The health care community (Nursing) is saturated with the term “hallmark sign”. In every stage of my learning career I have learned about hallmark signs. These are the so-called “If-this-then-that” sort of sign or symptoms. If you see a certain ‘hallmark sign’ then they have such and such illness/disease/disorder. So an educated inference is that, with all things remaining relatively equal, when you don’t see a certain ‘hallmark sign’ they probably don’t have the disease/disorder/illness. With me so far?
So here’s the ‘standard definition’ of the term hallmark (thank you merriam-webster).
Hallmark: (no not the card company)
1a : an official mark stamped on gold and silver articles in England to attest their purity
b : a mark or device placed or stamped on an article of trade to indicate origin, purity, or genuineness
2: a distinguishing characteristic, trait, or feature <the dramatic flourishes which are the hallmark of the trial lawyer — Marion K. Sanders>
For instance a Butterfly Rash (Malar Rash) on the face (see pic above) is supposedly indicative of someone diagnosed with Systemic Lupus Erythematosus (SLE). So therefore someone with SLE should/would have this malar rash. Or if you see the malar rash, that person may/could/would be diagnosed with SLE.
What if I told you that the presence of a malar rash was less than 50% sensitive to the diagnosis of SLE? Having to use percentages and deductive reasoning that means that greater than 50% of patients will have a false negative? What that means is that over 50% of patients who actually have the disease (SLE) do NOT have the malar rash. My “if-this-then-that” theorem is then blown to bits because “if malar rash then SLE” doesn’t hold up very well. Maybe I’m over simplifying the concept or I’m inaccurately trying to reverse the application? “If malar rash then SLE” is not the same as “if SLE then malar rash”.
It makes me ponder the concept of teaching hallmark signs. Sorry, the student in me is in over drive these days. We learn things like the ‘halo effect’ of Digoxin toxicity and this malar rash with SLE for standardized testing purposes. I mean when you read a standardized exam question (like the certification exam I’ll be sitting for soon) and somewhere in the description you read “facial rash” “malar rash” or “butterfly rash” and you will reflexively choose any answer that remotely resembles all things about SLE right?
When in reality malar rashes are not all that common in patients who actually are diagnosed with the disease?
Things that make you go hmm.
P.S. All this yammering is my vain attempt at trying to better understand the principles of testing selection. The whole sensitivity & specificity conundrum is frying my brain, it truly is.