Urinary 5-HIAA Unit Converter
Urinary 5-HIAA Unit Converter
Convert urinary 5-HIAA between mg/L, mg/dL, µmol/L and mmol/L — then turn the concentration into the 24-hour excretion your laboratory actually reports, using the collection volume.
Urinary 5-HIAA converter
Mass ⇄ molarUrinary 5-HIAA 8 mg/L in a 2.4 litre 24-hour collection
The units, and the step from concentration to excretion
derived from a molecular weight of 191.18 Da for 5-hydroxyindoleacetic acid (C₁₀H₉NO₃)
excretion (mg/24 h) = concentration (mg/L) × collection volume (L)
excretion (µmol/24 h) = concentration (µmol/L) × collection volume (L)
- × 5.23067
- the molar factor, from a molecular weight of 191.18 daltons. 5-HIAA is a small, well-defined molecule, so the conversion is exact: 10 mg/L is 52.31 µmol/L
- mg/dL = mg/L ÷ 10
- occasionally seen on North American reports. A 5-HIAA of 8 mg/L is 0.8 mg/dL
- × the volume, to get an excretion
- this is the step most readers actually need. A concentration of 8 mg/L in a 2.4 litre collection is 8 × 2.4 = 19.2 mg/24 h. Going the other way, a reported 12 mg/24 h from a 1.5 litre collection was a concentration of 8 mg/L
- why no reference interval is printed here
- because the published intervals are excretions, not concentrations, and the two are different quantities. Mayo’s cut-offs run from about 6 to 9 mg/24 h depending on age and sex; a concentration cannot be compared with them without the collection volume, so this page gives the arithmetic and the excretion table rather than a band the converter would apply to the wrong quantity
- an incomplete collection is the commonest error
- a missed void lowers both the total volume and the measured excretion, so the result reads falsely low. Measuring the urine creatinine on the same specimen is the standard check of collection adequacy — a 24-hour creatinine well below the expected value for the patient’s weight and sex means the collection, not the tumour, produced the number
Worked example
Urinary 5-HIAA 8 mg/L in a 2.4 litre 24-hour collection
8 mg/L × 5.23067 = 41.85 µmol/L, the molar concentration
8 mg/L = 0.800 mg/dL, and 41.85 µmol/L = 0.0419 mmol/L — the same result in the other two conventions
Now the step that matters: 8 mg/L × 2.4 L = 19.2 mg per 24 hours, and 41.85 µmol/L × 2.4 L = 100.4 µmol per 24 hours. That is the number the laboratory reports and the number the reference interval applies to
Against Mayo's 99th-centile cut-offs — for example ≤9.2 mg/24 h for a man of 50 — an excretion of 19.2 mg/24 h is roughly double the upper limit and warrants investigation
Two checks before believing it. Was the diet restricted? Serotonin-rich foods and several drugs raise 5-HIAA. And was the collection complete? A urine creatinine on the same specimen answers that, and an incomplete collection is the commonest cause of a falsely low result
Mayo’s 24-hour excretion cut-offs (99th centile, mg/24 h)
| Patient | Upper limit (mg/24 h) | Equivalent µmol/24 h |
|---|---|---|
| Female aged 20 | 6.1 | 31.9 |
| Male aged 20 | 6.9 | 36.1 |
| Female aged 50 | 7.7 | 40.3 |
| Male aged 50 | 9.2 | 48.1 |
What must be restricted before the collection, and why
| Restriction | Detail |
|---|---|
| Serotonin-rich foods | Bananas, avocado, pineapple, kiwi, melon, plums, walnuts, pecans and tomatoes are the classic offenders; they contain serotonin that is metabolised to 5-HIAA and excreted |
| Mayo’s instruction | For 48 hours before and during the collection, avoid or limit to one serving daily all fruits, all vegetables, all nuts and caffeinated food and drink |
| Drugs | Avoid paracetamol (acetaminophen) and tryptophan-containing supplements where medically feasible; several other drugs interfere in either direction, so review the medication list against the laboratory’s own list |
| Preservative | Mayo adds 25 mL of 50% acetic acid to the container at the start of the collection (15 mL for children under 5); alternative acid preservatives are acceptable |
| Completeness | Every void for the full 24 hours. A missed void lowers the result; a urine creatinine on the same specimen is the standard adequacy check |
A metabolite, a collection, and two different quantities
5-hydroxyindoleacetic acid is the principal metabolite of serotonin, produced by monoamine oxidase and aldehyde dehydrogenase and excreted in the urine. Because it integrates serotonin production over the whole collection period rather than sampling it at one instant, the 24-hour urinary 5-HIAA is the biochemical test of choice for midgut carcinoid, and it is a better test than blood serotonin for the same reason that IGF-1 is a better test than growth hormone: it averages rather than snapshots. Blood serotonin, converted on its own page here, is confounded by the platelet count and is a secondary measurement. Chromogranin A sits beside 5-HIAA as the general neuroendocrine marker.
