I recently had to have a COVID test because of an upcoming hospital procedure. After enduring the samplings—put a stick up your nose and twist it for ten seconds—I wondered how valuable this information was.
Certainly, I knew that I did not have COVID at that moment in time. However, my procedure was four days away.
- Could I get it in that period of time?
- Second how accurate is the test?
Specificity and Sensitivity
All tests—human and animal—are based on two terms: specificity and sensitivity.
Sensitivity measures the proportion of positives that are correctly identified (e.g., the percentage of sick people who are correctly identified as having some illness).
Specificity measures the proportion of negatives that are correctly identified (e.g., the percentage of healthy people who are correctly identified as not having some illness).
The John Hopkins study for COVID-19 showed that the false negatives could be as high as 1-in-5 as reported in the annuls of internal medicine.
Animal Lab Tests
All this demonstrates things to think about for lab tests in animals. For example, in culturing mastitis we typically run a culture when we see garget in the milk. We now know that this is usually late in the infection of the quarter.
I too have gotten frustrated with no growths from the lab, only then to realize that the bacteria was gone and I was just seeing the after math of the battle.
How about using PCR tests (a test that looks for genetic material from the pathogen)?
- They are very fast and very specific but can be too sensitive.
- Beyond 32 cycles, I don’t know if it means anything.
- It doesn’t prove that the organism is alive.
Okay, let's use milk cultures.
- Once again, the problem is timing. Even more so if you ship your sample off to be tested.
- Even if you are sure that the organism is alive, depending on the agar that was used, you can still miss the real issue.
What happens when you do not know?
You test again.
With repeat testing you get more accuracy. With an understanding of the biology and history you get more accuracy.