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Diagnosis of porphyria cutanea tarda (PCT) relies on the identification of characteristic symptoms, a comprehensive patient history, and a thorough clinical evaluation involving specialized tests.
Generally, the tests include:
Whilst blisters, crusted-over lesions, hyperpigmentation, and skin fragility on sun-exposed areas are all characteristic findings associated with PCT, these are not necessarily confirmation of the disorder. Thus, a skin biopsy will show features of sub-epidermal blister formation, but is not diagnostic for PCT.
Elevation in both urine and plasma porphyrin is expected in PCT patients. The specific pattern of porphyrin levels allows PCT to be distinguished from other porphyrias such as variegate porphyria and erythropoietic protoporphyria.
For example, a 24 hour urine porphyrin profile of a PCT patient would show that most of the excreted porphyrins are uroporphyrin and a 7-carboxyl porphyrin. With this aid, testing plasma porphyrin levels is considered more accurate because urine porphyrins are more subject to fluctuations induced by other conditions.
Additionally, urinary examination with a Wood's lamp can be used to show a coral pink fluorescence due to the excessive porphyrin concentration. This occurs because the porphyrins absorb black light and re-emit it at a longer wavelength, i.e. visible pink light.
A direct fluorometric assay of plasma is also a useful test. In PCT, the plasma porphyrins are elevated with maximal excitation and emission wavelengths at ~400 and ~620 nm, respectively. Following this fluorometric pattern, tests for levels of porphyrins and porphyrin precursors in urine can be used to confirm the diagnosis.
Serum ferritin analysis can be useful in gauging whether therapeutic phlebotomy is appropriate due to the known association between iron overload and PCT. The degree of iron accumulation can be both assessed by serum ferritin, and by liver biopsy just as in other conditions associated with iron accumulation including hemochromatosis and hepatitis C.
All patients with PCT should be tested for hepatitis C and HIV infections as these are factors known to precipitate the development of PCT. In fact, HCV appears to be such a strong trigger that in HCV-infected individuals PCT is developed at an earlier age than in uninfected persons with PCT.
Whilst not definitive, there are also reports that show involvement of HIV infection in PCT patients through viral activity that impairs the hepatic cytochrome oxidase system, thus hampering porphyrin metabolism. Testing for HIV might therefore be a wise decision.
Familial PCT can be diagnosed by a reduced amount of the UROD enzyme in erythrocytes. Molecular genetic testing for familial PCT is available and recommended if the diagnosis has been confirmed in the patient or a family member by porphyrin analysis and/or enzyme assay of UROD activity.
Mutations in the hereditary hemochromatosis gene (HFE) should be tested for because these may partly explain excess iron accumulation known to trigger PCT. For example, Cys282Tyr in the hemochromatosis gene is as a susceptibility factor for both acquired and familial PCT and those homozygous for this mutation with hemochromatosis have up to 60-fold greater risk of developing PCT53 and earlier onset.