Substance and claimed effects

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is the most common human enzyme defect, affecting an estimated 400–500 million people worldwide Cappellini and Fiorelli 2008Nkhoma et al. 2009. It is an X-linked recessive disorder of the G6PD gene on Xq28, with more than 200 known mutations producing functional variants of varying severity Luzzatto et al. 2023. G6PD catalyses the first step of the pentose phosphate (hexose monophosphate) pathway, generating NADPH; in mature red blood cells, which lack mitochondria and protein-synthesis machinery, this is the sole source of NADPH and therefore the sole engine for regenerating reduced glutathione that quenches reactive oxygen species. Without it, red cells cannot survive oxidative challenge. The entry covers the condition itself and the consequences that follow from it: vulnerability to acute haemolytic anaemia after exposure to oxidant drugs, fava beans, and certain infections; neonatal jaundice with kernicterus risk; chronic non-spherocytic haemolytic anaemia in severe variants; and the heterozygote advantage against falciparum malaria that explains the geographic distribution.

Evidence by addressing question

Mechanism

G6PD oxidises glucose-6-phosphate to 6-phosphogluconolactone, reducing NADP⁺ to NADPH. NADPH reduces oxidised glutathione (GSSG) to reduced glutathione (GSH), which in turn detoxifies hydrogen peroxide via glutathione peroxidase and reduces protein and membrane thiols oxidised by reactive oxygen species Luzzatto et al. 2020. Other cell types compensate for G6PD loss through NADPH from the malic enzyme and isocitrate dehydrogenase pathways; the mature erythrocyte cannot, because it lacks the nuclei and mitochondria to express those alternatives. When NADPH is depleted, haemoglobin is oxidised to methaemoglobin; thiol groups on globin chains crosslink and precipitate as Heinz bodies; the membrane skeleton becomes rigid; and the affected cells are cleared by the spleen, often haemolysing intravascularly when the oxidant load is high Cappellini and Fiorelli 2008.

The favism pathway is mechanistically distinct from the drug pathway only in the upstream chemistry. Fava beans contain the β-glucosides vicine and convicine at up to 2% of dry weight; intestinal hydrolysis releases divicine and isouramil, which redox-cycle in the bloodstream and generate superoxide and hydrogen peroxide that oxidise NADPH and GSH faster than the deficient cell can regenerate them Luzzatto et al. 2020. Cooking partially inactivates the glucosidases, which is part of why raw or undercooked beans carry the highest risk. Heinz-body anaemia after primaquine in 1956 was the inaugural pharmacogenetic syndrome and the discovery context for the enzyme itself.

Older red cells have the lowest residual G6PD activity (the enzyme decays through the cell's 120-day lifespan); acute haemolysis preferentially destroys them, leaving a younger population of cells with more residual enzyme. This explains the self-limited course of the A⁻ variant: haemolysis is essentially over within 7–14 days even with the trigger still present, because the surviving cells have enough enzyme to handle the load Frank 2005. The Mediterranean variant has so little residual activity (~4%) that this rebound mechanism fails, producing more severe and prolonged crises Luzzatto et al. 2023.

Evidence

Epidemiology. A systematic review of 280 prevalence estimates from 88 countries reported a global allele frequency of approximately 4.9% with a wide range across regions, peaking above 20% in parts of sub-Saharan Africa, the Arabian Peninsula, southern Italy and Greece, the Thai–Lao border, and the Solomon Islands Nkhoma et al. 2009. A model-based geostatistical map estimated 330–400 million affected individuals in malaria-endemic countries alone Howes et al. 2012. In the United States, prevalence among Black males is approximately 10%; among males of Mediterranean, Middle Eastern, and Southeast Asian ancestry it ranges from 5–25% depending on subpopulation Frank 2005.

Drug-induced haemolysis. The discovery of G6PD deficiency followed observations in 1956 that primaquine produced haemolysis in a subset of African-American soldiers. A 2010 evidence-based review classified medications by quality of evidence and concluded that only a short list — dapsone, methylene blue, nitrofurantoin, phenazopyridine, primaquine, rasburicase, and tolonium chloride — carried unambiguous evidence of haemolytic risk; many drugs historically labelled as contraindicated had weak or absent supporting evidence, often deriving from in vitro assays or single case reports Youngster et al. 2010. Tafenoquine, a long-acting 8-aminoquinoline, is contraindicated below 70% G6PD activity per FDA labelling CDC/Haston et al. 2019. Rasburicase, used for tumour lysis syndrome, carries an FDA boxed warning for haemolysis and methaemoglobinaemia in G6PD-deficient patients FDA 2020.

