Choosing and interpreting diagnostic tests for Lepto
Diagnostic testing is critical to disease management. If leptospirosis is diagnosed early it is treatable with antibiotics. Early detection is also critical to the identification of possible sources of exposure, which can help prevent both other animals and people from becoming exposed to this serious pathogen.
The gold standard in determining exposure to leptospires is the microscopic agglutination testing (MAT). This is a serological test that detects antibodies in the blood to leptospires. At the early stages of infection this test may be negative or only weakly positive (a vaccinated animal may have slightly elevated titers), but quickly, within 7-10 days after clinical signs appear, the titer rises, confirming the diagnosis. Therefore it is preferable to test an initial baseline sample at the time leptospirosis is suspected and then re-test 1 to 2 weeks later to confirm infection. This is important for epidemiological reasons and if there is an outbreak it is critical for tracking sources. However, MAT testing may not be positive right away due to a lag in the antibody response and it only tells you that the patient has been exposed. It does not accurately identify the infecting serovar; multiple serovars are included in the test in order to increase the chance of detecting an antibody response.
PCR (polymerase chain reaction) can be performed on urine, whole blood and kidneys (or other tissues suspected to be infected). This is a direct test for Leptospira DNA in the sample. It is an extremely sensitive test but, due to the fact that leptospires can be shed at low levels in the urine during the initial infection stage, it can often be negative. Urine is often acidic which leads to the rapid breakdown of DNA so it is important to get urine samples to the lab as soon as possible to have the best chance of detection of leptospires in the urine. Getting specimens before antibiotics are administered may also increase the chance of a positive test result. Leptospires in blood are at a much lower level than in urine but it is valuable to test blood in addition to the urine, because early in infection organisms may only be found in the blood, and later organisms are only found in the urine. Although leptospires can be hard to detect using PCR, when positive, this is the quickest possible way to confirm the diagnosis of leptospirosis.
Leptospires can be cultured from blood, urine and kidney tissue. Specimens for culture should be collected as soon as leptospirosis is suspected – before antibiotics are administered. If the patient has been treated with antibiotics during the acute stage of illness, this test is no longer an option. Culture can be difficult and has the best results if specimens are added to special media within several hours after being collected. A positive culture is an absolute confirmation of leptospirosis and serovar determination can be done once an organism has been isolated in culture. Currently, culture is the only accurate means to identify the infecting serovar.