Gene test interpretation: G6PD

INTRODUCTION — This monograph discusses the implications of genetic test results for the glucose-6-phospate dehydrogenase (G6PD) gene, which may be obtained during the genetic evaluation of various red blood cell (RBC) or nonhematologic disorders. It is not intended to replace clinical judgment in the decision to test or the care of the tested individual.

Most individuals with G6PD deficiency come to medical attention following an episode of hemolysis and biochemical testing that shows reduced enzyme activity. The indications for testing and the choice of test are discussed separately [1]. (See 'Resources' below.)

BASIC PRINCIPLES

How to read the report — Confirm that the result applies to the tested individual and determine whether testing was performed in a Clinical Laboratory Improvement Amendments (CLIA)-certified or other nationally certified laboratory. These and other caveats are summarized in the checklist (table 1).

●Variants in the G6PD gene that cause G6PD deficiency (reduced G6PD enzyme activity) are referred to as pathogenic or likely pathogenic variants (depending on certainty) or disease-causing variants (table 2) [2]. Affected individuals are predisposed to develop hemolytic anemia, which may be severe, upon exposure to a number of medications and other substances. (See 'G6PD variants' below.)

●Variants for which the likelihood of hemolysis is unclear may be termed variants of uncertain significance (VUS). A VUS can be reclassified when new information about its effect on G6PD activity becomes available. A variant initially classified as a VUS should be reevaluated periodically.

●Variants that are clearly not associated with hemolysis may not be reported (or may be reported as negative, benign, or likely benign).

Some genetic tests (sequencing, deletion/duplication analysis) evaluate the entire gene sequence; others only test for selected variants. If the individual has a variant not detected by the assay, they may receive a false-negative result. Thus, negative testing does not eliminate the possibility of G6PD deficiency in an individual with an appropriate clinical presentation, reduced G6PD enzyme activity, or a positive family history.

If there are questions or concerns about a genetic test report or variant classification, a hematologist or genetics expert should be consulted. (See 'Resources' below.)

G6PD variants — Over 200 pathogenic variants in G6PD have been reported.

Variants may be characterized according to the region of the world in which they arose and/or their enzyme classification. (See "Diagnosis and management of glucose-6-phosphate dehydrogenase (G6PD) deficiency", section on 'Variant classification'.)

Common variants include:

●Africa (G6PD A^-) – c.202G>A and c.376A>G

●Mediterranean region, Middle East, India (G6PD Mediterranean) – c.63C>T

●China

•G6PD Canton – c.1376G>T

•G6PD Kaiping – c.1388G>A

•G6PD Gaohe – c.95A>G

●Southeast Asia (G6PD Mahidol) – c.487G>A

The World Health Organization (WHO) has classified the different G6PD variants according to the magnitude of the enzyme deficiency and the severity of hemolysis:

●Class I – <10 percent activity; chronic hemolysis. Caused by variants that interfere with dimerization.

●Class II – <10 percent activity; intermittent marked hemolysis.

●Class III – 10 to 60 percent activity; intermittent mild to moderate hemolysis.

●Class IV – >60 percent activity; no hemolysis.

Chronic hemolysis occurs without a precipitating factor; intermittent hemolysis occurs only upon exposure to oxidant stress. (See 'Hemolysis and hemolytic anemia' below.)

CLINICAL IMPLICATIONS

Inheritance pattern — The G6PD gene is located on the X chromosome; inheritance is X-linked. Disease-causing variants can be transmitted from mothers to sons or daughters. Fathers can only transmit a disease-causing variant to daughters; father to son transmission does not occur. (See 'Counseling and testing of first-degree relatives' below.)

●Males have one X chromosome (they are hemizygous); a pathogenic or likely pathogenic variant in G6PD, if present, will affect all of their red blood cells (RBCs).

●Females have two X chromosomes. Typically, a pathogenic or likely pathogenic variant in G6PD will be present at only one allele; due to X-inactivation, approximately one-half of their RBCs will be affected. Severe hemolysis of these RBCs can occur. Skewed X-inactivation can alter these percentages, making females appear to be homozygous or to have normal enzyme activity.

In regions with high baseline prevalence of G6PD deficiency (figure 1), some females will inherit disease-causing variants from both parents, and all of their RBCs will be G6PD-deficient.

Prevalence — G6PD deficiency is a common genetic disorder. Its prevalence is greatest in individuals with ancestry from regions where malaria is or was endemic (figure 1) [3,4]. However, the presence or absence of G6PD deficiency cannot be inferred from race or ethnicity. (See 'G6PD variants' above.)

Clinically apparent disease may be more commonly appreciated in males than females because females typically have normal G6PD function in a subset of their RBCs, and some females may have clinically milder disease or less severe deficiency on biochemical testing. (See 'Inheritance pattern' above and 'Biochemical confirmation' below.)

Hemolysis and hemolytic anemia — The majority of affected individuals have no health consequences unless they are exposed to an oxidant stress. Insufficient G6PD enzyme activity coupled with high oxidative stress can lead to hemolysis; this is the typical presentation [5,6].

Common precipitants include (table 3) [5-7]:

●Medications – Includes sulfa-containing drugs (eg, dapsone), 8-aminoquinoline antimalarial drugs (primaquine, tafenoquine), and the anti-uricemic drug rasburicase.

●Fava beans (also called broad beans) – Contain a chemical that can cause hemolysis after being eaten (fresh beans or fava bean-containing foods). Children are more frequently affected than adults. Other types of beans (eg, chickpeas) are safe to eat.

●Chemicals – Examples include naphthalene (mothballs), isobutyl or amyl nitrate, and henna.

●Infections – Hemolysis can be further exacerbated if an oxidant antibiotic is used.

