At a glance
Also known as
Why get tested?
To help diagnose and sometimes to monitor porphyrias (a group of inherited disorders involving disturbance in the metabolism of haem, a component of haemoglobin)
When to get tested?
If a patient has symptoms that suggest an acute neurologic/psychiatric porphyria (such as abdominal pain, tingling in hands or feet, and/or confusion or hallucinations) or a cutaneous porphyria (such as reddening, blistering, or scarring on sun-exposed skin)
A blood sample drawn from a vein in your arm, a urine sample, preferably collected whilst symptoms are occurring, and a small stool sample
Test preparation needed?
What is being tested?
Porphyrin tests are assays that are used to help diagnose, and monitor a group of disorders called porphyrias. Most porphyrin tests detect and measure the by-products of haem synthesis. Haem is a part of haemoglobin (the protein inside red blood cells that allows them to transport oxygen) and a number of other proteins. The synthesis of haem is a step-by-step process that requires the sequential action of eight different enzymes. If there is a deficiency in one of these enzymes, a bottleneck forms and precursors build up in the body fluids and tissues and are excreted in urine and faeces. Which precursors build up depends on where the bottleneck is.
There are six major porphyrias, and each one is associated with a different enzyme deficiency. Most porphyrias are inherited in an autosomal fashion, with one normal and one affected gene. This results in about a 50% reduction in the activity of the haem-related enzyme. Enough haem is synthesised to prevent the affected person from becoming anaemic, but a large excess of one or more precursors is produced.
Porphyrias may be classified according to the:
- affected enzyme
- part of the body where the excess porphyrins are produced (hepatic - associated with the liver or erythropoietic - associated with red blood cell production), or
- and of the disease (neurological/psychiatric, cutaneous, or both).
Those porphyrias that cause neurological/psychiatric symptoms present with acute attacks that may last for days or weeks. They are associated with abdominal pain, nausea, constipation, depression, confusion, hallucinations and seizures. Attacks may be triggered by a variety of drugs (such as anticonvulsants, antibiotics and hormones) and environmental factors (such as dietary changes, stress and exposure to toxic substances).
The cutaneous porphyrias are associated with photosensitivity. Sunlight exposure, even through a glass window, has a toxic effect on the patient’s skin. This may cause redness, swelling and a burning sensation in some patients, while in others it leads to blistering, skin thickening, hyperpigmentation, and in some cases scarring.
Acute attack porphyrias (neurological/psychiatric) include:
- Acute intermittent porphyria (AIP), the most common of the neurological porphyrias
- Variegate porphyria (VP), which includes both neurological symptoms and photosensitivity
- Hereditary coproporphyria (HCP), which may present with neurological symptoms, photosensitivity, or both
Cutaneous porphyrias include:
- Porphyria cutanea tarda (PCT), the most common porphyria; unlike the other porphyrias, most cases are due to an acquired enzyme deficiency which is triggered by liver dysfunction.
- Protoporphyria (also called erythropoietic protoporphyria), which typically begins in childhood or adolescence; sun-exposed skin turns red accompanied by burning and itching, but scarring is uncommon.
- Congenital erythropoietic porphyria (CEP), a very rare autosomal disorder; over time, extreme photosensitivity leads to extensive and severe scarring of light-exposed areas.
In rare cases, a patient may have two different porphyrias, or a homozygous deficiency of one enzyme that produces a more severe form of porphyria.
Clinical laboratories measure porphyrins and their precursors in urine, blood, and faeces. These tests are listed below:
- Delta-aminolaevulinic acid (ALA) in urine
- Porphobilinogen (PBG) in urine
- Porphyrins in urine, faeces or blood
Specialised laboratories may offer testing for one or more of the affected enzymes. The most commonly measured enzyme is porphobilinogen deaminase (PBG-D) in red blood cells, which tests for patients with acute intermittent porphyria. Genetic testing for specific gene that cause porphyrias, is becoming more common and replacing measurement of the affected enzymes in many cases.
How is the sample collected for testing?
The sample collected depends on the porphyrin tests being ordered. It may be one or more of the following:
- A blood sample obtained by inserting a needle into a vein in the arm
- A (urine must be protected from light during collection)
- A fresh stool sample that is not contaminated with urine or water
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.