Diagnostic Approach for Acute Intermittent Porphyria

First-Line Diagnostic Tests

The essential blood work for diagnosing acute intermittent porphyria is plasma ALA and PBG measurement, though urinary ALA and PBG (normalized to creatinine) remain the gold standard first-line tests during acute symptoms, as recommended by the American Association for the Study of Liver Diseases 1, 2
Urinary porphobilinogen (PBG) is the most specific and essential test—if significantly elevated (typically >10 times upper limit of normal, or >10 μmol/mmol creatinine by mass spectrometry), it confirms an acute porphyria attack, with a high strength of evidence 3, 4
Urinary delta-aminolevulinic acid (ALA) should be measured alongside PBG, though it is less specific than PBG and can be elevated in lead poisoning and hereditary tyrosinemia, according to the European Association for the Study of the Liver 1, 2
Plasma ALA and PBG can be used as alternatives to urine testing and are particularly useful when urine collection is problematic, as stated by the American College of Gastroenterology 1, 2
Normal urinary or plasma PBG during acute symptoms excludes AIP as the cause (assuming proper sample handling), with a high level of certainty 3, 5

Protect all samples from light by covering tubes in aluminum foil to prevent falsely low results, as recommended by the Clinical and Laboratory Standards Institute 1
Use a spot urine sample (morning preferred) rather than 24-hour collection, with results normalized to urine creatinine, according to the National Institute of Diabetes and Digestive and Kidney Diseases 2
Refrigerate or freeze samples promptly, as PBG decreases within 24 hours at room temperature, with a moderate strength of evidence 2
Interpret results cautiously if urinary creatinine is below 2 mmol/L, as this can cause falsely elevated results, as noted by the American Association for Clinical Chemistry 2

Complete blood count (CBC) and ferritin levels at baseline and at least yearly to assess for iron deficiency, which is common in young women and causes chronic symptoms, as recommended by the American Gastroenterological Association 6
Comprehensive metabolic panel including electrolytes, as hyponatremia occurs in 25-60% of symptomatic cases during attacks, with a moderate strength of evidence 6
Estimated glomerular filtration rate (eGFR) at baseline and regularly thereafter, as AIP poses risk for chronic tubulointerstitial nephropathy, according to the National Kidney Foundation 6
Liver function tests (aminotransferases) at baseline, as they are elevated in ~13% of patients during acute attacks and in some asymptomatic patients, with a low strength of evidence 6

Avoid urinary total porphyrins as a first-line test—it is unhelpful and misleading for diagnosing acute porphyrias, as stated by the European Association for the Study of the Liver 1, 2
Do not rely on screening tests (qualitative PBG tests) due to low specificity and sensitivity; always confirm with quantitative assays, with a high strength of evidence 3, 5

In known AIP patients during attacks, expect PBG to increase significantly above their baseline, as PBG typically remains elevated for years after acute episodes, with a moderate strength of evidence 3, 4
Hemin treatment can lower or normalize PBG excretion if sampling occurs during or shortly after treatment, as noted by the American College of Gastroenterology 3, 5
The PBG/ALA ratio is approximately 2:1 in AIP patients with normal renal function, but increases with declining glomerular filtration, with a low strength of evidence 3, 5
Mass spectrometry methods have lower detection limits and correspondingly lower upper limits of normal compared to traditional ion-exchange methods, as recommended by the Clinical and Laboratory Standards Institute 1, 2

Once biochemical diagnosis is established, genetic testing (HMBS gene sequencing) is required to confirm AIP and enable family screening, as recommended by the American College of Medical Genetics and Genomics 6
Genetic testing should not be used as first-line testing due to lack of common mutations (except in Scandinavian populations with founder mutations), with a moderate strength of evidence 6

Order a random spot urine sample (not a 24‑hour collection) for porphobilinogen (PBG) and δ‑aminolevulinic acid (ALA) normalized to creatinine when a patient presents with neurovisceral symptoms such as abdominal pain or neuropsychiatric features; a PBG level >5–10 × the upper limit of normal confirms acute porphyria. [7][8]9
Urinary PBG is the most specific marker for acute porphyria; a concentration >10 µmol / mmol creatinine is diagnostic. 7
During an acute attack, both urinary ALA and PBG rise to at least five times the upper limit of normal. 9
A normal urinary PBG and ALA result during acute symptoms effectively rules out the acute porphyrias (AIP, VP, HCP, ALAD‑deficiency) provided that the specimen is collected and handled correctly. [7][8]10

Protect all urine specimens from light (e.g., wrap tubes in aluminum foil) because porphyrins degrade with light exposure, leading to falsely low results. [7][8]10
Refrigerate or freeze urine samples promptly; PBG concentrations can decline within 24 hours at room temperature. [7][8]
Interpret results cautiously when urinary creatinine is <2 mmol/L, as low creatinine can produce falsely elevated PBG/ALA ratios. [7][8]
Do not use total urinary porphyrin measurement as a first‑line test for acute porphyria; it is unhelpful and may be misleading. [7][8]10
Recognize that urinary ALA is less specific than PBG and may be elevated in conditions such as lead poisoning or hereditary tyrosinemia. [7][8]10
Be aware of the analytical method used by the laboratory: mass‑spectrometry assays have lower reference limits than traditional ion‑exchange techniques, which can affect interpretation. 7

In the acute phase, urinary ALA and PBG remain elevated for several days after symptom onset unless the patient has received hemin therapy. 9
Between attacks, patients with acute intermittent porphyria (AIP) may retain elevated urinary PBG for months to years. 9
In hereditary coproporphyria (HCP) and variegate porphyria (VP), urinary ALA and PBG levels tend to fall rapidly after an acute episode. 9
During remission, 15 %–44 % of individuals with sporadic AIP may have normal urinary ALA and PBG despite being asymptomatic. 9