The Most Sensitive Variants of G6PD Deficiency and Where They Are Found

Glucose-6-phosphate dehydrogenase deficiency, more commonly known as G6PD deficiency, is one of the most widespread enzyme disorders in the world. People born with this genetic condition have red blood cells that are more vulnerable to breaking apart (hemolysis) when exposed to certain triggers such as fava beans, infections, or medicines like some antimalarials and sulfa drugs.

Not all G6PD deficiencies are the same, however. Scientists have identified more than 200 different genetic variants of the G6PD gene, and some of these variants make the enzyme far weaker than others. This means that while some people may only have mild symptoms during strong exposures, others can experience dangerous, even life-threatening, hemolysis from relatively minor triggers.

The Most Sensitive Variants

The most sensitive variants are those that produce very little working enzyme. The World Health Organization (WHO) classifies these as Class I and Class II variants. Individuals carrying these forms usually have less than 10% of normal enzyme activity, which leaves their red blood cells extremely fragile under oxidative stress.

• Mediterranean Variant (Ser188Phe mutation)
  This is one of the most severe forms of G6PD deficiency. Enzyme activity is often nearly absent, which makes patients extremely sensitive to common triggers. Even something as ordinary as eating fava beans can set off a major hemolytic episode. The Mediterranean variant is most common around the Mediterranean basin — including Italy, Greece, Turkey, and the Middle East — but also occurs in parts of South Asia.
• Canton Variant (Arg459Leu mutation)
  Another highly sensitive form is the Canton variant, found most often in southern China, Hong Kong, and parts of Southeast Asia. This variant produces an unstable enzyme that breaks down quickly, leaving red blood cells unprotected.
• Kaiping Variant (Arg463His mutation)
  Like Canton, the Kaiping variant is mainly found in China and causes severe sensitivity.
• Viangchan Variant (Val291Met mutation)
  Particularly important in Southeast Asia, this variant is widespread in Laos, Cambodia, Vietnam, Thailand, and the Philippines. People with the Viangchan mutation often suffer strong reactions to antimalarial drugs like primaquine, which poses challenges in regions where malaria is still common.

Milder but Widespread Variants

By contrast, some variants are more common but generally less sensitive. The most well-known is the African A− variant, found across sub-Saharan Africa and among people of African descent in the Americas and Caribbean. Classified as Class III, this variant usually allows 10–60% of normal enzyme activity. People with the A− variant often tolerate daily life well but may still experience hemolysis during strong triggers like severe infections or certain medicines.

Why Location Matters

The distribution of G6PD variants is not random. Interestingly, many of the most sensitive variants are common in areas that historically struggled with malaria. Researchers believe that G6PD deficiency may have given carriers some protection against severe malaria, which allowed these genes to persist despite the risks. This is why severe variants like Mediterranean, Canton, Kaiping, and Viangchan are so concentrated in malaria-endemic regions.

The Takeaway

Not all people with G6PD deficiency face the same risks. Some variants, especially Mediterranean, Canton, Kaiping, and Viangchan, make individuals far more sensitive to triggers and are found mainly in Southern Europe, the Middle East, China, and Southeast Asia. Others, like the African A− variant, are milder but still important in public health.

Understanding these differences is vital. It helps doctors choose safe medications, guides newborn screening programs, and informs families about the kinds of exposures to avoid. As research continues, mapping the global distribution of G6PD variants remains essential for protecting vulnerable communities.