As a small animal internist, it is not uncommon for me to come across diagnostic test results that were misinterpreted or overlooked by other veterinary professionals.
This includes students, interns, residents and even veterinarians that have been in practice for many years. Everyone makes mistakes, and that won't ever change.
If anyone reading this disagrees on the above, there really are only two possibilities:
1. You have no clue you made a mistake because no one ever pointed it out (most common), or 2. You think you know everything and therefore there's no way you'd ever make a mistake. Since we work in the medical field, I'd run from anyone that has the latter belief!
As a board certified specialist, one of my duties is to teach. This includes both pet parents and veterinary professionals. Teaching and educating others does not (and must not) only happen in academia.
And yes, regardless of where we are in our career, we all have things to learn. More importantly, we can learn a lot from working with others, regardless of if one is a specialist or not.
PCR testing in our veterinary patients seems to be one of the most common sources of diagnostic testing errors. This is likely to be derived from a lack of understanding of the test itself (and therefore when it's appropriate to even submit it), and how to interpret a negative result. In my experience, this appears to be more prevalent with Leptospirosis PCR testing.
The basics
First things first - PCR stands for polymerase chain reaction and its inventor won the Nobel Prize in Chemistry in 1992. Since its development, it has been a crucial technique in molecular diagnostics.
While the the goal of this post is not to be an exhaustive resource on molecular diagnostics, here's the short version on the bare minimum you need to know. I strongly encourage you to find actual scientific reading material if you want to learn more. Dr. Sykes' Canine and Feline Infectious Diseases textbook (this is an affiliate link) has a great, easy to read chapter on this subject. If you'd like to look at our other recommended textbooks, click here.
For this technique to work, primers are used to identify the genetic material of interest (the primers used are complementary of the DNA sequence of interest) . If detected, a DNA polymerase is then involved in amplifying (copying) this genetic material. This amplification occurs during many cycles and in an exponential manner.
RNA can also be detected. If this is the goal, a reverse transcriptase-PCR (RT-PCR) must be used instead. Reverse transcription (RT) is a process converting RNA into complementary DNA (cDNA). This must occur since DNA polymerases can't amplify RNA. It's also important to know that RNA is highly susceptible to degradation, and therefore sample handling is crucial.
Bottom line, always follow the laboratory instructions for sample submission and reach out to them if you have questions. If you switch laboratories, they the sample s might need to be submitted differently!
Sample selection will depend on the disease we want to test for and where most organisms will be present - this can vary depending when infection has occurred!
How to interpret a positive result
While a positive result is easier to interpret, it does not necessarily mean a patient has the disease. Remember the following:
-A viable organism is not needed for testing. PCR testing will also detect genetic material of dead organisms.
-While there is genetic material present, it doesn't mean the organism is the cause of the clinical signs.
-Contamination is always possible (both during sampling/handling and at the lab). Only work with reputable laboratories.
-As with other test modalities, cross-reactions can occur. In the same manner, vaccine organisms can also be detected (depending on the vaccine).
How to interpret a Negative result
Please, do not ever interpret a negative PCR result as "my patient does not have this disease"! PCR testing can be negative due to many factors! Some include:
-Organism not present or in very few numbers in the sample provided
-Wrong sample submitted or inappropriate sampling timing
-Assay specific factors or incorrect sample handling (by you or the lab)
-Prior antibiotic therapy - one dose of antibiotics can be enough to lead to a negative result!
The current test for COVID-19
As of now, the test being used for COVID-19 is a RT-PCR test. As mentioned above, a negative result doesn't rule out infection!
Please follow the CDC guidelines and don't act like you're untouchable. You might be fine when this is all said and done, but your friends and family might not.
In the meantime, many more people will be infected and die from this disease. Do your part. Stay healthy, and keep others healthy as well.
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