Porphyrins

What is being tested?
Porphyrins are a group of compounds defined by their chemical structure. They are an integral part of certain proteins found in the body, such as haemoglobin, myoglobin and specific enzymes. Porphyrin tests are measurements used to help diagnose and monitor a group of diseases called porphyrias. Most porphyrin tests detect and measure by-products of haem synthesis. Haem, an iron-containing pigment, is part of haemoglobin and of a number of other proteins. Haem synthesis is a step-by-step process that requires the sequential action of 8 different enzymes. If one of these enzymes is deficient, a block occurs and precursors (porphyrins such as uroporphyrins, coproporphyrins and protoporphyrins) build up in the body’s fluids and tissues and are excreted in the urine and stool. Which precursors build up depends on where the enzyme block (or deficiency) occurs.

There are six major porphyrias, each associated with a different enzyme deficiency. Most porphyrias are inherited in an autosomal dominant pattern, with one normal gene and one defective gene. This results in a reduction of about 50% in the activity of the enzyme involved in haem synthesis. Enough haem is produced to prevent the affected person from becoming anaemic, but a large excess of one or more precursors is produced.

Porphyrias can be classified according to:

• The enzyme involved
• The part of the body in which the excess porphyrins are produced

• hepatic – related to the liver
• erythropoietic – related to red blood cell production

• The signs and symptoms of the disease (neurological/psychiatric, cutaneous, or both).

Porphyrias that cause neurological/psychiatric symptoms present with acute attacks that can last for days or weeks. They are associated with abdominal pain, nausea, constipation, depression, confusion, hallucinations and seizures. Attacks can be triggered by a range of medicines (such as anticonvulsants, antibiotics and hormones) and environmental factors (such as changes in diet, stress and exposure to toxic substances).

Cutaneous porphyrias are associated with photosensitivity. Exposure to sunlight, even through glass, has a toxic effect on the patient’s skin. In some patients this causes redness, swelling and a burning sensation, while in others it causes blistering, skin thickening, hyperpigmentation and in some cases scarring.

Porphyrias with acute attacks (neurological/psychiatric) include:

• Acute intermittent porphyria (AIP), the most common type of neurological porphyria
• Variegate porphyria (VP), which causes both neurological symptoms and photosensitivity
• Hereditary coproporphyria (HCP), which can cause neurological symptoms, photosensitivity or both

Cutaneous porphyrias include:

• Porphyria cutanea tarda (PCT), the most common porphyria; it is unlike the other porphyrias. Most cases are due to an acquired enzyme deficiency triggered by liver dysfunction.
• Protoporphyria (also called erythropoietic protoporphyria) typically begins in childhood or adolescence: sun-exposed skin becomes red, burns and itches, but scarring is uncommon.
• Congenital erythropoietic porphyria (CEP), a very rare autosomal recessive disorder; over time, extreme photosensitivity leads to extensive, severe scarring of sun-exposed areas.

In rare cases, a patient may have two different porphyrias or a homozygous enzyme deficiency that produces a more severe form of porphyria.

Individual tests
Clinical laboratories measure porphyrins and their precursors in urine, blood and stool. These tests are listed below:

• Urine delta-aminolevulinic acid (ALA)
• urine porphobilinogen (PBG)
• porphyrins, including uroporphyrins, coproporphyrins and protoporphyrins, in urine, stool and blood
• zinc protoporphyrin (or free erythrocyte protoporphyrin), a test for protoporphyrins in red blood cells

Specialist laboratories may offer tests for one or more of the enzymes involved. The most commonly measured enzyme is porphobilinogen deaminase (PBG-D) in red blood cells, which is used to investigate patients with acute intermittent porphyria. Some research laboratories offer genetic testing for mutations in specific genes that cause one of the porphyrias, but this remains mainly research work.

How is the sample collected for testing?
The sample collected depends on which porphyrin tests are requested. It may be one or more of the following:
A blood sample taken by inserting a needle into a vein in the arm
A random or 24-hour urine collection (the urine must be protected from light during collection)
A fresh stool sample not contaminated with urine or water.

How is it used?
When is it requested?
What does the test result mean?
Is there anything else I should know?

How is it used?
Porphyrin tests are used to help diagnose and sometimes monitor the porphyrias. Because the symptoms associated with these disorders can also be found in a variety of other conditions, the tests are also used to help rule out porphyria in someone who has neurological/psychiatric or cutaneous symptoms.
For acute attacks, urine porphobilinogen (PBG) and porphyrins may be requested on a random urine sample. If these are abnormal, they are followed by delta-aminolevulinic acid (ALA), PBG and/or porphyrin tests on a 24-hour urine sample. Faecal porphyrins may be requested to help distinguish between VP and HCP.

