The most common mistakes in leptospirosis diagnosis and how to avoid them

In this video, Jane Sykes, BVSc, DACVIM (SAIM), PhD, MPH, MBA, FNAP, a professor of medicine and epidemiology at theUniversity of California, Davis, School of Veterinary Medicine, explains why leptospirosis remains one of the most challenging diagnostic puzzles in veterinary practice. She outlines the most common pitfalls that lead to misdiagnosis, starting with the tendency to rule out the disease too early.

Because leptospirosis is an acute infection, many dogs have not yet mounted an antibody response at the time of testing. This can result in false-negative results on the microscopic agglutination test (MAT), often considered the diagnostic “gold standard,” as well as on point-of-care assays.

Sykes also cautions against the opposite mistake: overinterpreting a positive result. Subclinical exposure to Leptospira species is common, meaning a positive test—such as a visible red line or blue dot—may reflect prior exposure rather than the current cause of illness, which could be something else entirely.

She breaks down the key differences between immunoglobulin M (IgM)– and immunoglobulin G (IgG)–based assays and offers a reality check on polymerase chain reaction (PCR) testing. Although urine is generally a more appropriate sample than blood, factors such as intermittent shedding and prior antibiotic use can significantly reduce test sensitivity.

Below is a partial transcript, lightly edited for clarity.

dvm360: What are the most common pitfalls in diagnosing leptospirosis in clinical practice?

Sykes: This is one of the reasons why lepto is such a good disease to talk about to vets, is that it's really about understanding infectious disease diagnostic testing, and so we have two different groups of tests that we can use. We can use organism detection tests like PCR, or we can use antibody detection tests—serologic tests.

The kind of gold standard for testing for lepto has been a serologic test called the microscopic agglutination test, or the MAT test, and that reports out a group of serovars and the antibody titers to each of those serovars. But because lepto is an acute disease, when dogs come in to be evaluated by a veterinarian, they typically haven't had enough time to form antibodies yet, so serologic tests, including the MAT test, are often negative. In fact, we almost expect them to be negative or have very low positive titers early on in the course of illness.

So it's tempting, if you just do one serologic test and it comes back as being negative, to rule out leptospirosis when, in fact, there's not been enough time for an antibody response. And that's the same for other acute diseases, [such as] anaplasmosis in dogs or other acute infectious diseases. So always, we need to follow up that with a convalescent test that's done for lepto, ideally 1 to 2 weeks later—no later than that.

Many vets will wait for 3 to 4 weeks to do convalescent testing, although [oftentimes], convalescent testing is not done because owners don't return for another test, or their dog's better and they don't want to spend the money on it. So really, you need that follow-up titer to make the diagnosis and see seroconversion happening.