Laboratory Studies
Porphyrin analyses
Urinary porphyrin concentrations are increased 100-1000 times and involve predominantly uroporphyrin I. Urinary excretion of uroporphyrin III and coproporphyrin III is also elevated; however, the level is less than that of the isomer I porphyrins.
Urinary delta-aminolevulinic acid and porphobilinogen levels are not increased in erythropoietic porphyria.
Erythrocytes most often contain increased levels of uroporphyrin I; also, elevated zinc protoporphyrin is observed in some patients.
The combination of elevated urinary and erythrocyte isomer I porphyrin levels is specific for erythropoietic porphyria.
Coproporphyrin preferentially accumulates as fecal porphyrin after the decarboxylation of uroporphyrin.
Complete blood cell count
Excessive uroporphyrins in red blood cells appear to cause fragility; therefore, a hemolytic anemia is common. Consequent splenomegaly and hepatomegaly are observed.
A test to measure uroporphyrinogen III synthase activity is commercially available.
Mutation analysis of the uroporphyrinogen III synthase gene (ie, DNA testing) is performed at porphyria research units in several countries and has become commercially available in the United States. See the American Porphyria Foundation for further information.
Other Tests
Fluorescence microscopy of peripheral blood or bone marrow specimens. Red porphyrin fluorescence in intact erythrocytes and erythroid precursor cells can be observed in smears of bone marrow specimens illuminated by violet or blue light against a dark-field background. The brilliant fluorescence of nuclei in erythrocyte precursor cells is specific for erythropoietic porphyria.
Histologic Findings
Similar dermatopathologic changes can be found in all types of porphyria with photocutaneous manifestations. The characteristic feature is a subepidermal blister with a slight superficial perivascular lymphocytic infiltrate. Blood vessels in the superficial vascular plexus have markedly thickened, hyalinized walls that contain periodic acid-Schiff (PAS)–positive, diastase-resistant glycoproteins. Papillary dermal tips often festoon into the blister cavity due to the increased rigidity of the hyalinized vessel walls.
Caterpillar bodies, which are eosinophilic linear structures in the roofs of bullae composed of basement membrane material, are described in blisters of patients with several forms of porphyria. Direct immunofluorescence tests reveal linear C3 and immunoglobulin G staining around the superficial vessels and along the dermoepidermal junction. [7]
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Clinical classification of porphyrias.
