How to check what purity testing a fish oil has actually had

By Tom Francis·Updated 2026-09-03·3 sources·Editorial policy

Why this matters more for fish oil than for anything else

Most supplement contaminants are introduced in manufacture. Fish oil is different, because the contaminants of concern are in the raw material before anyone processes it.

Mercury, dioxins and PCBs accumulate up the marine food chain, so the species used and where they were caught both matter before refining starts. That is why purity testing is a genuine quality signal for a fish oil rather than a box tick, and why it deserves a page of its own.

What should actually be tested

Four categories, and a good manufacturer will name them rather than say tested for purity.

Per batch against per product

This is the distinction that separates a real statement from a marketing one, and almost nobody asks about it.

Per batch testing means samples from each production run are analysed. A single certificate of analysis obtained once for a product line tells you about one batch from some point in the past. Both can be described as independently lab tested, and only one of them tells you about the tub in your hand. Futuro Labs Omega 3 Fish Oil states per batch testing for heavy metals and microbiology.

What ISO 17025 actually means

It is the international standard for the competence of testing laboratories. A laboratory accredited to it has been assessed on its methods, equipment, staff and quality management.

It says nothing about what was tested for or what the results were. A product stating analysis to ISO 17025 standards is telling you the laboratory is credible, which is worth knowing and is not the same as telling you the results. The useful version names the standard and the tests together, which is what Futuro Labs Omega 3 Fish Oil does.

The words that mean nothing

Pure, ultra pure, purified, premium grade, pharmaceutical grade and molecularly distilled are all marketing terms without regulated definitions in this context.

Molecular distillation is a real process used to concentrate oils and reduce contaminants, and stating it tells you something about processing rather than about results. Pharmaceutical grade has no legal meaning for a food supplement at all. A statement worth reading names the contaminant, the frequency and the standard.

Oxidation, which nobody checks

Fish oil goes rancid. Peroxide value and anisidine value are the standard measures, and very few consumer products publish them.

The practical proxy is taste and smell. Strong fishy repeat, a smell on opening a tub, or an off taste from a broken capsule all suggest oxidation. Storage matters more than most people think: heat and light accelerate it, so a tub kept in a cupboard rather than on a windowsill or in a car will be better through to the end of its 90 days.

What purity testing does not cover

Banned substances. This is the recurring point on this site and it is worth repeating on a page specifically about testing.

Heavy metal and microbiological testing confirms the oil is clean of contaminants and says nothing about prohibited substances. Futuro Labs Omega 3 Fish Oil is batch tested for contaminants to ISO 17025 standards and is not batch certified for sport, and neither are Futuro Labs Magnesium Glycinate, Futuro Labs Marine Collagen or Futuro Labs Lion's Mane. A player subject to testing needs a certified product.

The whole check

• Look for named contaminants rather than the phrase tested for purity.
• Check whether testing is per batch or a one off certificate.
• Look for a laboratory standard such as ISO 17025.
• Treat pure, premium and pharmaceutical grade as marketing.
• Check whether oxidation markers are published, and use taste and smell as a proxy.
• Remember that none of it covers banned substances.
• Store the tub away from heat and light.

Where to go next

• How to read EPA and DHA on a fish oil labelThe dose side of the same label.
• How to spot an underdosed fish oilThe other way a fish oil disappoints.
• How to check whether a supplement is batch testedThe banned substance check, which purity testing does not cover.

Questions players ask

Sources

• EFSA register of nutrition and health claims
• Food supplements guidance, gov.uk
• UK Anti-Doping, supplements