Porphyrin testing is used to help diagnose and sometimes to monitor porphyrias. These disorders can be classified into two groups based on signs and symptoms: neurologic porphyrias and cutaneous porphyrias. As some porphyrias can have similar symptoms, testing is also used to help determine which type is present. A healthcare practitioner will choose individual tests based on the affected person's signs and symptoms as well as the medical and family history.

Neurologic porphyrias are associated with acute attacks involving the nervous system and/or gastrointestinal tract. They include acute intermittent porphyria (AIP), variegate porphyria (VP), hereditary coproporphyria (HCP), and ALA dehydratase deficiency porphyria (ADP).

Tests for neurologic porphyrias are given below:

• A urine test for porphobilinogen (PBG), a porphyrin precursor, is the primary test.
• If the result of the PBG test is abnormal, urine porphyrin testing, which measures uroporphyrin and coproporphyrin, may be ordered to provide additional information on the type of neurologic porphyria that is present.
• A test to detect porphyrins in stool may be used to help distinguish between VP and HCP.
• Aminolevulinic acid (ALA), another porphyrin precursor, is used to diagnose the rare ALA dehydratase deficiency porphyria. ALA may be ordered along with PBG as part of the diagnostic workup, as it is usually elevated in all four of the neurologic porphyrias.

Cutaneous porphyrias produce skin-related symptoms. This type includes porphyria cutanea tarda, (PCT), erythropoietic protoporphyria (EPP) and congenital erythropoietic porphyria (CEP).

Tests for cutaneous porphyrias are given below:

• Urine porphyrin testing is ordered to evaluate adults with blisters, scarring and hyperpigmentation in sun-exposed areas of the skin (suspected PCT).
• A blood test for porphyrins is the best test to diagnose patients with EPP, who present with redness, itching and swelling that begins soon after sun exposure.
• Both blood and urine tests for porphyrins are abnormal in CEP, a rare disorder that produces extreme sensitivity to light in children.

These tests may sometimes be used to monitor the disease in patients who have been diagnosed with a porphyria.

Enzyme testing may be used to confirm the diagnosis of acute intermittent porphyria (by measuring porphobilinogen deaminase in red blood cells). Genetic testing for porphyria is not widely available, but it is another way to establish the diagnosis. Both types of tests may be used to identify family members who have inherited a porphyria, even if they do not have signs and symptoms of the disease.

Porphyrin tests are ordered when an individual has signs and symptoms that a healthcare practitioner suspects are due to a porphyria. Typically, the person will have either neurologic symptoms or skin-related symptoms, but those with VP or HCP may have both.

Acute neurologic porphyrias—tests may be ordered when someone has acute attacks lasting for days to weeks. The attacks may be triggered by a variety of drugs or environmental factors such as dietary changes, stress, and exposure to toxic substances. The attacks may include signs and symptoms, such as:

• Abdominal pain, nausea
• Constipation
• Peripheral neuropathy—tingling, numbness, or pain in the hands and feet
• Muscle weakness
• Urinary retention
• Confusion, hallucinations

Cutaneous porphyrias—tests may be ordered when someone has one or more signs and symptoms affecting sun-exposed areas of the skin, such as:

• Blisters
• Scarring
• Hyperpigmentation
• Redness
• Itching
• Burning
• Swelling

When an individual has been diagnosed with a porphyria, testing may be performed on a regular basis to monitor the condition.

Care must be taken when interpreting porphyrin test results. Results are typically reported as below an established threshold (normal) or, if increased, reported as a specific level.

One or more of the porphyrins and their precursors are usually elevated in each of the porphyrias, and the pattern of elevation (which porphyrin is elevated in which sample) helps determine the diagnosis. For example, urine PBG is significantly increased in people with a neurologic porphyria. However, porphyrins may be increased up to several-fold in a variety of other conditions. In addition, levels may fall to near normal levels between acute attacks of a neurologic porphyria. Interpretation of the patterns can be difficult and should be done by a physician or laboratory scientist with expertise in porphyrias.

If the initial test results are negative, it means that it is unlikely that a person's symptoms are caused by a porphyria. Initial tests that are positive should be confirmed with follow-up testing.

People usually do not need to have all of these tests performed. The following table summarizes the patterns of results that are typical for each type of porphyria:

If enzyme testing is performed, a low level of the enzyme can help confirm a diagnosis. For example, a decreased level of porphobilinogen deaminase confirms the diagnosis of acute intermittent porphyria.

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

The diagnosis of a porphyria can be difficult. As a group, these diseases are uncommon or rare, and the signs and symptoms can be seen mimic other, more common diseases.

A variety of drugs, alcohol, and other environmental factors such as diets, stress, and illness, can trigger acute attacks of a neurologic porphyria in those with latent or inactive disease. By the same token, sun exposure can induce skin lesions in people with a cutaneous porphyria. Lifestyle modifications to avoid aggravating factors are the most effective way to minimize the impact of a porphyria.

Some reference laboratories can measure porphyrins in plasma, bile or other fluids, but this is not usually necessary to make a diagnosis.