This record converts a concentration, and that needs stating clearly because laboratories report something else. The conversion between mg/L and µmol/L is exact — 5-HIAA is a small molecule of 191.18 daltons, so multiplying by 5.23067 is all there is to it, and 10 mg/L is 52.31 µmol/L. But the reference intervals published for this test are excretions: milligrams or micromoles per 24 hours. A concentration and an excretion are different quantities, and the bridge between them is the volume of the collection. Multiply the concentration in mg/L by the collection volume in litres and the answer is the excretion in mg per 24 hours: 8 mg/L in a 2.4 litre collection is 19.2 mg/24 h. Divide the other way to go back. Because this page cannot know the collection volume, it deliberately prints no reference band — a band would be applied to the wrong quantity — and gives the arithmetic and the excretion table instead.
The commonest cause of a falsely low 5-HIAA is an incomplete collection. A missed void removes both urine and metabolite, and the reported excretion falls accordingly; in a patient with carcinoid that can turn a clearly abnormal result into an apparently normal one. The routine safeguard is to measure creatinine on the same specimen, because 24-hour creatinine excretion is reasonably predictable from weight, age and sex, and a creatinine well below the expected value marks the collection as short. Any 24-hour urine collected to answer an important question should be judged for adequacy before its result is believed.
Diet is the error in the opposite direction, and it is entirely avoidable. Serotonin-rich foods — bananas, avocado, pineapple, kiwi, melon, plums, walnuts, pecans, tomatoes — contain serotonin that is metabolised to 5-HIAA and appears in the urine, and several drugs, paracetamol and tryptophan supplements among them, interfere as well. Mayo Clinic Laboratories asks patients to avoid, or limit to one serving a day, all fruits, all vegetables, all nuts and caffeinated food and drink for 48 hours before and throughout the collection, and to collect into a container with acid preservative. An unrestricted collection produces an elevated 5-HIAA in somebody who has no tumour, which is an expensive and frightening result to have to walk back.
Frequently asked questions
How do I convert urinary 5-HIAA from mg/L to µmol/L?
Multiply by 5.23067, from a molecular weight of 191.18 daltons. A concentration of 10 mg/L is 52.31 µmol/L, and 8 mg/L is 41.85 µmol/L. The conversion is exact, because 5-HIAA is a small, well-defined molecule rather than a peptide with a nominal mass.
How do I turn a 5-HIAA concentration into a 24-hour excretion?
Multiply the concentration by the collection volume in litres. A 5-HIAA of 8 mg/L in a 2.4 litre collection is 8 × 2.4 = 19.2 mg per 24 hours, or 100.4 µmol per 24 hours. Laboratories report the excretion, and the reference intervals apply to the excretion, not to the concentration this page converts.
What is a normal 24-hour urinary 5-HIAA?
Mayo Clinic Laboratories publishes age- and sex-specific 99th-centile cut-offs of roughly 6 to 9 mg per 24 hours for adults — for example 6.1 mg/24 h for a woman of 20 and 9.2 mg/24 h for a man of 50. Because those are excretions, a concentration must be multiplied by the collection volume before it can be compared with them.
What foods must be avoided before a 5-HIAA collection?
Serotonin-rich foods: bananas, avocado, pineapple, kiwi, melon, plums, walnuts, pecans and tomatoes are the classic ones. Mayo asks patients to avoid, or limit to one serving daily, all fruits, all vegetables, all nuts and caffeinated food and drink for 48 hours before and during the collection, and to avoid paracetamol and tryptophan supplements where feasible.
Why might a 5-HIAA result be falsely low?
Almost always an incomplete collection — a missed void removes urine and metabolite together, so the reported excretion falls. It is the commonest cause of a falsely low result. Measuring creatinine on the same specimen checks collection adequacy, because 24-hour creatinine excretion is predictable from weight, age and sex.
Is urinary 5-HIAA better than blood serotonin for carcinoid?
Yes, and it is the preferred test. 5-HIAA integrates serotonin production across the whole collection period, whereas whole-blood serotonin is largely a platelet measurement and moves with the platelet count. Chromogranin A is measured alongside 5-HIAA as the general neuroendocrine tumour marker.
Related calculators
References
- Mayo Clinic Laboratories. Test HIAA: 5-Hydroxyindoleacetic Acid, 24 Hour, Urine — age- and sex-specific 99th-centile reference values in mg/24 h; 25 mL of 50% acetic acid preservative; dietary and drug restrictions for 48 hours before and during collection. Accessed 2026.
- Adaway JE, Dobson R, Walsh J, et al. Serum and plasma 5-hydroxyindoleacetic acid as an alternative to 24-h urine 5-hydroxyindoleacetic acid measurement. Ann Clin Biochem. 2016;53(5):554–560.
- O’Toole D, Grossman A, Gross D, et al. ENETS consensus guidelines for the standards of care in neuroendocrine tumors: biochemical markers. Neuroendocrinology. 2009;90(2):194–202.
- Feldman JM, Lee EM. Serotonin content of foods: effect on urinary excretion of 5-hydroxyindoleacetic acid. Am J Clin Nutr. 1985;42(4):639–643.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.