Classic erythropoietic protoporphyria (EPP) is usually caused by reduced activity of ferrochelatase, an enzyme needed to make heme. The resulting buildup of protoporphyrin can cause painful reactions to light. Diagnosis relies chiefly on a blood test that measures erythrocyte protoporphyrin and distinguishes its free form from the zinc-bound form; genetic testing can clarify the cause.
What causes classic EPP?
Heme is an essential component of hemoglobin and other proteins. In the final step of heme production, ferrochelatase inserts iron into protoporphyrin. In classic EPP, reduced ferrochelatase activity impairs that step, so protoporphyrin accumulates, especially in red-blood-cell precursor cells and the liver.
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The enzyme is encoded by FECH. The usual inheritance mechanism is more complex than a single disease-causing variant: an affected person commonly has a pathogenic FECH variant together with a common low-expression allele, which further reduces enzyme activity. More rarely, pathogenic changes affecting both copies of FECH are involved. A finding of one FECH variant alone therefore needs interpretation in context.
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EPP often becomes noticeable in childhood. Light-exposed skin may start to burn, sting, itch, tingle, swell, or hurt severely soon after exposure. Visible-light exposure can trigger the reaction. Unlike many conditions people associate with sun sensitivity, EPP may cause intense pain with little blistering or scarring. The absence of a dramatic rash does not rule it out.
What blood test is used to diagnose EPP?
When EPP or a related protoporphyria is suspected, clinicians generally order whole-blood or erythrocyte protoporphyrin testing. The key is not simply whether total protoporphyrin is elevated: the laboratory method should distinguish metal-free (free) protoporphyrin from zinc-complexed protoporphyrin.
Classic EPP characteristically produces a marked increase in free erythrocyte protoporphyrin relative to the zinc-bound fraction. GeneReviews describes markedly increased free erythrocyte protoporphyrin as the most sensitive and specific biochemical diagnostic test for EPP. Assays that report only a combined or total value can obscure this distinction, so the test method and fractionation should be considered when interpreting a result.
A 2025 review states that metal-free erythrocyte protoporphyrin at least three times the upper limit of normal confirms a protoporphyria diagnosis. That is the review’s criterion; it is not a universal stand-alone cutoff for every laboratory or a replacement for fractionation and clinical interpretation.
How do clinicians distinguish EPP from similar test results?
Elevated erythrocyte protoporphyrin is not unique to classic EPP. Iron-deficiency anemia, lead exposure, anemia of chronic disease, and hemolytic disorders can also raise protoporphyrin, often with zinc-complexed protoporphyrin predominating. A clinician interprets the fractions alongside symptoms and other relevant findings rather than treating any elevation as proof of EPP.
Classic EPP and X-linked protoporphyria
X-linked protoporphyria is associated with gain-of-function variants in ALAS2, rather than reduced function of FECH. It tends to raise both free and zinc-complexed erythrocyte protoporphyrin, unlike the predominantly free pattern characteristic of classic EPP. Molecular testing can help distinguish these conditions.
Other rare causes
CLPX-related protoporphyria is described as a separate, rare erythropoietic protoporphyria. It should not be conflated with classic, usually FECH-related EPP.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What can genetic testing establish?
Genetic testing can identify the molecular cause and help distinguish classic EPP from X-linked or other related conditions. Testing may start with FECH sequencing; if the biochemical and clinical picture remains compelling but sequencing does not explain it, deletion/duplication analysis may be considered. A multigene panel or broader genomic testing may be appropriate when initial testing leaves the cause unresolved.
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GeneReviews describes EPP as established by markedly increased free erythrocyte protoporphyrin and/or identification of biallelic pathogenic FECH variants on molecular testing. The result should be interpreted with the inheritance mechanism and biochemical findings in mind; genetic testing does not make fractionation irrelevant.
What should someone do with a suspected result?
Symptoms or an abnormal protoporphyrin result warrant discussion with a clinician familiar with porphyria. Ask whether the assay separates free from zinc-bound protoporphyrin, and whether other causes of an elevated result have been considered. This article explains the diagnostic approach but cannot determine an individual diagnosis.
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