Neonatal hyperbilirubinaemia and kernicterus. G6PD deficiency is a major risk factor for severe neonatal hyperbilirubinaemia, with risk arising from both increased haemoglobin breakdown and impaired bilirubin conjugation. A Sardinian screening programme that tested ~34,000 newborns found 18.4% G6PD-deficient (24.5% boys, 11.8% girls); zero developed kernicterus over the programme's follow-up, against a baseline rate of preventable cases in the years prior. The 2022 AAP guideline for hyperbilirubinaemia management names G6PD deficiency as one of the principal hyperbilirubinaemia neurotoxicity risk factors and recommends testing in any infant with hyperbilirubinaemia poorly responsive to phototherapy or with consistent family ancestry Kemper et al. 2022Kaplan and Hammerman 2010.

Malaria selection. G6PD deficiency confers heterozygous-female protection against severe Plasmodium falciparum malaria. In a Kenyan case-control and cohort study of >6,000 children, heterozygous G6PD A⁻ females had reduced severe-malaria risk and reduced mortality, while hemizygous males showed no protection against severe malaria and possibly elevated risk of severe malarial anaemia Uyoga et al. 2015. An earlier landmark Nature paper documented natural selection acting on both hemi- and heterozygotes in Nigeria Ruwende et al. 1995; Luzzatto's 2015 commentary integrates the human-genetic evidence with a balanced-polymorphism model and concludes that female heterozygote advantage is the dominant selection mechanism, with hemizygous males paying the cost Luzzatto et al. 2020.

Protocol

The clinical action is single-step: test once, document the result, then apply a lifelong avoidance list. Quantitative spectrophotometric assay of erythrocyte G6PD activity (the reference standard) or fluorescent spot testing (a qualitative screen) makes the diagnosis Frank 2005. Both tests are unreliable during or immediately after an acute haemolytic episode, because the deficient older red cells have been destroyed and the residual younger population has near-normal activity; retesting 2–3 months after the crisis is the conventional workaround Cappellini and Fiorelli 2008. Point-of-care quantitative biosensors (e.g. SD Biosensor STANDARD G6PD; CareStart) now provide enzyme activity at the bedside within minutes and have WHO prequalification for use prior to primaquine and tafenoquine prescribing. Documentation matters as much as the test itself: medic-alert tags, electronic health record problem-list entries, and pharmacist awareness prevent the foreseeable failure mode of an emergency-room provider prescribing a sulfonamide or nitrofurantoin to an unidentified patient.

Contraindications (triggers to avoid)

The high-evidence drug list, drawn from Youngster et al. 2010 and contemporary FDA labelling: primaquine, tafenoquine (8-aminoquinoline antimalarials), dapsone (leprosy, dermatitis herpetiformis, Pneumocystis prophylaxis), rasburicase (tumour lysis prophylaxis — FDA boxed warning), methylene blue / methylthioninium chloride (methaemoglobinaemia, sentinel-node mapping — paradoxically the standard methaemoglobinaemia antidote that becomes contraindicated here), nitrofurantoin (urinary antiseptic), phenazopyridine (urinary analgesic), tolonium chloride / toluidine blue. Sulfonamides as a class are conventionally listed; real-world risk varies considerably across agents, with sulfamethoxazole/trimethoprim and sulfasalazine showing lower haemolytic risk than the historical label implies, while sulfanilamide and sulfacetamide retain clear association Youngster et al. 2010. Other agents commonly listed but with weaker evidence include high-dose aspirin (>1 g/day), high-dose vitamin C (intravenous gram-doses), chloramphenicol, and some quinolones; clinical risk depends on dose, variant severity, and concurrent oxidative load.

Non-drug triggers: raw or undercooked fava beans (mainly Mediterranean variant; Vicia faba exposure includes ingestion and inhalation of bloom pollen in classical case reports); naphthalene (mothballs); henna; infections, particularly viral hepatitis, typhoid, pneumonia, and severe sepsis — infection-triggered haemolysis is in fact the most common cause of crisis in many series Cappellini and Fiorelli 2008. Diabetic ketoacidosis can also precipitate haemolysis through endogenous oxidative stress.

