How to Read the Oxidation Values on a Dog Joint Supplement COA

A 9-year-old Labrador mix came in for a recheck last spring with a new jar of joint chews and an owner who was doing everything right. She had switched foods, added short morning walks, kept his weight at 82 pounds instead of the 88 pounds on his chart from November, and she wanted to know why he still turned away from the chews after two weeks. The label listed glucosamine, chondroitin, and fish oil, and the fish oil line was the part I circled. When marine oils oxidize, dogs often notice before we do, and the National Animal Supplement Council reminds buyers that quality systems matter as much as ingredient names for that exact reason.

You do not need a chemistry degree to read that part of a Certificate of Analysis. You need to know which three numbers describe freshness, what units they use, and when to ask the company for a newer lot.

So what is the lab actually measuring?

A COA for a joint product with fish oil, krill oil, or green-lipped mussel oil will often list identity and potency first, then a small oxidative-stability section. Think of it as a freshness report for fat.

Peroxide value (PV) captures primary oxidation, the first hydroperoxides that form when oxygen attacks EPA and DHA. It is reported as milliequivalents per kilogram (mEq/kg). Low means recently intact oil. Very low is what you want to see on a fresh lot.

Anisidine value (AV) captures secondary oxidation, the aldehydes and ketones that create rancid odor and off taste after peroxides break down. It has no units, reported as a plain number. This is the number that tracks with that painty, fishy smell.

TOTOX combines them into one score for total oxidation. The formula is simple:

TOTOX = (2 x peroxide value) + anisidine value

Acid value is related but different. It reflects free fatty acids from hydrolysis, reported as mg KOH/g. High acid value can mean poor refining, moisture exposure, or age, and it often travels with poor palatability.

For context on why EPA and DHA deserve that fuss, the NIH Office of Dietary Supplements omega-3 fact sheet reviews how readily these long-chain fats participate in normal inflammatory pathways and why dose and form matter for supporting normal function.

What counts as fresh on paper?

Joint brands rarely print limits on the front label, so compare the COA numbers against the voluntary monograph widely used for marine oils. The GOED Omega-3: GOED Voluntary Monograph sets commonly cited limits for refined oils at peroxide value under 5 mEq/kg, anisidine value under 20, TOTOX under 26, and acid value under 3 mg KOH/g.

Those are human-food oil limits, not a legal pet-supplement standard, and I use them as a screening line. A fresh lot often runs well below them, with peroxide around 1 to 3 and anisidine in single digits. A TOTOX in the teens on a recent manufacture date is reassuring. A TOTOX creeping toward that upper limit on a product with 14 months left on its best-by date deserves a question, not instant rejection.

My own evidence-bounded rule is blunt. I prioritize low oxidation numbers over a slightly higher milligram claim of EPA plus DHA, because oxidized oil does not support normal joint comfort and mobility as intended and dogs are less likely to eat it consistently. The skeptical friend asks whether that is fussy, and my answer is practical: consistent intake at a verified dose beats a heroic label dose the dog sniffs once and abandons.

The second case that taught me that involved a 5-year-old Australian shepherd who loved training treats and hated one particular salmon-based joint chew. Her owner had bought the large 120-count jar to save money, stored it above the stove through July and August, and by week six the chews smelled sharp every time she opened the lid. She had logged 41 training sessions with those chews and only 11 clean takes in the final two weeks. Heat and repeated oxygen exposure will move those numbers even when the starting COA looked fine, which is why I now tell owners to judge the COA plus storage and package design together.

How do I read the COA line by line without getting fooled?

Start with lot match and date. A COA belongs to one lot, not to a brand forever. Check that the lot number matches your jar, note the manufacture date and test date, and look for a third-party lab name rather than testing done only in-house. My explainer on cGMP Certification vs. NASC Membership: What Each Actually Verifies on a Dog Supplement Label walks through why those quality programs support label accuracy but do not by themselves prove freshness.

