Porphyrin Screen

The porphyrias are inherited or acquired disorders of haem biosynthesis in which a deficient enzyme causes accumulation of porphyrins and their precursors, delta aminolaevulinic acid (ALA) and porphobilinogen (PBG).

Acute porphyrias (acute intermittent porphyria, variegate porphyria, hereditary coproporphyria and the rare ALA dehydratase deficiency) cause neurovisceral attacks. Cutaneous porphyrias (porphyria cutanea tarda, erythropoietic protoporphyria, congenital erythropoietic porphyria) cause photosensitivity. Some forms (variegate porphyria and hereditary coproporphyria) cause both.

The screen is a set of tests on urine, faeces, plasma and red cells. Urine PBG, measured on a fresh random sample, is the first and most urgent test in suspected acute porphyria. A plasma porphyrin fluorescence scan, urine total porphyrins with fractionation, faecal porphyrins, and red cell protoporphyrin are added depending on the presentation.

Acute porphyria is easy to miss and potentially fatal if untreated. The symptomatic prevalence is about 1 in 100 000, with a clinical penetrance of under 10% in gene carriers, and attacks occur mostly in women of reproductive age. Porphyria cutanea tarda is the commonest porphyria, with a prevalence of about 1 in 10 000.

Indications.

• Acute attack: severe abdominal pain without a surgical cause, vomiting, constipation, tachycardia, hypertension, hyponatraemia, motor neuropathy, seizures, psychiatric disturbance, red or dark urine.
• Cutaneous: blistering, fragility and scarring of sun exposed skin, hypertrichosis, or immediate burning pain in sunlight with no visible lesions.
• Family members of an affected person.

Specimen. Urine: a fresh random sample (10 to 20 mL) protected from light, sent promptly to the laboratory, and refrigerated if delayed. Do not add preservatives to a PBG sample. Plasma: EDTA or heparin, protected from light, for the fluorescence scan. Faeces: a random sample, protected from light. Red cells: EDTA whole blood, protected from light. Record the day of symptoms, medicines and alcohol.

Reference ranges are method specific and are expressed relative to creatinine for random urine. Local ranges take precedence.

Urine PBG. Normal in health. During an acute attack of acute intermittent porphyria, variegate porphyria or hereditary coproporphyria, urine PBG is raised to more than 10 times the upper reference limit, usually 20 to 200 times. A semi-quantitative rapid test gives a result within an hour and is followed by a quantitative assay. A normal PBG in a patient with current severe abdominal pain effectively excludes acute porphyria as the cause.

Urine ALA. Raised together with PBG in the acute porphyrias, and raised with a normal PBG in lead poisoning, hereditary tyrosinaemia type 1 and ALA dehydratase deficiency porphyria.

Urine total porphyrins and fractionation. Raised in the acute porphyrias during an attack, in porphyria cutanea tarda (uroporphyrin and heptacarboxylic porphyrin predominate), in hepatic disease, and in secondary coproporphyrinuria (alcohol, lead, liver disease, some drugs, and many illnesses). Fractionation separates uroporphyrin from coproporphyrin. A raised total with a normal PBG needs interpretation of the pattern.

Faecal porphyrins. Protoporphyrin and coproporphyrin are raised in variegate porphyria (protoporphyrin over coproporphyrin) and hereditary coproporphyria (coproporphyrin, especially isomer III). Faecal results stay abnormal between attacks and are the best way to distinguish variegate porphyria from hereditary coproporphyria.

Plasma porphyrin fluorescence scan. Emission peak at 624 to 628 nm is characteristic of variegate porphyria. A peak at 618 to 622 nm points to porphyria cutanea tarda, erythropoietic protoporphyria (peak about 634 nm), and other forms. It is positive in variegate porphyria even in remission and is a rapid screen.

Red cell protoporphyrin. Raised in erythropoietic protoporphyria (free protoporphyrin, metal free), X-linked protoporphyria, lead poisoning and iron deficiency (zinc protoporphyrin). Fractionation distinguishes the zinc bound form from the metal free form.

Red cell porphobilinogen deaminase (HMBS) activity. Reduced to about half of normal in most acute intermittent porphyria. It is unreliable, with overlap with normal and with other conditions, and genetic testing has largely replaced it.

