What is the role of lab tests in the workup of acute intermittent porphyria (AIP)?

Updated: Jul 08, 2020
  • Author: Thomas G DeLoughery, MD; Chief Editor: Emmanuel C Besa, MD  more...
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The fundamental step in diagnosing acute intermittent porphyria (AIP) is to demonstrate increased urinary porphobilinogen secretion. If a patient has no increased secretion of porphobilinogen, (ie, a level of 0-4 mg/L during acute symptoms), acute porphyria is eliminated as a cause of the neurovisceral symptoms. [9, 10]

A spot urine test for porphobilinogen can rapidly provide the diagnosis; these tests detect porphobilinogen at levels greater than 6 mg/L. A common error is to order a urine porphyrin screen. Porphobilinogen, a porphyrin precursor, usually is not included in a urine porphyrin screen; it must be ordered specially.

AIP patients have elevated porphobilinogen between attacks. However, in some patients with a remote (years ago) history of attacks, porphobilinogen can return to the reference range.

Elevation of urine porphyrins, especially coporphobilinogen, is observed. This is caused by spontaneous polymerization of porphobilinogen in the urine. Nonspecific (1-2 times reference range) elevation of urine porphyrins, especially coproporphyrins, is common and is not specific for porphyria. Stool porphyrins are within the reference range or mildly elevated.

Other nonspecific signs in an attack of AIP include the following:

  • Hyponatremia
  • Syndrome of inappropriate secretion of antidiuretic hormone (SIADH)
  • Mild leukocytosis

Although a defective enzyme causes AIP, measuring the activity of porphobilinogen deaminase is of little value. Approximately 10% of AIP patients will have normal activity because a different form of the enzyme is expressed in the hematopoietic tissues. The vast majority of patients with the defective enzyme do not have any symptoms of the disease.

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