Rheumatoid factor (RF) in dogs and cats
Rheumatoid factor (RF) is an antibody that can appear in the blood in immune-mediated diseases. In dogs it is tested when the vet suspects rheumatoid arthritis, a rare erosive immune-mediated joint disease. RF is neither a reliable yes nor a reliable no: many dogs with erosive arthritis test negative, and other joint diseases can raise it, so RF alone is not used to make the diagnosis.
What it shows
Immune-mediated polyarthritis is inflammation of several joints driven by the immune system rather than by infection. It comes in two broad forms: nonerosive, where the joint surfaces stay intact, and erosive, where the bone at the joint is gradually destroyed. In dogs, rheumatoid arthritis is an erosive form.
RF is one of the blood antibody tests that can help confirm immune-mediated joint disease. Some studies in dogs look at RF of the IgM class. The main evidence for erosive arthritis, however, comes from elsewhere: joint fluid with increased cells, mostly neutrophils, and X-rays that show bone loss inside the joint. RF is an extra piece of the picture.
Higher than reference — what it may be a sign of
- Rheumatoid arthritis. Dogs with rheumatoid arthritis had higher rheumatoid factor levels than healthy dogs, both in serum and, to a lesser degree, in joint fluid.
- Other joint diseases. Increases were generally also seen in dogs with other joint diseases; the study compared dogs with infective arthritis and osteoarthritis. Researchers read RF in non-rheumatoid joint disease as a sign of nonspecific inflammation.
Neither rheumatoid factor nor antinuclear antibody titers alone are used to diagnose rheumatoid arthritis or lupus.
Lower than reference
A negative RF does not rule out erosive arthritis. In one series of 13 dogs with erosive immune-mediated polyarthritis, only 1 of the 4 dogs tested was RF-positive; the authors cite another study in which 6 of 22 dogs with erosive polyarthritis (27.3%) had RF of the IgM class. So a dog can have the disease with a negative test, and the diagnosis rests on joint fluid and X-rays.
Look at together with
- Joint fluid analysis — the key test: cell count, cell types and viscosity, plus a check for infection. In the series above, a count of 3,000 cells/µL or more defined immune-mediated polyarthritis.
- X-rays of several joints, including both wrists — erosive changes and joint collapse are what separate the erosive form.
- ANA — to look at lupus, a nonerosive cause of polyarthritis.
- CRP and globulins — general markers of inflammation; one reported dog with rheumatoid arthritis had raised gamma globulins on electrophoresis.
Units and conversion
Results have no unit. RF is reported as positive or negative, sometimes as a titer — the highest dilution of serum, written 1:N, at which the test still reacts; the larger N, the more RF. In the app the result is stored as text.
Reference range
The reference range is on your lab’s form: the lab states the titer or cut-off from which it calls RF positive. Methods and cut-offs differ between labs, so a result is read against its own form, and always together with joint fluid, X-rays and the dog’s signs.
Cats and dogs
Cats also get immune-mediated polyarthritis. Their erosive forms are called feline rheumatoid-like arthritis and feline chronic progressive polyarthritis; nonerosive forms include lupus, idiopathic and reactive polyarthritis. In cats the diagnosis is based on joint fluid analysis, which shows high cell counts, mainly neutrophils when there is no infection. The open sources describe RF testing mainly in dogs.
Canine rheumatoid arthritis is rare. It starts as lameness that shifts from leg to leg with swelling around the joints, then settles in particular joints over weeks or months; the wrists, ankles and toes are most often affected. Early X-rays show soft-tissue swelling and thinner bone near the joint, later erosions and deformity. Erosive disease typically affects small-breed dogs, nonerosive disease usually large-breed dogs.
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
- Merck Veterinary Manual — Fever of unknown origin in animals
- Merck Veterinary Manual — Hypersensitivity diseases in animals (canine rheumatoid arthritis)
- Merck Veterinary Manual — Immune-mediated polyarthritis in dogs and cats
- Shaughnessy et al., JAVMA 2016 — Clinical features and pathological joint changes in dogs with erosive immune-mediated polyarthritis: 13 cases (PMC)
- Carter et al., Ann Rheum Dis 1989 — Immune complexes and rheumatoid factors in canine arthritides (PMC)
- Merck Veterinary Manual — Disorders involving immune complexes (type III reactions) in dogs
- Heuser, Can Vet J 1980 — Canine rheumatoid arthritis (PMC)
- VCA Animal Hospitals — Testing for lameness
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.