Diagnostic Use
The hepatic porphyrias (Acute Intermittent Porphyria, Hereditary Coproporphyria, Variegate Porphyria) are genetic conditions that may present with acute porphyric crisis. In this situation, the porphyrin precursors ALA and PBG are produced in great excess, and their neurotoxic effects produce an acute neuropathy that may have components that are any or all of:
autonomic neuropathy (acute abdominal pain)
peripheral (motor-sensory) neuropathy
central neuropathy (acute delirium)
ALA and PBG will be present in high concentrations in urine in an acute porphyric crisis because they are the cause of the acute syndrome. This screening test for PBG in urine is sensitive to concentrations of about 20umol/L (normal urine would have PBG up to 9 umol/L), and if the patient has current symptoms due to acute porphyric neuropathy then the test will be positive; a negative screening test effectively rules out porphyria as a cause of symptoms.
The screening test is not entirely specific, and there are literature reports of occasional false positive screens due to drugs. This test is not sensitive to detect very low amount of PBG which can be seen in asymptomatic patients or at the recovery phase of an acute attack.
Interpretation
For suspected acute attack of porphyria (abdominal pain, with or without autonomic or neurological abnormalities or skin lesions):
- A casual urine is all that is needed to confirm or rule out an acute attack (a negative urine PBG taken during abdominal pain excludes acute porphyria)
- To determine the type of porphyria, both blood and faecal samples will also be needed.
For a patient with skin lesions:
- Urine PBG is not raised
- Urine porphyrins are all that is needed to diagnose PCT, which is the commonest porphyria.
- Both blood and urine porphyrins are needed to diagnose EPP.
- Both urine and faecal porphyrins are needed to diagnose hereditary coproporphyria and variegate porphyria.
Reference Intervals
Negative (negative screening test effectively rules out acute porphyria as a cause of symptoms)
Limitations / Interference
Note a very dilute urine (i.e. urine creatinine < 2.6mmol/L) can give a false negative result.
This qualitative screening test is not sensitive enough to detect very low amount of PBG, which can be seen in asymptomatic patients with acute hepatic porphyria. PBG screening test at MMH lab is only to be used in acute symptomatic patients, to assess if the presenting symptoms are due to acute porphyria attack. Most urine samples received at Middlemore Hospital Laboratory for PBG analysis will also be sent to LabPLUS for urine PBG by an alternative and slightly more sensitive method (longer turn around time). If that is raised, urine porphyrin profile will also be test added.
Uncertainty of Measurement
Not applicable- qualitative method, reported as positive or negative