PCT

Diagnosis

The best screening test when PCT is suspected may be a plasma total porphyrin measurement. A normal result excludes active PCT. This test will also detect any other type of porphyria that is causing skin problems. Further testing is then needed to establish the type of porphyria. Urine porphyrins are commonly measured for screening for PCT and other porphyrias. But it must be remembered that increases in urinary porphyrins can occur in medical conditions other than porphyria, especially conditions that affect the liver and bone marrow.

The excess porphyrins in plasma and urine in PCT are mostly uroporphyrin (octacarboxylporphyrin) and heptacarboxylporphyrin. These porphyrins have 8 and 7 carboxyl groups, respectively, on the porphyrin molecule. Coproporphyrin (tetracarboxylporphyrin - with 4 carboxyl groups) is also increased in urine, but this is not a finding that is specific for PCT or even for porphyria. Fecal total porphyrins are usually only slightly increased, but a predominance of isocoproporphyrins, which are measured only in some specialty laboratories, is very specific for PCT.

UROD can be measured in red blood cells to establish whether or not a patient has inherited a deficiency of the enzyme. This is not essential in all cases, because the treatment of patients with familial PCT is not fundamentally different from those who do not have an inherited enzyme deficiency.

Some patients with "pseudoporphyria" have skin lesions that are essentially identical to those in PCT. These patients are most readily distinguished from PCT by finding normal levels of porphyrins in plasma. A photosensitizing drug is found to cause the sensitivity to light in some of these individuals.