For cutaneous porphyrias, the most frequently requested tests are whole blood and urine porphyrins. They are used to diagnose a porphyria and may be used to monitor it. Enzyme tests, such as PBG deaminase, may be requested to help detect latent porphyrias in the relatives of a patient who has been diagnosed with a porphyria.

When is it requested?
PBG and porphyrin tests may be requested on a random urine sample when a patient has symptoms suggesting an acute porphyria: abdominal pain, nausea, constipation, peripheral neuropathy (tingling, numbness or pain in the hands or feet), muscle weakness, urinary retention, confusion and hallucinations. When the initial tests are abnormal, they should be repeated on a 24-hour urine collection to determine how much PBG, ALA and porphyrins are being excreted. Faecal porphyrin tests may be requested to help distinguish between the porphyrias.

Depending on the patient’s age and symptoms, a urine porphyrin test and/or a free erythrocyte protoporphyrin test should be requested when a patient presents with blisters, scarring, redness or other skin lesions on sun-exposed areas. A positive urine test should be followed by analysis of a 24-hour urine collection to determine which porphyrins are present.

Enzyme tests are most commonly requested to confirm a diagnosis of acute intermittent porphyria (porphobilinogen deaminase in red blood cells). This test is particularly useful for identifying relatives of a patient who have inherited the disease but have not yet developed signs or symptoms. Some laboratories can measure porphyrins in plasma, bile or other fluids, but this is not usually needed to make a diagnosis.

What does the test result mean?
Great care must be taken when interpreting porphyrin test results. Some porphyrins or their precursors may be mildly to moderately raised in patients with other diseases or conditions. In addition, ALA, PBG and porphyrin levels may fall almost to normal between the acute attacks of a neurological porphyria. While negative test results mean that the patient’s symptoms are unlikely to be caused by a porphyria, positive initial tests should be confirmed by further testing.
ALA and PBG are significantly increased in most patients with acute porphyria. ALA is less specific than PBG, as it can also be high in other conditions. Specific porphyrins are raised in each of the porphyrias, and the pattern of elevation (which porphyrin is raised in which sample) determines the diagnosis. Urine, blood and faecal porphyrins may increase several-fold in a variety of other conditions. Interpreting these conditions can be difficult and should be done by a doctor or a laboratory specialist with expertise in this field.

An abnormal enzyme test or the detection of a gene mutation indicates that a family member has inherited a porphyria. However, enzyme and genetic tests cannot determine whether that individual will develop symptoms of porphyria or, if so, how severe they will be. Fortunately, most gene carriers never have an attack.

Findings seen in specific porphyrias include:

Type of porphyria	Urine ALA and PBG*	Urine porphyrins	Faecal porphyrins	Erythrocyte porphyrins
Acute intermittent porphyria	Increased*	URO* increased	Normal	Normal
Variegate porphyria	Increased	COPRO increased	PROTO, COPRO increased	Normal
Hereditary coproporphyria	Increased	COPRO increased	COPRO increased	Normal
Porphyria cutanea tarda	Normal	URO, heptacarboxyl porphyrin increased	Isocoproporphyrin increased	Normal
Protoporphyria	Normal	Normal	PROTO increased	PROTO increased (zinc protoporphyrin)
Congenital erythropoietic porphyria	Normal	URO, COPRO increased	COPRO increased	URO, COPRO increased

URO = uroporphyrins; COPRO = coproporphyrins; PROTO = protoporphyrins
*may only be increased during an acute attack

Is there anything else I should know?
A variety of medicines, alcohol and other environmental factors such as diet, stress and illness can trigger acute attacks of neurological porphyria in people with latent or inactive disease. Likewise, sun exposure will cause skin lesions in patients with a cutaneous porphyria. Changing lifestyle to avoid aggravating factors is the most effective way to minimise the impact of a porphyria.

1. Will a latent porphyria affect my health?
2. Can porphyrin tests be done at home?
3. Why does my doctor need a 24-hour urine sample if a random sample has already been tested?

1. Will a latent porphyria affect my health?
In most cases the answer is no, and the porphyria remains quiescent. It is important, however, to identify a latent porphyria if you have a family history, so that your doctor can adapt any treatment to avoid medicines and situations that could trigger the porphyria.

2. Can porphyrin tests be done at home?
No. They require specialist equipment to perform and great care in interpretation. Some can be done in a local hospital laboratory, while others may need to be sent to a reference laboratory.

3. Why does my doctor need a 24-hour urine sample if a random sample has already been tested?
The rate at which porphyrins are produced and excreted can vary, so they may or may not be raised in a random sample. A 24-hour urine sample allows all the porphyrins excreted over a 24-hour period to be assessed.