Urine Arsenic Unit Converter
Urine Arsenic Unit Converter
Convert urine arsenic between µg/L, ng/mL, µg/dL, nmol/L and µmol/L — and find out why a TOTAL urine arsenic raised a day after a shellfish meal is the commonest false positive in occupational medicine.
Urine Arsenic converter
Mass ⇄ molarUrine total arsenic 34.9 µg/L, ARUP’s upper reference limit
The conversion, and the measurand it hides
µg/L = nmol/L × 0.0749216
µmol/L = µg/L × 0.0133473 · µg/dL = µg/L ÷ 10
- 13.3473
- from arsenic’s atomic weight, 74.921595. The factor is the same on a urine and a blood report, because the element has not changed — only the specimen and the interval have
- total vs speciated
- the real measurand question. Total arsenic sums inorganic arsenite and arsenate, their methylated metabolites MMA and DMA, and the non-toxic arsenobetaine and arsenocholine from seafood
- per litre, not per day
- this page converts a concentration. ARUP publishes the same test three ways and the three cannot be converted into each other without the collection volume or the creatinine
Worked example
Urine total arsenic 34.9 µg/L, ARUP's upper reference limit
34.9 × 13.3473 = 465.8 nmol/L
= 34.9 ng/mL = 3.490 µg/dL = 0.4658 µmol/L
At ARUP's limit for total arsenic, and at the ACGIH biological exposure index figure of 35 µg/L — but the BEI counts only the toxic species, so the two numbers are not comparable without speciation
The same test, published four ways, for four different quantities
| Source | Figure | Quantity |
|---|---|---|
| ARUP 0025000 | ≤ 34.9 µg/L | Total arsenic, per litre |
| ARUP 0025000 | ≤ 49.9 µg/d | Total arsenic, per 24 hours |
| ARUP 0025000 | ≤ 29.9 µg/g creatinine | Total arsenic, ratio |
| Mayo ASU24, adults | < 35 µg/24 h | Total arsenic, per 24 hours |
| Mayo SPASU | < 35 µg/L | Toxic species only, per litre |
| ACGIH BEI (via Mayo ASOU) | < 35 µg/L, end of work week | Toxic species, occupational surveillance |
What a seafood meal does to a total urine arsenic
| Observation | Figure | Source |
|---|---|---|
| Urine arsenobetaine, median | 51.6 nmol/L (3.87 µg/L) | SAS |
| Urine arsenobetaine, 95th centile | 1,690 nmol/L (127 µg/L) | SAS |
| Total urine arsenic after a seafood meal | over 300 µg/24 h | Mayo ASU24 |
| Seafood-rich diets | "several thousand nmol/L" | SAS |
| Recommended abstinence before collection | 48 – 72 h (ARUP); 5 days (SAS) | ARUP, SAS |
Total urine arsenic is the wrong test, and the conversion will not save it
Urine arsenic in µg/L becomes nmol/L on multiplication by 13.3473, the same factor the blood arsenic page uses, because the element is the same. The arithmetic is the easy part of this test. The hard part is that two assays go by the same name and only one of them is interpretable.
A total urine arsenic sums everything arsenical in the sample. That includes the toxic inorganic species, arsenite and arsenate, and their partially detoxified metabolites monomethylarsonic acid and dimethylarsinic acid — and it also includes arsenobetaine and arsenocholine, the organic arsenic in fish and shellfish, which Mayo describes as "relatively nontoxic" and the SAS laboratory as excreted unchanged. The magnitude of the confounding is not subtle. Mayo states that after a seafood meal urinary arsenic "may increase to over 300 mcg/24 h", against an upper reference limit of 35. The SAS laboratory’s own speciation data put urinary arsenobetaine at a median of 51.6 nmol/L and a 95th centile of 1,690 — a 33-fold spread driven by diet alone — and notes that seafood-rich diets have produced "several thousand nmol/L".
This is why the speciated test exists and why it is what an occupational physician orders. Mayo’s total-arsenic test reflexes automatically to speciation at 10 µg/L; ARUP fractionates any elevated total. The ACGIH biological exposure index is written against "the sum of all the toxic species" rather than against total arsenic, for exactly this reason. Mayo’s speciation report adds that inorganic arsenic at or above 20 µg/L is considered toxic, and that there is no upper limit for the organic forms because they are not toxic.
