Anemia Defisiensi G6PD

01Pathophysiology

G6PD is the rate-limiting enzyme of the hexose monophosphate shunt (also called the pentose phosphate pathway). This pathway is the sole source of NADPH in red blood cells. NADPH is essential because it serves as the cofactor for glutathione reductase, which converts oxidized glutathione (GSSG) back into reduced glutathione (GSH). Reduced glutathione is the primary defense mechanism that neutralizes reactive oxygen species and hydrogen peroxide inside the erythrocyte.

When G6PD activity is deficient, the red blood cell cannot regenerate adequate NADPH. Without NADPH, glutathione remains in its oxidized form and cannot scavenge free radicals. The result is that hemoglobin is oxidized and denatures, forming precipitates known as Heinz bodies. These Heinz bodies are rigid inclusions that attach to the inner surface of the red cell membrane, making the cell stiff and unable to traverse the narrow sinusoids of the spleen. Splenic macrophages recognize and partially phagocytose these cells, "biting off" the portion containing the Heinz body. This produces the characteristic bite cells (degmacytes) seen on peripheral smear.

The reason hemolysis is episodic rather than chronic is that under baseline conditions, even a reduced level of G6PD activity may be sufficient to handle normal oxidative load. It is only when an exogenous oxidative stress is introduced (infection, drug, or fava beans) that the demand for NADPH outstrips supply, and hemolysis occurs. Importantly, the hemolysis is intravascular (complement-independent, direct oxidative lysis) and extravascular (splenic removal of damaged cells), which is why you see both hemoglobinuria and elevated indirect bilirubin.

Another critical concept is that reticulocytes and young red blood cells have higher G6PD activity than older cells. During an acute hemolytic episode, the older, more enzyme-deficient cells are destroyed first. The bone marrow responds with a reticulocytosis, flooding the circulation with young cells that have relatively normal G6PD levels. This is why the G6PD enzyme assay can be falsely normal if drawn during or immediately after an acute crisis, a point that exam writers exploit repeatedly.

In neonates, G6PD deficiency is an important cause of pathologic neonatal jaundice and can lead to kernicterus if unrecognized. The neonatal liver has immature conjugation capacity, and when combined with accelerated hemolysis, bilirubin levels can rise rapidly.

02Classification and Clinical Manifestation

WHO Classification

Clinical Manifestation Patterns

Common Oxidative Triggers

03Diagnostic Workup

The first step when encountering a patient with sudden-onset anemia, jaundice, and dark urine is to confirm that hemolysis is occurring. The combination of elevated indirect bilirubin, elevated LDH, decreased haptoglobin, and elevated reticulocyte count establishes a hemolytic process. The urinalysis showing hemoglobinuria (dipstick positive for blood without red blood cells on microscopy) confirms an intravascular component.

The next step is to determine whether the hemolysis is immune-mediated or non-immune. The direct Coombs test (direct antiglobulin test) is the pivotal branch point. A negative Coombs test in the setting of hemolysis points away from autoimmune hemolytic anemia and toward intrinsic red blood cell defects: hemoglobinopathies, membranopathies, or enzymopathies such as G6PD deficiency.

The peripheral blood smear then provides the morphologic clue. Heinz bodies are best visualized with a supravital stain (crystal violet or methyl violet), not on a standard Wright-Giemsa stain. Bite cells are visible on routine smear and are essentially pathognomonic in the right clinical context (episodic hemolysis + oxidative trigger + Coombs-negative).

The G6PD enzyme assay is the confirmatory gold standard. However, the single most tested concept about this assay is its timing. During an acute hemolytic crisis, the most enzyme-deficient (oldest) red blood cells have already been destroyed, and the circulating reticulocytes are young cells with higher G6PD levels. Therefore, the assay may return a falsely normal result during the crisis. The test should be deferred until 2 to 3 months after the episode when the red blood cell population has normalized. If clinical suspicion is high but the initial assay is normal, repeat testing after recovery is mandatory.

For female carriers suspected of heterozygous G6PD deficiency (who may have a mosaic population of normal and deficient cells due to X-inactivation), the standard quantitative assay may also be falsely reassuring. Cytochemical assays or flow cytometry-based tests can detect the two distinct red cell populations and are more sensitive in this setting.

