Leptospirosis tests (MAT and PCR) in dogs and cats

What it shows

• MAT (microscopic agglutination test) — a laboratory test for antibodies. The dog’s serum is diluted step by step and mixed with live leptospires of several types (serovars); the result is the highest dilution at which antibodies still clump them. Antibodies take time to form, so in the first week of illness the titer is often negative or low. The standard approach is two samples: one at the start and one 7–14 days later. A rise of four times or more between them confirms recent infection.
• Leptospira PCR — looks for the DNA of the bacteria. It can be positive before antibodies appear. Blood is more useful early in illness and urine later, so both are often sent. Antibiotics can make PCR negative, which is why samples are taken before they are started.

Higher than reference — what it may be a sign of

• A rising MAT titer on paired samples in a dog with fitting signs — fits leptospirosis.
• A single positive MAT — not enough for a diagnosis. By the ACVIM consensus, a single positive titer lacks specificity unless it is very high (above 1:3200), and a diagnosis based on one MAT is not recommended; even a single titer above 1:800 picks up only about half of cases. Vaccination also produces MAT titers, which can persist, and so can a past infection without illness. Because types cross-react, the type with the highest titer does not reliably show which one infected the dog.
• Positive blood PCR in a dog with fitting signs — supports the diagnosis.
• Positive urine PCR — read together with signs and other tests: up to 20% of apparently healthy dogs can shed leptospiral DNA in urine. Inactivated leptospirosis vaccines are not expected to make PCR positive.

Lower than reference

A negative MAT or PCR does not rule out leptospirosis. Early in illness antibodies may not have formed yet; PCR can miss the infection when there are few bacteria or shedding is intermittent — in one study PCR detected 25% of cases in blood and 69% in urine. This is why a second sample is often taken 7–14 days later. The vet decides on repeat testing.

Look at together with

• Kidneys — creatinine, urea, phosphorus, potassium and sodium can change.
• Urine — low specific gravity, protein, and glucose with normal blood glucose.
• Liver — ALP rises, usually more than ALT, and bilirubin can rise.
• Platelets — a mild to moderate fall; clotting times such as PT and aPTT can be prolonged.
• Vaccination record — the date of the last leptospirosis vaccine matters for reading the MAT.

Units and conversion

Both results are qualitative. The MAT is reported as a titer, written 1:N (for example 1:800), often for each serovar on the lab’s panel; a higher N means more antibody. PCR is reported as “detected” or “not detected” (sometimes “positive” / “negative”). In the app these results are stored as text, as printed on the form.

Reference range

The reference range is on your lab’s form; for PCR it is “not detected”, and for the MAT the lab states which titer it considers negative. A result is read together with the signs, the vaccination date, kidney and liver values and, for the MAT, a second sample.

Cats and dogs

Sources

1. Sykes JE et al. Updated ACVIM consensus statement on leptospirosis in dogs (J Vet Intern Med, 2023)
2. Merck Veterinary Manual — Leptospirosis in dogs
3. CDC — Leptospirosis in animals