Lyme disease tests (C6 antibody and Quant C6) in dogs and cats

What it shows

Both lines on the form are antibody tests: they look for the body’s answer to the bacterium, not for the bacterium itself.

• C6 antibody — a screening test for antibodies against C6, a small part of a surface protein of B. burgdorferi that stays the same between strains. It is usually the first test, often done in the clinic as part of a combined tick-borne disease test. C6 is not part of Lyme vaccines, so a vaccinated dog does not test positive because of the vaccine. Older tests for antibodies to the whole bacterium could react to vaccine antibodies and to related bacteria.
• Quantitative C6 (Quant C6) — the same antibody, but measured as a level in U/mL. It is usually ordered after a positive screening test. The manufacturer states that the C6 level reflects how many live bacteria there are, and that it falls sharply after successful therapy, so a baseline level lets the vet follow the dog over time.

Higher than reference — what it may be a sign of

• Positive C6 — the dog has been exposed to B. burgdorferi. It does not prove that the bacterium causes the dog’s current signs: most seropositive dogs have no signs at all.
• Signs that fit Lyme disease — fever, lameness that shifts from leg to leg and swollen joints, usually 2–5 weeks after the tick passed the bacterium on. In such a dog, a positive test supports the diagnosis.
• Kidneys — a rare protein-losing kidney disease has been linked to the bacterium in dogs, although a causal link has not been proven. This is why the vet checks urine for protein in a seropositive dog.
• Quant C6 level — IDEXX, the maker of the test, reads 30 U/mL or more as supporting Lyme disease when signs or other results also fit. It suggests repeating the test 6 months after the course prescribed by the vet ends; a fall of 50% or more from the first level is read as a good response.

Lower than reference

A negative result is the expected one. Early after infection, in the first month, antibodies may not have formed yet, so a test can be falsely negative. A Quant C6 level below 30 U/mL is read by the maker as not supporting Lyme disease, though a low level can also appear early in infection. The vet decides whether to repeat the test.

Look at together with

• Other tick-borne infections — the same ticks can carry other agents, so a positive dog is checked for anaplasma and ehrlichia; combined in-clinic tests often report them together.
• Urine protein and the urine protein-to-creatinine ratio — to look for protein loss through the kidneys.
• Creatinine, SDMA and a complete blood count — part of the usual work-up of a seropositive dog.

Units and conversion

The C6 screening test is qualitative and has no unit: it is reported as positive or negative. The Quant C6 test prints a number in U/mL, and the lab’s comment says how it reads that number. In the app both results are stored as text, as printed on the form.

Reference range

The reference range is on your lab’s form; for the screening test it is “negative”. For the quantitative test, the form gives the lab’s cut-off. Any result is read together with the dog’s signs, other tick-borne tests and a urine test.

Cats and dogs

Sources

1. Merck Veterinary Manual — Lyme borreliosis in animals
2. CAPC (Companion Animal Parasite Council) — Lyme disease guidelines
3. IDEXX — Lyme disease management with the Lyme Quant C6 Test
4. IDEXX — Lyme disease: next steps to interpret results