Methaemoglobinaemia in a G6PD-deficient patient is a therapeutic conundrum: methylene blue, the standard antidote, is itself contraindicated. Ascorbic acid (intravenous), high-dose vitamin C combined with cobalamin, exchange transfusion, and hyperbaric oxygen are second-line options; onset is slower than methylene blue.

Misconceptions

Three common errors. First, G6PD deficiency is not a sulfa allergy — the mechanism is direct pharmacological oxidation of erythrocyte glutathione, not IgE-mediated hypersensitivity, and the clinical pattern (delayed haemolysis 24–72 hours after exposure, dark urine, jaundice) differs from urticaria/anaphylaxis Frank 2005. Second, the condition is not strictly "male-only." Female heterozygotes have a mosaic red-cell population due to X-inactivation; depending on the lyonisation pattern, they can have anywhere from 5% to 95% deficient red cells and can experience haemolytic crises indistinguishable from hemizygous males Luzzatto et al. 2020. The fluorescent spot test, which measures averaged enzyme activity, frequently misclassifies these women as normal. Third, fava beans are not always dangerous to anyone with G6PD deficiency: many people with the African A⁻ variant eat them without incident, while the Mediterranean variant carries a much higher per-exposure risk. The conservative clinical position is total avoidance for any confirmed deficient individual, but the population-level reality is variable.

Audience

Highest carrier and hemizygote frequencies in: sub-Saharan African ancestry (African A⁻; ~10–20% male prevalence in many populations), Mediterranean ancestry (G6PD Mediterranean / B⁻; especially Sardinia, Greece, southern Italy, Cyprus), Middle Eastern Arab ancestry (~2–31% across countries with consanguinity-driven enrichment), and Southeast Asian populations (G6PD Mahidol, Viangchan, Kaiping; especially Thailand, Laos, Cambodia, southern China, the Philippines) Howes et al. 2012Nkhoma et al. 2009. Ashkenazi Jewish populations carry the Mediterranean variant at appreciable frequency. Affected ancestry alone is not a clinical diagnosis: New York State's universal newborn screening implementation found that 13% of identified deficient infants had parents identifying as white or Ashkenazi Jewish and 22% as Puerto Rican or Dominican — risk-factor-based screening would have missed roughly 44% of cases.

Failure modes

Most preventable harm in G6PD deficiency comes from documentation and recognition failures rather than from any inherent unmanageability of the condition. (1) The qualitative fluorescent spot test misclassifies female heterozygotes whose lyonisation pattern produces an averaged red-cell activity above the deficiency cutoff but whose deficient subpopulation can still haemolyse; quantitative testing is preferable when female status matters clinically Luzzatto et al. 2023. (2) Testing during acute haemolysis returns a false-normal result because the deficient cells have already been destroyed; the diagnosis is missed exactly when it is most clinically important, and patients are sent home thinking they are normal until the next crisis. (3) Emergency-department and urgent-care prescribing of nitrofurantoin (for cystitis), sulfamethoxazole-trimethoprim (for cellulitis or sinusitis), or dapsone (for various dermatologic indications) without history-taking is a routine trigger when status is undocumented or unknown to the patient. (4) Neonatal discharge before peak bilirubin (typically days 3–5) can miss kernicterus risk, particularly in infants with no obvious haemolysis but baseline impaired bilirubin clearance from deficient hepatic G6PD Kemper et al. 2022.

Stakes

Most G6PD-deficient people pass through life with no episodes, because they happen not to encounter a trigger and not to be prescribed a contraindicated drug. The minority who do encounter a trigger experience acute intravascular haemolysis 24–72 hours after exposure: jaundice, dark cola-coloured urine (haemoglobinuria), tachycardia, abdominal or back pain, and progressive weakness over days; severe crises require hospitalisation, transfusion, and occasionally cause acute kidney injury from haemoglobin nephropathy Frank 2005. Neonatal stakes are higher per event: untreated severe hyperbilirubinaemia produces kernicterus with mortality ~10% and long-term morbidity (deafness, cerebral palsy, intellectual impairment) in ~70% of survivors Kemper et al. 2022. The Mediterranean and severe Asian variants account for the bulk of fatal favism cases, historically a recognised public health problem in Sardinia and southern Italy before screening programmes.

