Heme Synthesis, Porphyrias, and Bilirubin

Laboratory evaluation of heme metabolism includes measurements of proteins, pathway precursors, and pigments. Plasma free hemoglobin indicates release from erythrocytes; urine 5-aminolevulinic acid and porphobilinogen reveal acute precursor accumulation; erythrocyte protoporphyrin supports selected porphyria evaluations; and serum bilirubin reflects heme breakdown and hepatic processing. Interpretation depends on using the proper specimen and understanding which part of the pathway the method measures.

Heme proteins in laboratory testing

Hemoglobin carries four heme groups within an erythrocyte, while myoglobin carries one heme group within striated muscle. Their cellular physiology is covered in Hematopoiesis, Erythropoiesis, and Erythrocyte Physiology. Chemistry testing measures these proteins outside their usual cells. The specimen determines what each method can establish and which interferences matter.¹²³

Plasma free hemoglobin methods have assay-specific specimen, interference, and reporting rules. One current eight-wavelength method uses EDTA plasma separated within 2 hours and reports total plasma hemoglobin or oxyhemoglobin according to the pigment pattern. Its limits and reference interval apply to that procedure.¹ Serum myoglobin also remains method-specific. Its concentration reflects muscle release and renal clearance, and its broad tissue distribution limits etiologic interpretation.² Current chest-pain guidance uses serial cardiac troponin I or T, with high-sensitivity assays preferred.³

The detailed urine heme workflow appears in Urine Reagent-Strip Chemistry and Confirmatory Testing.

Heme synthesis and porphyria patterns

Heme is an iron-containing tetrapyrrole used by hemoglobin, myoglobin, cytochromes, catalase, and other proteins. Synthesis occurs in most nucleated cells and is most active in erythroid precursors and hepatocytes. The pathway begins in mitochondria, continues through four cytosolic reactions, then returns to mitochondria for its final three reactions. ALAS1 supplies the first step in nonerythroid cells; ALAS2 is the erythroid form. Ferrochelatase completes the pathway by inserting ferrous iron into protoporphyrin IX.⁴

Porphyrins have strong absorption near 400 nm and red fluorescence. Carboxyl groups affect their solubility and specimen distribution. The highly carboxylated uroporphyrins appear readily in urine; coproporphyrins occur in urine and feces; hydrophobic protoporphyrin is measured in erythrocytes and feces. These properties help determine which specimen can reveal a pathway block.⁵

Each enzyme defect has a dominant biochemical direction, while the full pattern depends on presentation and specimen.⁴⁵

Porphyrias are classified by the principal site of precursor overproduction and by their usual clinical pattern. Acute intermittent porphyria, hereditary coproporphyria, variegate porphyria, and ALA dehydratase deficiency porphyria are the acute hepatic porphyrias. Hereditary coproporphyria and variegate porphyria can also produce cutaneous disease. Porphyria cutanea tarda is usually sporadic; a minority of cases have familial uroporphyrinogen-decarboxylase deficiency. Erythropoietic protoporphyria, X-linked erythropoietic protoporphyria, and congenital erythropoietic porphyria arise mainly from erythroid precursor production. Biochemical patterns overlap, so the enzyme table guides test selection and cannot establish a subtype by itself.⁵

Selecting porphyria tests

Clinical presentation determines the first biochemical tests. A generic urine porphyrin order may exclude ALA and PBG, which are porphyrin precursors. The laboratory should state the measured analytes and refer complex patterns to a porphyria testing center.⁵⁶

Quantitative porphyrin testing commonly uses high-performance liquid chromatography with fluorescence detection. Fecal porphyrins are extracted and fractionated chromatographically to separate coproporphyrin isomers and protoporphyrin. Erythrocyte hydroxymethylbilane synthase activity can support an acute intermittent porphyria or family evaluation, but overlapping activities limit its independent diagnostic value. A total urine porphyrin result must be paired with urine ALA and PBG during evaluation of an acute neurovisceral attack.⁵

During an acute attack, PBG and ALA usually rise substantially in acute intermittent porphyria, hereditary coproporphyria, and variegate porphyria. A PBG result more than 5 times the upper limit of normal strongly supports an acute hepatic porphyria in the appropriate setting. ALA dehydratase deficiency produces an ALA-predominant pattern. Lead exposure and hereditary tyrosinemia can also increase ALA. Properly handled PBG and ALA results within their reference intervals during current symptoms make an attack from acute intermittent porphyria, hereditary coproporphyria, or variegate porphyria unlikely. Timing, renal function, recent hemin treatment, and specimen quality remain part of the interpretation.⁵⁶

