Acetylcholine receptor antibodies (myasthenia gravis test) in dogs and cats

What it shows

When the antibodies bind to the receptors, the receptors are blocked or destroyed and the nerve signal no longer reaches the muscle well. Typical signs are generalised weakness and fatigue after mild exercise; in dogs the gullet often widens and loses tone (megaesophagus), which causes regurgitation. There is also a focal form, with weakness of the muscles used to bark, chew and swallow while the legs are spared.

The test measures how much antibody against acetylcholine receptors is in the serum, using a method called immunoprecipitation radioimmunoassay. It is species-specific and is considered the gold standard for acquired myasthenia gravis. It is a specialised test sent to a reference laboratory; results usually take several working days.

Higher than reference — what it may be a sign of

• Above the lab’s cut-off — supports acquired myasthenia gravis; false positives are rare.
• Height of the titer — across animals, the level of antibody does not match how severe the disease is. In one animal, however, the titer follows the course of the disease well, which is why it is measured repeatedly.
• A thymus tumour — acquired myasthenia gravis can accompany a thymoma, a tumour in the chest. A thymoma is found in approximately 3% of dogs and 30% of cats with myasthenia gravis. If the titer stays high for more than a year, a search for a tumour such as a thymoma is advised.

Lower than reference

• Negative in a dog with signs — about 98% of dogs with generalised myasthenia gravis are seropositive; approximately 2% are seronegative, and the disease is still clinically suspected in them.
• Early in the disease — false negatives happen especially early in the course. If signs began recently and the test is negative, retesting in about 3 weeks is suggested.
• After immunosuppressive medication — corticosteroids at immunosuppressive levels for longer than 7–10 days lower the antibody level and can make the test negative. Medicines of the anticholinesterase group do not affect the test.
• Inherited form — the test does not detect congenital myasthenia gravis, which is not antibody-driven.
• Remission — as a dog goes into clinical remission, the titer falls into the reference range.

Look at together with

• T4 and TSH — a screen for concurrent problems such as hypothyroidism is advised.
• Chest X-rays — to look for megaesophagus, aspiration pneumonia and a mass in the chest.
• A short drug-response test in the clinic — weakness that improves within about 30 minutes after a short-acting medicine also supports the diagnosis.

Units and conversion

The result is given in nmol/L (nanomoles per litre). This is the only unit in use for the test, so no conversion is needed; in the app the value is stored and charted in nmol/L.

Reference range

The reference range is on your lab’s form, and the cut-off differs between dogs and cats. At the UC San Diego neuromuscular laboratory, a titer above 0.6 nmol/L is positive in the dog, and one of its case reports gives the feline reference as below 0.3 nmol/L. A value close to the cut-off, or a negative value with typical signs, is a reason for the vet to look at the whole picture and possibly retest.

Cats and dogs

Sources

1. UC San Diego Comparative Neuromuscular Laboratory — Frequently asked questions (sample and AChR antibody test)
2. UC San Diego Comparative Neuromuscular Laboratory — Most commonly asked questions about acquired myasthenia gravis
3. UC San Diego Comparative Neuromuscular Laboratory — Genetic analysis of dogs with acquired myasthenia gravis
4. UC San Diego Comparative Neuromuscular Laboratory — Glossary for myasthenia gravis
5. UC San Diego Comparative Neuromuscular Laboratory — Myasthenia gravis and cranial mediastinal mass in a cat
6. Merck Veterinary Manual — Inflammatory disorders of the peripheral nerves and neuromuscular junction in animals
7. VCA Animal Hospitals — Myasthenia gravis in dogs