Payoff

Documented status confers near-complete avoidance of foreseeable crises. The Sardinian screening programme essentially eradicated kernicterus from G6PD deficiency over two decades; Singapore similarly reports zero G6PD-attributable newborn deaths since universal screening implementation. For adults, the payoff is a stable problem-list entry that prevents the foreseeable prescribing mistake — a small ongoing administrative cost (telling new clinicians, pharmacists, dentists) in exchange for a low-probability, high-severity averted event.

The credibility range

Optimist case (the maximalist position on G6PD relevance). G6PD deficiency is one of the most prevalent inherited conditions on Earth, and the affected population is concentrated in precisely the regions where antimalarials are routinely prescribed. Universal newborn screening is cheap, fast, and demonstrably eliminates kernicterus where implemented; the failure of high-income countries to adopt it is regulatory inertia rather than scientific debate. The drug list is firmly established for the high-evidence agents, and every emergency department should treat G6PD status as a routine vital piece of history. The condition is benign only conditionally — it becomes lethal in the presence of a trigger, and our pharmacopoeia continues to add candidate triggers (rasburicase, tafenoquine, immunotherapy combinations).

Skeptic case (the deflationary position). Most G6PD-deficient people are asymptomatic their entire lives. Many drugs historically labelled as contraindicated have weak supporting evidence — single case reports, in vitro assays, mechanistic plausibility rather than clinical events Youngster et al. 2010. The drug-induced haemolysis literature predates modern pharmacovigilance and conflates real triggers with co-administered drugs. Universal newborn screening produces high false-positive rates, generates familial anxiety, and most identified infants would never have had a crisis. Sulfa drugs in real-world data carry less haemolytic risk than their label implies, and methylene blue's contraindication is routinely overridden in emergencies without obvious harm. The condition is genuinely benign for most people; the clinical-attention budget is better spent elsewhere.

Author's call. The deflationary view captures something real — overcaution on weak-evidence drugs and on universal screening in low-prevalence populations is overcaution. The high-evidence drug list (the seven absolute contraindications) is the load-bearing piece and should be treated with bright-line discipline. Beyond that list, prescribing decisions should weigh dose, variant severity, and clinical alternative. Documentation of status remains the highest-leverage action: it costs essentially nothing once known, and it converts a high-severity unpredictable event into a managed problem. Newborn screening in high-prevalence populations passes any sensible cost-benefit threshold; in low-prevalence populations the case is debated but trending toward universal as point-of-care quantitative testing becomes cheaper. Evidence quality is high overall; controversy is low and concentrated at the margins of the drug list and screening policy.

Stakeholder and incentive map

• Antimalarial programmes — the dominant institutional stakeholder. WHO, national malaria control programmes, MMV (Medicines for Malaria Venture), and PATH have driven point-of-care G6PD testing adoption to enable safe primaquine and tafenoquine use for P. vivax radical cure. Incentive: end malaria transmission; cannot use the only relapse-prevention drugs without knowing G6PD status.
• Newborn screening programmes — Greece, Israel, Singapore, Sardinia, Philippines, and increasingly U.S. states have G6PD on the panel; the AAP and AAFP advocate selective testing in high-risk infants. Incentive: prevent kernicterus.
• Pharmacovigilance and drug labelling — FDA, EMA, NICE maintain contraindication labels; the rasburicase boxed warning is a notable post-marketing addition FDA 2020. Incentive: liability and regulatory safety.
• Patient advocacy — small but established (G6PD Deficiency Foundation, regional favism societies). Influence is modest given the condition's low public profile in the U.S.
• Counter-pressure — there is no significant commercial counter-incentive. Drug manufacturers occasionally push back on listing of marginal agents but generally accept G6PD labelling. The deflationary pressure is mostly academic — calls for restricting the drug list to high-evidence agents to avoid undertreatment of common infections.