Acute intermittent porphyria (HMBS). Attack with raised ALA and PBG (PBG remains raised for months, often years, between attacks), no skin disease. Urine can darken on standing in light, because PBG converts to porphobilin and porphyrins.

Variegate porphyria (PPOX) and hereditary coproporphyria (CPOX). Attacks resemble those of acute intermittent porphyria, with the addition of photosensitive skin disease. PBG rises during an attack and falls to normal within days to weeks, so a late sample may be normal. Faecal and plasma tests remain abnormal. Variegate porphyria has a founder effect in people of Dutch descent in South Africa, with a frequency of about 1 in 1000.

Porphyria cutanea tarda (UROD). Urine uroporphyrin and heptacarboxylic porphyrin raised, plasma peak at about 620 nm, and normal PBG. Triggers: alcohol, oestrogen, hepatitis C, HIV, iron overload (HFE variants), and smoking. Check hepatitis C, HIV, ferritin and HFE genotype.

Erythropoietic protoporphyria (FECH). Immediate painful photosensitivity in childhood without blistering, raised erythrocyte free protoporphyrin, normal urine, and liver disease in a minority.

Congenital erythropoietic porphyria (UROS). Severe blistering, mutilating scarring, red urine and teeth in infants, and haemolytic anaemia.

Secondary coproporphyrinuria. Mildly raised urine coproporphyrin with normal PBG in alcohol use, liver disease, lead poisoning, haemolysis and many drugs, which is not porphyria.

Triggers of acute attacks. Porphyrinogenic drugs (barbiturates, sulphonamides, some anticonvulsants such as carbamazepine and phenytoin, rifampicin, many hormonal contraceptives, ergot alkaloids, some antibiotics), alcohol, fasting and low carbohydrate diets, infection, surgery, stress, and the luteal phase of the menstrual cycle.

Critical values and findings needing immediate action.

• A raised PBG with abdominal pain, vomiting, tachycardia, hyponatraemia or neuropathy: acute attack, which needs urgent treatment with intravenous haem arginate and glucose, withdrawal of triggers, and specialist advice. Treatment does not wait for confirmation if the screen is positive.
• Rapidly progressive motor neuropathy with respiratory weakness, requiring ventilatory support.
• Hyponatraemia under 125 mmol/L or seizures during an attack.
• Dark urine with abdominal pain and a high clinical suspicion, whatever the first result, in a patient who is deteriorating.

Interferences and pitfalls.

• PBG degrades in light and at room temperature, so delay gives false low results.
• A sample taken days after an attack in variegate porphyria or hereditary coproporphyria can be normal for PBG. A plasma scan and faecal porphyrins then diagnose.
• Total urine porphyrin screens with a normal PBG do not diagnose acute porphyria, and secondary coproporphyrinuria is common.
• Some drugs and metabolites colour urine and give false positive spot tests (rifampicin, phenazopyridine, methyldopa metabolites).
• Positive tests in lead poisoning mimic acute porphyria by raising ALA, and a blood lead level is needed.
• Genetic testing confirms the diagnosis and identifies relatives, since many carriers are asymptomatic.
• Alcohol, fasting and hormone therapies may provoke attacks in carriers, and counselling is needed.

Special groups.

• Women of reproductive age: attacks often cluster in the premenstrual phase, and pregnancy carries risk with fasting and vomiting.
• Children: clinically manifest acute porphyria before puberty is rare, and skin disease dominates in childhood forms.
• Older adults: porphyria cutanea tarda appears with iron overload, hepatitis C and alcohol.
• Regions: South Africa has a high frequency of variegate porphyria, northern Sweden has a high frequency of acute intermittent porphyria, and hepatitis C influences prevalence of porphyria cutanea tarda.

Follow up tests. Quantitative urine PBG and ALA, urine and faecal porphyrin fractionation, plasma fluorescence scan, red cell protoporphyrin, HMBS, PPOX, CPOX, UROD and FECH gene sequencing, sodium and magnesium, liver function tests, glucose, creatinine, blood lead level, hepatitis C and HIV tests, ferritin and HFE genotype, and family screening.