A 35-year-old man with known porphyria cutanea tarda (PCT) presents with blistering lesions on sun-exposed areas and hepatomegaly. Laboratory work shows elevated liver enzymes and low serum ceruloplasmin. Which investigation is most specific for confirming the diagnosis of PCT and determining its type?

Explanation

## Diagnostic Confirmation of Porphyria Cutanea Tarda **Key Point:** PCT is the most common porphyria and is characterized by cutaneous photosensitivity with blistering and fragility on sun-exposed areas. The diagnostic hallmark is a specific urine porphyrin pattern. ### Why Urine Porphyrin Profile? **High-Yield:** PCT is a cutaneous porphyria caused by deficiency of uroporphyrinogen decarboxylase (UROD) in the liver. The characteristic finding is: - **Massive elevation of uroporphyrin (>80% of total urinary porphyrins)** - Elevated coproporphyrin (type III) - This pattern is pathognomonic for PCT and distinguishes it from other porphyrias **Clinical Pearl:** Urine in PCT is often **dark red or port-wine colored** due to high uroporphyrin content — visible on inspection without need for light exposure (unlike PBG in acute porphyrias). ### Classification of PCT | Type | Mechanism | UROD Activity | Inheritance | Frequency | | --- | --- | --- | --- | --- | | **Type I (most common)** | Acquired; liver UROD deficiency | Low in liver only | Sporadic | ~80% | | **Type II** | Familial; germline UROD mutation | Low in all tissues | Autosomal dominant | ~15% | | **Type III** | Sporadic; undefined defect | Normal | Sporadic | ~5% | **Mnemonic:** **PCT = Porphyria Cutanea Tarda = Uroporphyrin in Urine** (UPO in U). ### Associated Conditions **Clinical Pearl:** PCT is strongly associated with: - Hepatitis C infection (40–80% of cases) - HIV infection - Alcohol use disorder - Iron overload (hemochromatosis) - Estrogen use (oral contraceptives, HRT) These are risk factors but not diagnostic tests; urine porphyrin profile is the gold standard. ### Why Other Options Are Not First-Line | Investigation | Role in PCT | | --- | --- | | **Serum iron/ferritin** | Identifies iron overload (a risk factor), not diagnostic | | **Hepatitis C testing** | Identifies associated condition, not diagnostic | | **UROD activity** | Useful for type classification (I vs II) after diagnosis confirmed; requires liver biopsy or leukocyte testing | | **Fecal porphyrins** | Less sensitive than urine; not preferred |