Population variability

• Variant severity dominates clinical presentation. WHO Class A (chronic non-spherocytic haemolytic anaemia, no triggers required) is rare and severe; Class B (the common polymorphic variants, including African A⁻ and Mediterranean) is overwhelmingly the clinical reality and produces episodic trigger-dependent haemolysis Luzzatto et al. 2023. Within Class B, the Mediterranean variant has ~4% residual activity and is associated with favism; African A⁻ has ~12–20% residual activity and a milder, more self-limited pattern.
• Sex. Hemizygous males express the variant uniformly. Heterozygous females have mosaic red-cell populations from X-inactivation, producing a continuous distribution of effective enzyme activity. The conventional dichotomy "carrier vs. affected" understates female risk; women with skewed X-inactivation can be clinically affected and can be missed by qualitative screening Luzzatto et al. 2023.
• Age. Neonates are uniquely vulnerable to hyperbilirubinaemia regardless of haemolysis, because hepatic G6PD is also reduced and bilirubin conjugation is impaired Kemper et al. 2022. Older adults can present with first crisis in late life, often triggered by a newly prescribed urinary antibiotic.
• Concurrent oxidative load. Infection, diabetic ketoacidosis, and combinations of mild oxidant exposures can summate beyond what any single trigger would produce. The same drug at the same dose can be tolerated once and trigger haemolysis another time depending on background.

Knowledge gaps

• Many drugs on conventional avoidance lists have weak underlying evidence; head-to-head pharmacovigilance data are sparse, and the field has not systematically re-evaluated the historical list with modern methods Youngster et al. 2010.
• The real-world incidence of clinically significant haemolysis from sulfamethoxazole-trimethoprim and similar agents is uncertain; reported case rates are far below what the contraindication label suggests.
• Whether universal newborn screening in low-prevalence populations passes a cost-benefit threshold is contested; the relevant trial would be expensive and is unlikely to be run.
• The Mediterranean variant's selective advantage against P. vivax is suggestive but not as firmly established as the African variant's advantage against P. falciparum.
• Mechanisms of variation in trigger sensitivity — why the same person can eat fava beans without incident in one exposure and crash in another — remain unexplained.

Scope coverage relative to the brief. The input description named four threads — red-cell vulnerability to oxidative triggers (drugs and foods), hemolysis risk, and prescribing precautions. All four are covered end to end (mechanism, evidence, contraindications, failure-modes). No narrowing.

Category placement. Filed under medical rather than screening. The blood test is single-event and the lifelong work is medication-aware living — a clinical-condition pattern more than a recurring screening regimen. Re-filing to screening would be defensible; flagging in case a reviewer prefers it.

Drug list framing. Led with the high-evidence short list (seven agents per Youngster et al. 2010) rather than the traditional long list. This is mildly contrarian — many clinical references still publish 30+ drug lists. The risk of the conservative long list is that it leads to under-treatment of common infections; the risk of the short list is that an idiosyncratic case from a weak-evidence agent gets missed. Chose to anchor on the evidence-based review and explicitly note that sulfa drugs and weaker-evidence agents need pharmacist input. Reviewer call: would also accept a longer warning list if the editorial preference is maximally conservative.

Audience scoping decisions. Did not narrow audience.gender to male despite X-linked inheritance. Female heterozygotes can be clinically affected through skewed X-inactivation; the fluorescent spot test misses many of them. Narrowing to male would mis-signal that women in high-prevalence families are safe. The audience addressing section names the four high-prevalence ancestries explicitly within the body.

Rating difficulties. Most benefit dimensions are 0 because the condition itself confers no benefit. The non-zero scores (health_short_term 3, longevity 2) reflect the avoided crisis when status is known and acted on, not a positive effect of having the gene. Considered scoring 0 across the board with a high evidence rating, but that would understate the practical health stakes; landed on the avoided-harm framing.

Future links to wire in once they exist.

• Sickle cell trait — geographic and ancestral overlap, same one-time-knowledge action pattern
• Pharmacogenetic testing more broadly (CYP2D6, TPMT, DPYD) — same actionable-genetics frame
• Lp(a) testing — different topic, same one-test-once-life pattern that this entry exemplifies
• Newborn screening — if the catalogue grows a parent-facing entry on what the heel-prick panel covers

Separate-entry candidates surfaced during the write.

• Methylene blue and methaemoglobinaemia management — covered briefly here, but the broader topic (toxic exposures, blue-baby syndrome, anaesthetic complications) could warrant its own entry under medical
• Fava beans / favism — interesting as a folkloric-to-genetic story; could be its own short entry under food, cross-linked from here
• Point-of-care diagnostics — the G6PD biosensor story is one example of a broader shift; could anchor a productivity- or medical-category entry

Action and cadence. know + once. Considered test + once since the test is the gating step, but the substantive lifelong work is the knowing and the documentation, not the testing itself. know better captures that the reader's job is awareness rather than recurring measurement.