Porphyria cutanea tarda (PCT) is the most common porphyria resulting from a partial deficiency of hepatocyte and/or erythrocyte uroporphyrinogen decarboxylase. PCT is classified into 3 subtypes. Type I is the most common subtype. It occurs sporadically after exposure to precipitating factors such as alcohol. Patients exhibit normal enzyme activity in erythrocytes but decreased hepatic activity. Type II PCT is inherited as an autosomal dominant disorder and has moderately decreased enzyme activity in both erythrocytes and hepatocytes. Type III is a rare familial form of PCT in which patients have normal erythrocyte uroporphyrinogen decarboxylase activity in erythrocytes and decreased hepatic activity. Type III PCT is distinguished from type I by the history of other affected family members.
Hepatoerythropoietic porphyria (HEP) is a rare autosomal recessive form of porphyria that typically presents in early childhood. Patients have a severe deficiency of uroporphyrinogen decarboxylase, with activity levels 10% of normal in both hepatocytes and erythrocytes.
PCT and HEP result in accumulation of uroporphyrin in skin, subcutaneous tissues, and liver. Patients ususally present with cutaneous photosensitivity and scarring on sun-exposed surfaces. Patients also have elevated liver enzymes and an increased risk of developing cirrhosis.
Specimen requirement is a green or lavender top tube of whole blood. Testing is performed on saline washed red blood cells. See porphyrins for a list of the preferred tests for each type of porphyria.