Diagnostic Strategy in Canine Leptospirosis: What to Trust, What to Question

Diagnosing leptospirosis in dogs is rarely straightforward. The disease often presents with non-specific early signs and overlaps with other systemic conditions, while available diagnostic tests have important limitations. As a result, clinicians should rely on a combination of clinical suspicion and strategic testing, rather than expecting a single definitive result¹. In practice, misinterpretation of results is a more common issue than lack of testing. 

Clinical Suspicion: The Starting Point 

Leptospirosis should be considered in any dog presenting with acute kidney injury (AKI), especially when accompanied by hepatic dysfunction or thrombocytopenia. Importantly, the absence of classical exposure history does not rule out disease. Urban cases without water exposure are increasingly recognized, making clinicopathologic patterns more reliable than history alone. Even in vaccinated dogs, leptospirosis cannot be completely excluded, although other differentials may be more likely¹. 

MAT: Valuable but Frequently Misinterpreted 

The microscopic agglutination test (MAT) remains the reference standard for serology, but its interpretation requires caution. In early infection, antibody levels may be too low to detect, resulting in false-negative results. The sensitivity of a single acute MAT titer >1:800 is only about 50%^1,2, meaning early disease can easily be missed. 

On the other hand, a single positive titer is not confirmatory. Titers may reflect previous exposure, recent vaccination, or subclinical infection, and can persist for months. Additionally, MAT cannot reliably identify the infecting serovar due to cross-reactivity¹. 

The most reliable use of MAT is through paired serology, where a fourfold rise in titer over 7–14 days confirms infection. However, follow-up sampling is often missed in practice, limiting its diagnostic value. 

Rapid Antibody Tests: Supportive, Not Definitive 

Point-of-care tests such as SNAP Lepto and WITNESS Lepto provide quick results and can support early clinical decisions. However, like MAT, they depend on antibody production. This leads to false negatives in early infection and false positives due to vaccination or prior exposure^1,3. 

These tests are best used as adjunct tools. A negative result does not exclude leptospirosis, and a positive result must always be interpreted alongside clinical findings.  

PCR/NAAT: Early Detection with Important Caveats 

PCR-based tests detect leptospiral DNA and are particularly useful during the early leptospiremic phase, before antibodies develop. However, their sensitivity depends on both timing and sample type. Blood PCR is more sensitive in the first few days of illness, while urine PCR becomes more useful later when organisms are shed via the kidneys¹. 

A key practical point is to submit both blood and urine samples to improve diagnostic yield¹. 

Interpretation remains critical. False negatives can occur due to intermittent shedding or prior antibiotic use. Conversely, a positive urine PCR does not always indicate active disease, as clinically healthy dogs may shed leptospiral DNA¹. Therefore, PCR results must always be correlated with clinical signs. 

A Combined Approach Improves Accuracy 

No single diagnostic test is sufficient on its own. Serology reflects immune response, while PCR detects the organism. Using them together helps overcome individual limitations and improves diagnostic confidence. 

Structured case definitions that combine clinical signs with laboratory abnormalities further support decision-making and reduce overreliance on isolated test results. 

Key Clinical Takeaway 

The most important principle in managing suspected leptospirosis is that treatment should not be delayed while awaiting confirmation. Given the disease’s severity and zoonotic risk, early intervention based on suspicion is justified. 

In clinical practice, successful diagnosis depends less on the test chosen and more on how well the results are interpreted in context. 

Reference 

1. Sykes JE, Francey T, Schuller S, Stoddard RA, Cowgill LD, Moore GE. Updated ACVIM consensus statement on leptospirosis in dogs. Journal of veterinary internal medicine. 2023 Nov;37(6):1966-82. https://onlinelibrary.wiley.com/doi/pdfdirect/10.1111/jvim.16903?utm_source=consensus 

2. Reagan KL, Deng S, Sheng J, Sebastian J, Wang Z, Huebner SN, Wenke LA, Michalak SR, Strohmer T, Sykes JE. Use of machine-learning algorithms to aid in the early detection of leptospirosis in dogs. Journal of Veterinary Diagnostic Investigation. 2022 Jul;34(4):612-21. https://journals.sagepub.com/doi/pdf/10.1177/10406387221096781 
3. Lizer J, Grahlmann M, Hapke H, Velineni S, Lin D, Kohn B. Evaluation of a rapid IgM detection test for diagnosis of acute leptospirosis in dogs. Veterinary Record. 2017 May;180(21):517-. https://bvajournals.onlinelibrary.wiley.com/doi/pdfdirect/10.1136/vr.104134