Two practical consequences. A raised total arsenic in a patient who ate fish within three days is not evidence of exposure and should be repeated on a seafood-free collection or speciated — rice also contributes. And check which quantity your report gives: ARUP publishes this test per litre, per 24 hours and per gram of creatinine, with three different limits.
Frequently asked questions
How do I convert urine arsenic from µg/L to nmol/L?
Multiply by 13.3473, from arsenic’s atomic weight of 74.921595. A urine arsenic of 34.9 µg/L is 465.8 nmol/L. The factor is the same as on a blood report — the element has not changed, only the interval.
Why does seafood raise a urine arsenic result?
Fish and shellfish contain arsenobetaine, an organic arsenic compound that is non-toxic and excreted unchanged, and a total-arsenic assay counts it. Mayo reports that urinary arsenic may exceed 300 µg/24 h after a seafood meal, against an upper limit of 35. ARUP asks for 48 to 72 hours of abstinence and the SAS laboratory asks for five days.
What is the difference between total and speciated urine arsenic?
Total arsenic sums every arsenical present, toxic and non-toxic. Speciation separates inorganic arsenite and arsenate and their methylated metabolites MMA and DMA — the toxic sum — from arsenobetaine and arsenocholine. Mayo reflexes to speciation when total arsenic reaches 10 µg/L.
Can I convert a urine arsenic concentration to a daily excretion?
Not from the concentration alone. A µg/24 h figure needs the measured volume of the complete collection; a µg/g creatinine figure needs the creatinine on the same specimen. ARUP publishes all three with different limits — 34.9 µg/L, 49.9 µg/d and 29.9 µg/g creatinine — because they are different quantities.
Is the ACGIH biological exposure index a normal range?
No. A BEI is a surveillance figure for a workforce measured at a defined point in the working week, not a reference interval and not a clinical threshold. The arsenic BEI as Mayo states it is under 35 µg/L of the toxic species at the end of the work week.
Related calculators
References
- ARUP Laboratories. Arsenic, urine with reflex to fractionated. Laboratory Test Directory 0025000; 2026. "Less than or equal to 34.9 microg/L", "…49.9 microg/d" and "…29.9 microg/g CRT"; "For specimens with elevated total arsenic results, fractionation is automatically performed".
- Mayo Clinic Laboratories. Arsenic with reflex, 24 hour, urine (ASU24) — reference values and cautions. Test catalogue, test ID 48537; 2026. "≥18 years: <35 mcg/24 h"; speciation reflexes at 10 mcg/L; "After a seafood meal…the urine output of arsenic may increase to over 300 mcg/24 h".
- Mayo Clinic Laboratories. Arsenic speciation, 24 hour, urine (SPASU). Test catalogue, test ID 609383; 2026. "Toxic arsenic (combined inorganic and methylated forms): <35 mcg/L"; "Concentrations of 20 mcg inorganic arsenic per liter or higher are considered toxic"; "There is no limit to the normal range for the organic forms…since they are not considered toxic".
- Mayo Clinic Laboratories. Arsenic occupational exposure, random, urine (ASOU). Test catalogue, test ID 608890; 2026. "Biological Exposure Indices (BEI): <35 mcg/L at end of work week", being "the sum of all the toxic species (inorganic arsenic plus methylated arsenic metabolites)".
- Supraregional Assay Service, Trace Element laboratory. Arsenic. sas-centre.org; accessed October 2026. Whole blood 14–95 nmol/L, serum or plasma 20–100 nmol/L; "Diets rich in seafood give misleading results (several thousand nmol/L have been reported) due to presence of non-toxic organo-arsenic compounds"; urine arsenobetaine median 51.6 nmol/L, 95th centile 1690 nmol/L.
- Commission on Isotopic Abundances and Atomic Weights, IUPAC. Standard Atomic Weights 2024. ciaaw.org; accessed October 2026. As 74.921595(6), Mn 54.938043(2), Al 26.9815384(3), Cr 51.9961(6), Co 58.933194(3), Tl [204.382, 204.385]; for the interval elements the IUPAC abridged values Tl 204.38, C 12.011, N 14.007 are used.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/