The use of polymerase chain reaction (PCR) assays to diagnose veterinary diseases is an exciting new development in the world of veterinary medicine. By amplifying the genetic material of a specific infectious agent that is found in an animal’s stool or blood sample, PCR assays can catch infectious diseases much sooner than traditional testing methods. For example, PCR assays have lower detection limits than fluorescent tests – sometimes in the range of a single cell.
So, is PCR the future of veterinary diagnostics? And should you be using PCR assays in your veterinary practice?
Obviously, earlier diagnosis is the big benefit of PCR testing. With traditional veterinary diagnostic tests, a sick animal often needs to be “sick enough” before the test can detect the presence of the disease agent. If an animal’s immune system has reacted to and “bound up” the antigens from the disease in question, an antigen/antibody-based test may have trouble detecting the presence of the disease due to a lack of physical availability. It is only when the infection has spread to a certain level that these tests can detect it. With genetic-based PCR assays, on the other hand, the DNA of interest in the sample can be amplified significantly, by as much as 107. The use of amplification makes effective diagnostic testing possible sooner and results in healthier animals.
Diagnosis using PCR assays is quite reliable as well. Each primer is 15-20 bases long, which provides a very high level of confidence that the DNA of interest will be the only thing amplified by the test. Generally speaking, if even one base is not a match, the primer will not bind.
While there are some situations in which the primer can bind to a non-desired target and result in a false positive (for example, if a live virus is introduced with the same genetic code as the probes used to begin the PCR reaction), this can be corrected for by making sure you use the correct primer and by optimizing the thermal profile of the testing procedure. When used correctly, PCR assays can be a good diagnostic approach when testing for genetic material that is different than the host (i.e., bacteria, fungus, cancer, etc.). The technology has been used in the human diagnostic field for many years now and, if applied appropriately, could help veterinary diagnostics advance as well.
PCR assays offer faster results than some traditional diagnostic methods. For example, with PCR assays, there is no need to wait for a culture growth. Finally, PCR assays also have a large throughput level and are mostly automated, making it possible for a lab to easily screen a huge number of assays in one day. If your practice runs a high-volume lab, PCR technology could help you speed up your processes and reduce backlog.
But PCR does have its drawbacks.
While PCR is generally very robust, there are cases in which it may not be an effective diagnostic tool. For example, PCR may not be the right choice for diagnosing prions (an illness characterized by the misfolding of proteins), since there is nothing genetically unique about that disease. Contamination of source DNA, which can occur when amplified DNA molecules are “carried over” from one round of reactions to the next, can also be a challenge when using PCR assays. There are also cases in which endpoint PCR assays could be too sensitive, since certain bacterial species (such as bacteria in the gut) can be beneficial below certain levels. In these cases, qPCR may be useful to quantify the bacteria present and determine whether an animal has healthy bacterial levels.
The biggest drawback to PCR, though, is the cost and skill required to run the tests. An entry level PCR machine starts at around $10,000, while a high-volume machine will cost around $30,000; add to this the cost of consumables and reagents, and the price tag rises quickly, making the cost of entry too high for most veterinary clinics. In addition, extracting DNA from a sample requires special training which, in many cases, veterinary staff do not have. PCR techniques using cell-free DNA (DNA that has leached out of the bacteria into the blood/serum) reduce some of the steps (and knowledge) necessary to run the assay, but the sensitivity of these techniques remains low.
PCR assays are an exciting technology and will likely be the future of veterinary diagnostics. However, the use of commercial PCR tests is not yet feasible for many of today’s vets. At the end of the day, the best diagnostic approach remains the one that is simplest and most cost-effective to run. If PCR assays do not meet those criteria for your veterinary practice, it may not be the right time to adopt the technology. If, however, your lab is growing in volume and you can commit the time and money to upgrade to the latest technology, PCR assays may be a good option for you.