Hemolysis may be symptomatic with fatigue, tachycardia, dark urine, and jaundice, or there may be an asymptomatic drop in the hemoglobin or an increased reticulocyte count. Children with favism (hemolysis after eating fava beans) may be quite ill with pallor, jaundice, abdominal pain, and fever [7].

Laboratory evidence of hemolysis includes anemia, reticulocytosis, elevated lactate dehydrogenase (LDH) and bilirubin, and low or absent haptoglobin (table 4). The blood smear may show classic bite cells or blister cells.

Neonatal jaundice — Females and males can be affected. The peak bilirubin is usually on day 3, when many newborns have already been discharged home. The mechanism is not fully understood, but usually significant hemolysis is not present [5].

BIOCHEMICAL CONFIRMATION — Biochemical testing is usually the first test used to identify G6PD deficiency. In individuals who present with initial genetic test results, it is useful to verify G6PD deficiency with a quantitative enzyme assay, as depicted in the algorithm (algorithm 1).

Results of biochemical testing are usually available in one to two days. Rapid-turnaround point-of-care tests are under investigation. (See "Diagnosis and management of glucose-6-phosphate dehydrogenase (G6PD) deficiency", section on 'Point of care tests under investigation'.)

Appropriate settings for confirmatory testing include:

●Clinical findings such as unexplained neonatal jaundice, favism (hemolysis after eating fava beans), or acute onset of hemolysis with a negative direct antiglobulin (Coombs) test.

●Positive screening assay (qualitative biochemical test).

●First-degree relative with G6PD deficiency.

●Positive genetic testing (pathogenic variant, likely pathogenic variant, or variant of uncertain significance [VUS] in the G6PD gene).

Biochemical testing is most important in individuals who have had a hemolytic episode and/or who anticipate the need to alter their medical care (eg, use a different antibiotic) or diet.

Some individuals may omit biochemical testing if it would confer an undue burden and/or if they are unlikely to require a change in medical care or diet based on the result. Individuals who omit biochemical testing initially may need to undergo testing at a later date if they require a known oxidant drug. In the absence of a confirmatory test, avoidance of substances known to trigger hemolysis seems prudent.

Discordance between genetic and biochemical results is not expected when genetic testing is performed in a Clinical Laboratory Improvement Amendments (CLIA)-certified laboratory, but biochemical testing may be falsely negative in some settings (algorithm 1):

●During an acute hemolytic episode, if most of the G6PD-deficient cells have hemolyzed, only red blood cells (RBCs) and reticulocytes with higher G6PD activity may remain. This is most likely in women (who are usually heterozygous) and individuals of African ancestry (who usually have A^- variants). (See 'Inheritance pattern' above and 'G6PD variants' above.)

In such cases, the biochemical test should be repeated approximately three months after recovery from hemolysis.

●Some females with skewed X-inactivation may also have falsely normal or borderline biochemical results.

PREVENTION AND TREATMENT OF HEMOLYSIS

Avoidance of implicated medications, foods, and other substances — Individuals with G6PD deficiency should avoid exposure to medications, foods, and chemicals that could precipitate hemolysis (table 5), especially when a reasonable alternative is available.

Rare exceptions may include use of a potentially life-saving medication, especially in an individual with a mild variant that is likely to produce transient hemolysis. (See "Diagnosis and management of glucose-6-phosphate dehydrogenase (G6PD) deficiency", section on 'Avoidance of unsafe drugs and chemicals' and "Diagnosis and management of glucose-6-phosphate dehydrogenase (G6PD) deficiency", section on 'Dietary restrictions'.)

Some individuals may have had one of these exposures without experiencing hemolysis. The need to avoid these substances in the future is individualized with shared decision-making.

Treatment of symptomatic patients — Management is individualized (table 5):

●For acute hemolytic episodes, red blood cell (RBC) transfusions are frequently required.

●For jaundiced neonates, phototherapy may be needed. Most cases are not associated with significant hemolysis.

Details are presented separately. (See 'Resources' below.)

COUNSELING AND TESTING OF FIRST-DEGREE RELATIVES — The following first-degree relatives of an individual with confirmed G6PD deficiency are at risk for carrying the familial disease-causing variant:

●Mothers of an affected child (male or female)

●Fathers of an affected daughter

●Daughters of an affected parent (male or female)

●Sons of an affected mother

●Male or female siblings of an affected individual

These individuals should be informed about the familial variant (table 5) and can discuss with their clinicians the value of testing and the best test.

●Biochemical testing is most likely to be most useful.

●Genetic testing may be reasonable if a familial variant has already been identified, or for carrier detection in a female with a family history of G6PD deficiency and a normal biochemical test result.

Testing may reasonably be deferred in otherwise healthy children until adolescence or adulthood.

RESOURCES — Information and resources are available on websites of the G6PD Deficiency Association (Associazione Italiana Favismo) in Italy (G6PD.org) and the Genetic and Rare Diseases Information Center (GARD; rarediseasesinfo.nih.gov) in the United States.

UpToDate topics include:

●G6PD deficiency

•Pathogenesis – (See "Genetics and pathophysiology of glucose-6-phosphate dehydrogenase (G6PD) deficiency".)

•Evaluation and management – (See "Diagnosis and management of glucose-6-phosphate dehydrogenase (G6PD) deficiency".)

●Hemolytic anemias

•Neonates, evaluation – (See "Unconjugated hyperbilirubinemia in neonates: Etiology and pathogenesis".)

•Neonates, management – (See "Unconjugated hyperbilirubinemia in term and late preterm newborns: Initial management".)

•Children – (See "Overview of hemolytic anemias in children".)

•Adults, evaluation – (See "Diagnosis of hemolytic anemia in adults".)

•Adults, management – (See "Drug-induced hemolytic anemia", section on 'Management'.)