04Management and Treatment

Acute episode management centers on removing the oxidative trigger. In drug-induced cases, the offending medication must be stopped immediately. In most patients with G6PD A- (the African variant), hemolysis is self-limited because only the older red blood cells are destroyed. The reticulocyte response peaks around 5 to 7 days, and hemoglobin stabilizes within 1 to 2 weeks. In contrast, patients with G6PD Mediterranean may experience more prolonged and severe hemolysis because their enzyme deficiency is more profound across all red cell ages, and transfusion is more often required.

Transfusion is indicated when the patient shows signs of cardiovascular compromise (tachycardia, hypotension, dyspnea at rest) or when the hemoglobin level drops below approximately 7 g/dL. There is no role for corticosteroids, splenectomy in the acute setting, or any pharmacologic agent to "boost" G6PD activity. The treatment is entirely supportive.

Folic acid at a dose of 1 mg orally daily is given to support the bone marrow's compensatory erythropoiesis. Iron supplementation is generally not needed unless there is a coexisting iron deficiency, because hemolysis releases iron back into the body's stores (unlike blood loss anemia where iron is lost externally).

In the neonatal setting, management follows standard hyperbilirubinemia protocols. Phototherapy is first-line. If total serum bilirubin rises to exchange transfusion thresholds (which vary by gestational age and hours of life per the Bhutani nomogram), exchange transfusion is performed.

A crucial long-term consideration is screening before drug prescription. Before initiating primaquine for Plasmodium vivax or Plasmodium ovale malaria (or for malaria prophylaxis), G6PD status must be checked. This is a high-yield point because primaquine is the most tested drug association. Another commonly tested contraindication is rasburicase (recombinant urate oxidase used in tumor lysis syndrome), which generates hydrogen peroxide as a byproduct and is absolutely contraindicated in G6PD deficiency. Similarly, methylene blue, the standard treatment for methemoglobinemia, relies on NADPH to function and is therefore ineffective and potentially harmful in G6PD-deficient patients.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most exploited trap on this topic is the timing of the G6PD enzyme assay. Test writers construct a vignette with a classic presentation: a young man develops jaundice and dark urine two days after starting an antimalarial. The question asks for the most appropriate next step. The tempting answer is "G6PD enzyme assay." However, if the patient is still in the acute hemolytic phase, the correct answer is to draw the hemolysis labs and peripheral smear now, but defer the G6PD assay until after recovery. Ordering it acutely yields a falsely normal result that may cause you to miss the diagnosis entirely. Many students lose points here because they jump to the confirmatory test without considering the clinical nuance of when it is valid.

A second common trap involves the drug associations. Vignettes love to present a patient prescribed primaquine for P. vivax radical cure who then develops hemolysis. The tested concept is not just recognizing the association, but understanding the clinical implication: you must screen for G6PD before prescribing primaquine. Similarly, test writers will place a G6PD-deficient patient into a tumor lysis syndrome scenario and ask you to choose between rasburicase and allopurinol. The correct answer is allopurinol (or febuxostat), because rasburicase is absolutely contraindicated.

The methylene blue contraindication is another favorite. A patient presents with methemoglobinemia (chocolate-colored blood that does not correct with supplemental oxygen). The reflexive answer is methylene blue. But if the vignette mentions G6PD deficiency or drops a clue (recent hemolytic episode, African or Mediterranean descent), methylene blue will not work and can worsen hemolysis. The alternative treatment is ascorbic acid (vitamin C), which acts as a direct reducing agent, though it is much slower and less effective.

Finally, remember that G6PD deficiency is the most common human enzymopathy worldwide. It confers a selective survival advantage against Plasmodium falciparum malaria, which is why its geographic distribution overlaps with the malaria belt. This evolutionary biology link is occasionally tested in basic science-oriented questions.

The core competency being assessed in G6PD questions is the ability to recognize a Coombs-negative, episodic hemolytic anemia triggered by an identifiable oxidative stress, correctly sequence the diagnostic workup (especially knowing when NOT to order the confirmatory test), and apply drug contraindications in clinical decision-making scenarios.