Then read potency before oxidation. Confirm EPA plus DHA per chew in milligrams, not just total fish oil. A panel that says 300 mg fish oil but only 36 mg combined EPA and DHA tells a different story than one delivering 150 mg combined EPA and DHA. Our guide to Proprietary Blend Versus Fully Disclosed Ingredients on Dog Probiotic Labels makes the same point for a different category, and the logic holds here: undisclosed splits hide weak doses.

Then read the oxidation block:

• Look for units. Peroxide without mEq/kg or acid value without mg KOH/g is incomplete.
• Calculate TOTOX yourself if the lab omitted it. Twice peroxide plus anisidine takes ten seconds.
• Compare oils fairly. Krill and mussel matrices read differently than refined triglyceride fish oil, so use trends over time rather than demanding identical numbers across oil types.
• Watch missing AV. A COA that reports only peroxide is showing the early half of the story. Secondary oxidation is where palatability lives, so ask for anisidine if it is absent.

If you want a broader filter for what else hides behind a pretty front label, my notes on Red Flag Fillers to Avoid on a Dog Supplement Ingredient List pair well with this freshness check.

The methods behind those numbers are standardized titrations and spectrophotometric tests maintained by groups such as AOCS Methods for fats and oils, which is why results should be comparable across competent labs when sampling and storage are handled correctly.

A third situation bends the usual advice. A hospice foster I advise cares for a 13-year-old beagle mix with a sensitive stomach who needs joint support but cannot tolerate a full fish-oil load. Her regular COA looked excellent, with a TOTOX of 12 on a lot made 3 months earlier, yet even fresh oil at the full 2-chew serving caused soft stool within 36 hours. We kept the fresh lot, dropped to half a chew with breakfast for 10 days, then reassessed mobility on a 0.4-mile loop she knows by heart. Freshness still mattered, but tolerance set the dose, and her veterinarian adjusted the plan to support normal comfort without pushing through GI upset.

When should you keep the jar, ask questions, or move on?

Keep the jar when the lot matches, dates are recent, EPA and DHA are stated clearly, and peroxide, anisidine, TOTOX, and acid value sit comfortably under monograph limits.

Ask questions when numbers sit near limits, when anisidine is missing, when the COA is older than the lot in your hand, or when the chews smell strongly fishy despite decent paper. Ask for the finished-product COA rather than only a raw-oil COA, ask how the company flushes or seals packaging with nitrogen, and ask about recommended storage after opening.

Move on when the company cannot produce a lot-specific COA, when TOTOX exceeds limits by a wide margin, or when palatability has clearly failed. For a deeper dose-by-dose view of what else to demand from the glucosamine and chondroitin side of the formula, see The Best Joint Supplement for Dogs: A Vet’s Honest, Dose-by-Dose Breakdown.

Fresh oil will not rebuild cartilage overnight. It simply gives the rest of a sensible joint plan — weight control, measured exercise, and veterinary follow-up — a fair chance to support normal mobility without fighting rancidity at every meal.

Frequently asked questions

Where do I find peroxide and anisidine values for my dog's joint chew?

Ask the brand for the finished-product COA for your exact lot number. Many companies email it on request, and third-party-tested brands often post it by lot on their website.

What TOTOX should I hope to see on a fresh joint supplement?

Lower is better, and many fresh refined fish oils test well under the commonly cited GOED limit of 26. I get cautious when TOTOX approaches or exceeds that line, especially with months left before expiration.

Can a good COA still smell fishy at home?

Yes. Heat, air, and time after opening can oxidize oil after a clean lab result. Reseal tightly, store cool and dark, use within the stated in-use period, and contact the company if strong odor develops quickly.

Sources

1. Omega-3 Fatty Acids Fact Sheet — NIH Office of Dietary Supplements
2. Quality and Safety Standards — National Animal Supplement Council
3. GOED Voluntary Monograph for Omega-3 Oils — GOED Omega-3
4. Methods for Analysis of Fats and Oils — AOCS