Specialty Testing Guide › Heavy Metals Testing
Can seafood make a urine arsenic test look high? Yes, often by a lot. Fish and shellfish contain organic arsenic, mainly arsenobetaine, which the body passes into urine without harm. A total arsenic test counts it anyway, so a meal of fish in the days before testing can produce a result that looks alarming but reflects dinner, not poisoning.
The fix is speciation: a lab test that separates harmless seafood arsenic from the inorganic arsenic that matters for health. Avoiding seafood for a few days before collection also helps.
Key takeaways
- Arsenobetaine from fish and shellfish raises total urine arsenic without causing harm.
- A high total arsenic result should be followed by speciation before anyone calls it a problem.
- Seaweed and some shellfish can raise a form called DMA, which muddies even a speciated result.
- Telling your clinician what you ate in the days before collection is part of reading the result correctly.
Two kinds of arsenic
Arsenic comes in forms with very different risks:
- Inorganic arsenic: the more harmful kind, found in some groundwater, including some private wells. The body converts it to MMA and DMA, which are measured in urine.
- Organic seafood arsenic: mainly arsenobetaine in fish and shellfish, which is not considered toxic and is passed out in urine.
- Arsenosugars: found in seaweed and some shellfish. The body breaks them down partly into DMA.
Urine is the usual specimen for arsenic. Testing is commonly used to assess arsenic exposure, but only a speciated result tells you which kind you are looking at.1
How much seafood can change a result
The effect is large. In a review of patients at an occupational and environmental medicine clinic, eating seafood within two to three days of testing raised total urine arsenic.2 Among patients tested before and after avoiding seafood, total arsenic averaged 291 µg/L without avoidance and 9 µg/L after.2 In paired speciated samples from the same clinic, no inorganic arsenic was found at all, even though the totals were high.
The authors concluded that the elevations were due to harmless seafood arsenic, and that labs should automatically speciate high totals before reporting them, to spare patients needless worry and testing.
Why speciation is needed
A 2023 review of metal biomarkers stated that total urinary arsenic reflects both organic and inorganic exposure, so speciation and adjustment for arsenobetaine are needed in people who eat seafood.3 In practice, a speciated report usually lists:
- inorganic arsenic (arsenite and arsenate);
- MMA and DMA, the body's breakdown products of inorganic arsenic;
- arsenobetaine, and sometimes other organic forms.
The health concern centers on the inorganic forms plus MMA and DMA. A high arsenobetaine level on its own means you ate seafood.
The seaweed and shellfish catch
Speciation is not perfect. Seaweed and some shellfish contain arsenosugars, and the body converts them partly to DMA, the same product made from inorganic arsenic. In one study, volunteers who ate seaweed showed substantial rises in urinary DMA, and the authors warned that standard inorganic arsenic markers are unreliable when arsenosugars are eaten.4
In another volunteer study, eating seafood including seaweed every day for six days raised urinary arsenic, mostly as DMA, and levels returned to baseline by day 10.5 If you eat seaweed snacks, sushi or shellfish often, a raised DMA may reflect diet rather than water or work.
How to prepare for an arsenic test
General points to discuss with your clinician:
- Seafood and seaweed: many labs ask you to avoid fish, shellfish and seaweed for a few days before collection. Ask our team for the exact instructions for the clinic's test.
- Supplements: kelp and other seaweed products contain arsenosugars. Bring a list of everything you take.
- Your water source: if your home is on a private well, say so. Arsenic occurs naturally in some rocks and sediments in Pennsylvania, and a USGS analysis found that nearly 8% of groundwater samples statewide were at or above the federal limit of 10 µg/L.6 Testing the well often tells you more than testing your body.
- Write down your meals: a note of what you ate in the three days before collection makes the result easier to read.
What to do with a high result
If total arsenic is high, ask whether the lab speciated it. If it did not, a repeat sample after avoiding seafood, with speciation, is usually the next step. If inorganic arsenic, MMA or DMA are confirmed high without a dietary explanation, the work turns to finding the source, starting with drinking water. Provoked urine tests, done after a chelating drug, cannot be compared with normal reference values and should not be used to judge arsenic levels.
Next step
For timing, seafood and supplements, read Preparing for a heavy metals test. The clinic's unprovoked test is an $89 add-on; ask our team whether it includes arsenic speciation. More is on the heavy metals testing hub.
Frequently asked questions
Is arsenobetaine dangerous?
It is not considered toxic. It is the main form of arsenic in fish and shellfish, and a high arsenobetaine result tells you about your diet rather than a health problem.
How long should I avoid seafood before an arsenic test?
Seafood eaten within two to three days can raise total arsenic, so many labs ask for a few days without it. Heavy seaweed or shellfish eating can affect results for longer. Follow the instructions for your specific test.
Is a blood test better for arsenic?
Blood can reflect arsenic exposure too, but urine is the specimen most commonly used, and a speciated urine result is what clinicians usually rely on to separate seafood arsenic from inorganic arsenic.
Related reading
This article is for general education and is not medical advice. Decisions about testing and treatment should be made with your clinician. If you have severe symptoms, call 911. For a suspected poisoning, call Poison Control at 1-800-222-1222.