Biochemical evidence comes before broad genetic testing in a symptomatic patient. A pathogenic variant can identify susceptibility in a relative or confirm the type after a diagnostic biochemical pattern. Active precursor accumulation establishes that current symptoms fit an acute attack.⁵

Specimen handling

Porphyria testing is vulnerable to preanalytic loss and dilution effects. Current international porphyria laboratory guidance recommends the following handling practices.⁵

1. Collect the symptom-appropriate specimen. A preservative-free random or morning urine is used for initial acute testing, with a 24-hour collection discouraged.
2. Cover urine, plasma, blood, and fecal specimens immediately after collection. Porphyrins are light-sensitive.
3. Refrigerate or freeze according to the performing laboratory’s instructions. PBG begins to decrease during room-temperature storage, and repeated freeze-thaw cycles can cause further loss.
4. Report urine PBG and ALA relative to creatinine. Interpret a very dilute specimen cautiously. A urine creatinine below 2 mmol/L, about 22.6 mg/dL, can produce a misleading ratio. Each laboratory sets its rejection criteria.
5. Follow the referral laboratory’s requirements for pH, container, anticoagulant, transport time, and allowable storage. These conditions vary by analyte and method.

Lead and secondary porphyrin increases

Lead inhibits ALA dehydratase and ferrochelatase. ALA and erythrocyte protoporphyrin rise, and much of the excess erythrocyte protoporphyrin binds zinc. Zinc protoporphyrin (ZPP) also increases with iron-restricted erythropoiesis, inflammation, and other disorders, and its sensitivity is poor at lower blood-lead concentrations. A venous whole-blood lead measurement establishes exposure; ZPP supplies supporting evidence of impaired heme synthesis.⁷

Liver disease, renal disease, alcohol exposure, medications, infection, toxins, and inherited bilirubin disorders can produce secondary porphyrin abnormalities. Fractionation across urine, plasma, erythrocytes, and feces helps separate these common secondary patterns from a porphyria. Clinical context and expert laboratory interpretation help evaluate small or isolated increases.⁵

Bilirubin formation and fractions

Macrophage heme oxygenase opens the heme ring and produces biliverdin while releasing iron and carbon monoxide. Biliverdin reductase forms unconjugated bilirubin. This hydrophobic pigment travels bound to albumin, enters hepatocytes, and is conjugated with glucuronic acid by UGT1A1. Conjugated bilirubin enters bile. Intestinal bacteria convert bilirubin products to urobilinogen; most pigment leaves in feces, while a smaller portion is reabsorbed and a small residual reaches urine.⁸

Routine fraction names describe the analytical reaction as well as bilirubin chemistry.⁸

The direct result therefore differs from a chemically pure conjugated-bilirubin measurement. The worked subtraction and its limits appear in Core Chemistry Calculations.

Bilirubin measurement and specimen quality

Diazo methods couple bilirubin with diazotized sulfanilic acid or another diazonium reagent to form a colored azopigment. An accelerator such as caffeine-benzoate or another solubilizer releases albumin-bound unconjugated bilirubin for total measurement. Direct methods omit that total-bilirubin accelerator or use different reaction conditions. Oxidative methods use bilirubin oxidase or vanadate and measure the loss of bilirubin absorbance. Each approach has its own response to unconjugated, conjugated, delta, and phototherapy-related bilirubin species.⁸

Classic Malloy-Evelyn procedures use methanol to accelerate the diazo reaction. Jendrassik-Grof procedures use caffeine-benzoate as the accelerator and measure the resulting azopigment. Exact pH, wavelength, reaction time, blank correction, and interference limits belong to the validated method. Transcutaneous bilirubinometry uses reflectance measurement for newborn screening and monitoring; serum or plasma measurement supplies confirmation when the clinical protocol requires it.⁸

A higher-order reference system supports total bilirubin measurement. The Joint Committee for Traceability in Laboratory Medicine lists the Doumas reference measurement procedure, which uses the Jendrassik-Grof diazo principle for human serum.⁹ NIST Standard Reference Material 916b supplies certified high-purity bilirubin for calibration and traceability work.¹⁰ Direct bilirubin remains a method-defined fraction, and routine total-bilirubin comparability still depends on calibration and the molecular forms present.

