What is the role of urine aminolevulinic acid (ALA) testing in diagnosing porphyria, particularly in patients with suspected acute intermittent porphyria (AIP)?

Urine ALA Testing for Porphyria

Urine aminolevulinic acid (ALA) must be measured alongside porphobilinogen (PBG) and creatinine in a random urine sample as the first-line biochemical test for diagnosing acute hepatic porphyria, with both markers typically elevated at least 5-fold above normal during acute attacks. 1

Essential Testing Protocol

Initial biochemical diagnosis requires simultaneous measurement of:

• Urinary ALA
• Urinary PBG (the most specific marker)
• Urinary creatinine (for normalization)
• All measured from a single random urine sample, preferably morning spot urine 1, 2

A 24-hour urine collection is not recommended and should be avoided — the elevations during acute attacks are sufficiently high that a random sample is adequate. 1, 3

Critical Sample Handling

Protect samples from light immediately by wrapping collection tubes in aluminum foil, as porphyrins and their precursors are photosensitive and will degrade, causing falsely low results. 2, 3

Refrigerate or freeze samples promptly — PBG decreases within 24 hours at room temperature, which can lead to false-negative results. 2

Normalize results to urinary creatinine to adjust for variations in urine concentration. Interpret cautiously if creatinine is below 2 mmol/L, as this can cause falsely elevated results. 2

Diagnostic Interpretation

During Acute Attacks

Both ALA and PBG are elevated at least 5-fold (often >10-fold) above the upper limit of normal during acute attacks, making the diagnosis straightforward. 1, 3 The mean excretion can reach 50-fold above normal during attacks. 4

PBG is the most specific marker — if significantly elevated (>10 times upper limit of normal, or >10 μmol/mmol creatinine), it confirms an acute porphyria attack with high certainty. 2

ALA is less specific than PBG and can be elevated in other conditions including lead poisoning and hereditary tyrosinemia, which is why both must be measured together. 2

During Remission or Between Attacks

Testing interpretation becomes more complex outside of acute attacks:

• In patients with recurrent acute attacks, ALA and PBG typically remain elevated even at baseline between attacks 1
• In sporadic AIP patients when asymptomatic, 15-44% can have normal urine ALA and PBG values 1
• ALA and PBG can remain elevated for months to years after an acute attack in AIP patients 1
• However, ALA and PBG levels can fall quickly after an acute attack in patients with hereditary coproporphyria (HCP) or variegate porphyria (VP) 1

If both ALA and PBG are normal during symptoms suggestive of acute attacks, AHP is effectively ruled out as the etiology (with the rare exception of ALAD deficiency porphyria where only ALA is elevated). 1, 3

Critical Pitfalls to Avoid

Never use urine porphyrins alone as a screening test — this is the most common diagnostic error. Mild and nondiagnostic elevations in urinary porphyrins (secondary porphyrinurias) are often incorrectly interpreted as indicating AHP and lead to erroneous overdiagnosis. 1, 3

Do not rely on qualitative tests (Watson-Schwartz or Hoesch tests) alone — always use quantitative ALA and PBG measurements for definitive diagnosis. 1, 3

Hemin treatment can lower or normalize PBG excretion if sampling occurs during or shortly after treatment, potentially causing false-negative results. 2, 3

In known AIP patients during attacks, expect PBG to increase significantly above their baseline — even a 2-fold increase in PBG from baseline, independent of the absolute concentration, is likely disease-related and not due to biological variation alone (probability >80%). 5

Confirmatory Steps

Once biochemical testing confirms elevated ALA and PBG, genetic testing is mandatory to confirm the diagnosis and identify the specific type of acute hepatic porphyria by sequencing ALAD, HMBS, CPOX, and PPOX genes. 1, 3

Genetic testing should never be used as first-line screening (except in Scandinavian populations with founder mutations) because most carriers of pathogenic variants never develop symptoms. 1, 2

First-degree family members require genetic screening once the pathogenic variant is identified in the proband. 1, 3

Clinical Context

Consider AHP in any patient, especially women of childbearing age, presenting with unexplained recurrent severe abdominal pain, as diagnosis is frequently delayed by an average of 15 years from symptom onset. 1

Both ALA and PBG can be measured with high sensitivity and specificity using modern LC-MS/MS methods, though results often require 1-2 weeks from large reference laboratories. 1, 6 A new rapid qualitative PBG test (Teco Diagnostics) was recently approved in the United States for point-of-care use. 1