Embden-Meyerhof (nonoxidative) glycolysis cycle. When defective G-6-PD status is superimposed on the older erythrocyte, use of the pentose phosphate shunt is lost. This cycle is apparently necessary to protect the integrity of the RBCs against certain chemicals. Currently it is thought that these chemicals act as oxidants and that reduced nicotinamide-adenine dinucleotide phosphate (NADPH) from the pentose cycle is the reducing agent needed to counteract their effects. At any rate, exposure to certain chemicals in sufficient dosage results in destruction of erythrocytes with a sufficiently severe G-6-PD defect. About 11% of African-American males are affected. Their defect is relatively mild, since the younger RBCs contain about 10% of normal enzyme activity, enough to resist drug-induced hemolysis. As the RBCs age they lose nearly all G-6-PD activity and are destroyed. In affected persons of Mediterranean ancestry, all RBCs regardless of age contain less than 1% G-6-PD activity.

As previously noted, susceptible persons do not have anemia before drug exposure. After a hemolytic drug is given, acute hemolysis is usually seen on the second day, but sometimes not until the third or fourth day. All the classic laboratory signs of nonspecific acute hemolysis are present. The degree of anemia produced in African Americans is only moderate, because only the older cell population is destroyed. If the drug is discontinued, hemolysis stops in 48-72 hours. If the drug is continued, anemia continues at a plateau level, with only a small degree of active hemolysis taking place as the RBCs advance to the susceptible cell age. Whites have a more severe defect and therefore more intense hemolysis, which continues unabated as long as the drug is administered.

Many drugs have been reported to cause this reaction in G-6-PD–defective persons. The most common are the antimalarials, sulfa drugs, nitrofurantoin (Furadantin) family, aspirin, and certain other analgesics such as phenacetin. Hemolysis induced by various infections has been frequently reported and may also occur in uremia and diabetic acidosis.

Screening tests for G-6-PD deficiency. Several different tests are available, among which are methemoglobin reduction (Brewer’s test), glutathione stability, dye reduction, ascorbate test, and fluorescent spot tests. G-6-PD assay procedures can also be done. During hemolytic episodes in African Americans, dye reduction and glutathione stability tend to give false normal results. Blood transfusions may temporarily invalidate all G-6-PD deficiency tests in African Americans and Europeans.

The same caution applies to G-6-PD that was applicable to the hemoglobinopathies. Hemolytic anemia in populations known to have a high incidence of G-6-PD defect should always raise the suspicion of its presence. However, even if a patient has the defect, this does not exclude the possibility that the actual cause of hemolysis was something else.

Other red blood cell enzyme deficiencies

There are numerous enzymes in both the anaerobic Embden-Meyerhof glycolytic pathway and the aerobic pentose phosphate shunt. Deficiency in any of these enzymes could result in clinical abnormality. After G-6-PD deficiency, by far the most common is pyruvate kinase deficiency, accounting for 90% of those hemolytic anemias from congenital RBC enzyme defects that are not produced by G-6-PD. Actually, pyruvate kinase deficiency is uncommon, and clinical abnormality from pyruvate kinase or other glycolytic enzymes is rare. Clinical abnormality from pyruvate kinase or other Embden-Meyerhof glycolytic enzyme deficiencies is usually manifest as a Coombs’-negative hemolytic anemia without the relationship to drugs or infections that is associated with G-6-PD.