For routine testing, use serum or plasma validated by the method, separate it promptly, and protect it from light in an amber container or equivalent cover. Fasting is usually unnecessary for current automated assays; a Malloy-Evelyn or other lipemia-sensitive method follows its validated specimen requirement. The laboratory applies its analyzer’s limits for hemolysis, lipemia, icterus, storage, and bilirubin photoproducts. Hemoglobin can produce spectral and chemical interference, with the direction and size set by the assay. Current reference-laboratory instructions require prompt separation and light protection for both total and direct serum bilirubin.¹¹

Reference intervals also depend on method, age, and population. Jaundice is usually visible when serum bilirubin exceeds about 3 mg/dL, although skin tone, lighting, and the observer affect recognition.¹² Newborn bilirubin is interpreted against age in hours, gestational age, and risk-based decision thresholds. The Blood Banking page on Hemolytic Disease of the Fetus and Newborn owns that clinical context.

Interpreting bilirubin and urobilinogen patterns

Fractionated bilirubin results are interpreted with urine bilirubin and urobilinogen, aminotransferases, alkaline phosphatase, gamma-glutamyl transferase, and hemolysis studies. The performing laboratory’s interval defines whether each result is increased.¹³

An elevated total bilirubin should be fractionated. A conjugated or direct-predominant increase supports hepatocellular disease or cholestasis in most settings; an unconjugated-predominant increase supports increased bilirubin production or impaired uptake or conjugation.¹³ Urine patterns provide supporting evidence and vary with partial obstruction, urine stability, strip chemistry, and interfering substances. Detailed pad reactions and follow-up testing are covered in Urine Reagent-Strip Chemistry and Confirmatory Testing.

Quantitative manual urobilinogen methods use Ehrlich reagent, p-dimethylaminobenzaldehyde, to form a red product. Incomplete recovery limits precision, so many laboratories use a semiquantitative strip. Protect fresh urine from light, test promptly, and read the reaction within the method’s stated window. Porphobilinogen, selected drugs, pigments, storage, and delayed reading can alter a result. Reference intervals and reaction limits are method-specific.⁸

Selected cholestasis evaluations use serum bile acids with indication- and method-specific reference intervals. Fractionated bilirubin and the accompanying liver chemistries provide the general pigment pattern.⁸

References

1. Mayo Clinic Laboratories. Plasma Free Hemoglobin, Plasma. Accessed August 29, 2026.
2. Mayo Clinic Laboratories. Myoglobin, Serum. Accessed August 29, 2026.
3. Gulati M, Levy PD, Mukherjee D, et al. 2021 AHA/ACC guideline for the evaluation and diagnosis of chest pain. Circulation. 2021;144(22):e368-e454. doi:10.1161/CIR.0000000000001029
4. Belot A, Puy H, Hamza I, Bonkovsky HL. Update on heme biosynthesis, tissue-specific regulation, heme transport, relation to iron metabolism and cellular energy. Liver Int. 2024;44(10):2235-2250. doi:10.1111/liv.15965
5. Aarsand AK, To-Figueras J, Whatley S, Sandberg S, Schmitt C. Practical recommendations for biochemical and genetic diagnosis of the porphyrias. Liver Int. 2025;45(3):e16012. doi:10.1111/liv.16012
6. Wang B, Bonkovsky HL, Lim JK, Balwani M. AGA Clinical Practice Update on Diagnosis and Management of Acute Hepatic Porphyrias: Expert Review. Gastroenterology. 2023;164(3):484-491. doi:10.1053/j.gastro.2022.11.034
7. Agency for Toxic Substances and Disease Registry. Lead Toxicity: Clinical Assessment, Diagnostic Tests and Imaging. Accessed August 29, 2026.
8. Guerra Ruiz AR, Crespo J, López Martínez RM, et al. Measurement and clinical usefulness of bilirubin in liver disease. Adv Lab Med. 2021;2(3):352-361. doi:10.1515/almed-2021-0047
9. Joint Committee for Traceability in Laboratory Medicine. NRMeth 29: Doumas reference method for total bilirubin. Accessed August 29, 2026.
10. Nelson MA, Brown Thomas J, Lang BE, et al. Certification of Standard Reference Material 916b: Bilirubin. NIST Special Publication 260-212. National Institute of Standards and Technology; 2021.
11. Mayo Clinic Laboratories. Bilirubin, Serum. Accessed August 29, 2026.
12. Nelson M, Mulani SR, Saguil A. Evaluation of jaundice in adults. Am Fam Physician. 2025;111(1):25-30. American Family Physician.
13. Kwo PY, Cohen SM, Lim JK. ACG Clinical Guideline: Evaluation of Abnormal Liver Chemistries. Am J Gastroenterol. 2017;112(1):18-35. doi:10.1038/ajg.2016.517