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Immunology & Autoimmune
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Currently under review
Pending specialist review and validation.
This test measures the balance between two forms (isomers) of coproporphyrin, called coproporphyrin III and coproporphyrin I, in a 24 hour urine collection. Coproporphyrins are natural byproducts of heme production, and their amounts and relative proportions in urine can change when the liver, bile transport, or heme pathway are not working normally.
The result is reported as a ratio of coproporphyrin III to coproporphyrin I. Your healthcare provider often orders this test along with other porphyrin studies to help clarify the cause of abnormal porphyrin excretion and to distinguish different conditions that can look similar clinically.
The coproporphyrin III to I ratio helps your care team evaluate certain liver transport disorders, inherited conditions that affect bilirubin handling, and porphyrias that involve the heme pathway. It can aid in differentiating disorders such as Dubin-Johnson syndrome or Rotor syndrome from other causes of jaundice, and it can support the evaluation of suspected hereditary coproporphyria or other porphyrin disorders.
Clinicians use this ratio with your symptoms, physical exam, routine liver tests, and other porphyrin measurements to narrow the diagnosis and guide next steps. It can be useful when jaundice is unexplained, when there is long-standing conjugated hyperbilirubinemia, or when symptoms suggest a porphyria. The test involves collecting all urine over a full day, which is low risk but requires careful handling to ensure accurate results.
Your result is interpreted in the context of your age, medical history, symptoms, and any medications or exposures. A ratio that differs from the expected pattern may point toward a liver transport defect, cholestasis, drug effects, or a porphyria, but it does not by itself make a diagnosis. If the collection was incomplete or the sample was exposed to light or stored improperly, your clinician may recommend repeating the test.
Depending on the pattern, your provider may order additional tests such as total urinary porphyrins, fecal porphyrin profile, plasma fluorescence scanning, routine liver function tests, or genetic testing. If results are abnormal, your care team will discuss what they likely mean for you and whether further evaluation or follow-up is needed.
Reference intervals vary by laboratory, analyzer, methodology, population, and units. The ranges shown here are for education only. Always interpret your results against the reference interval printed on your own lab report.
An incomplete 24 hour collection, exposure to light, or improper storage can degrade porphyrins and distort the ratio. Keep urine refrigerated or on ice as instructed and protect the container from light.
Certain drugs and exposures can alter porphyrin production or excretion, including rifampin, estrogens or hormonal therapies, barbiturates, probenecid, and alcohol. Always share a full medication and supplement list.
Cholestasis, viral or autoimmune hepatitis, biliary obstruction, and inherited bilirubin transport disorders can shift the relative excretion of coproporphyrin isomers and change the ratio.
In porphyrias, porphyrin patterns can vary during symptomatic periods compared with baseline. Testing during or soon after symptoms may be more informative than testing when well.
Reduced kidney function can change urinary elimination of porphyrins, which may affect measured concentrations and the observed ratio.
Physiologic changes in pregnancy and in infants or children can influence porphyrin excretion patterns, so results may be interpreted with age and pregnancy status in mind.
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