What diagnostic tests are available?
Are the diagnostic tests the same for all porphyrias?
There are many laboratory tests available for the porphyrias, and the correct tests to order depend on the type of porphyria the doctor suspects. It is often difficult to decide which tests should be chosen, and the results may be difficult to interpret. The tests vary in sensitivity and specificity. If a test is “sensitive”, it is unlikely to be falsely negative (that is, fail to diagnose porphyria in a patient who has the disorder). If a test is “specific,” it is unlikely to be falsely positive (that is, diagnose porphyria in a patient who does not have the disorder). Certain tests are both sensitive and specific in patients who have symptoms that are suggestive of a porphyria. It is advisable to have the 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 is made. When abdominal and neurological symptoms suggest an acute porphyria, the best screening tests are urinary aminolevulinic acid (ALA) and porphobilinogen (PBG).
When there are cutaneous symptoms that suggest porphyria, the best screening test is a plasma porphyrin assay. If one of these screening tests is abnormal, more extensive testing, including urinary, fecal, and red blood cell porphyrins, are often 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 specific enzyme deficiency. The table below summarizes the tests to be done for each type of porphyria.