Porphyria Overview

Q: What diagnostic tests are available?

A: There are many laboratory tests available for porphyrias, and it is often difficult to decide which should be chosen. Many of these tests are expensive. The results are often difficult to interpret. The tests vary in sensitivity and specificity. If a test is sensitive, it is unlikely to be falsely negative (i.e., fail to diagnose porphyria in a patient who has the disease). If a test is specific, it is unlikely to be falsely positive (i.e., diagnose porphyria in a patient who does not have porphyria). Certain tests are both sensitive and specific in patients who have symptoms that are suggestive of porphyria. When abdominal and neurological symptoms suggest an acute porphyria, the best screening tests are urinary ALA and PBG. When there are cutaneous symptoms that suggest porphyria, the best screening test is a plasma porphyrin determination. If one of these screening tests is abnormal, more extensive testing, including urinary, fecal and red blood cell porphyrins, are then indicated. Urinary, fecal, and red blood cell porphyrin measurements are not very useful for initial screening, because they lack either sensitivity or specificity and, therefore, are often difficult to interpret. Measurement of heme biosynthetic enzymes in red blood cells or lymphocytes is not appropriate for screening, unless it is part of a family study that is done after someone in the family is already known to have a particular enzyme deficiency. It is advisable to have testing performed by a laboratory that has expertise in the clinical aspects of porphyria and can provide a valid interpretation of the test results.

If testing has been performed in laboratories other than porphyria laboratories, consultation with a porphyria expert is advised before a final diagnosis of acute porphyrias is accepted.

For more information, please see Testing.