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Diagnosing infectious disease
Using antibody titers to diagnose diseases can be helpful, but make sure you understand their limitations


A note about PCR testing

The advent of DNA testing using PCR amplification has allowed veterinarians to confirm active infection, Breitschwerdt says. "PCR testing is analogous to obtaining a DNA fingerprint at a crime scene," he says. "It can confirm that the organism is present in a sample obtained from the patient on the same day the sample was obtained. However, PCR has its limitations, too, and is not 100 percent accurate for every patient or for every infectious disease."

In most acute illnesses, such as with anaplasomis, ehrlichiosis and leptospirosis, a PCR test is likely to detect the organism's DNA, but with many chronic diseases, such as babesiosis, bartonellosis and leishmaniasis, PCR can miss the organism in an infected animal. This is not to say that PCR is not a potentially useful test for these infections, only that each test has limitations.

"PCR can be the best diagnostic tool to detect a needle in haystack, but if you only have one needle and the haystack is too big, it can be hard to find, which equates to a negative PCR result in an infected patient," Breitschwerdt says. "Whenever possible, it is best to do both serology and PCR testing."

"Serology can tell you all kinds of information if you ask the right questions and have the right samples," Hesse says.


Veterinarians often ask which test they should run when they suspect an infection. The answer is more than one. Whenever possible, perform both a serologic test to determine whether there was exposure, and then do a culture, biopsy or PCR test to confirm that the patient is, in fact, infected with that organism. Obtaining samples before treatment greatly enhances a diagnostic laboratory's ability to assist the veterinarian with obtaining an accurate diagnosis.

Perhaps most important, before any testing is done, it's always necessary to perform a complete physical examination and get a detailed patient history, including vaccinations, travel and activities. The animal's history along with clinical signs will direct which tests are likely to be most beneficial. Results of serologic testing, a PCR test or any other diagnostic testing must be evaluated along with the patient's physical and historical findings to make an accurate diagnosis. "You need to look at the whole picture and not just the snapshot," Hesse says.

Ellen Jensen is a freelance writer and editor in Lawrence, Kan.


Source: DVM360 MAGAZINE,
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