“You have psoriasis,” I say all the time. I mean it when I say it, of course. But I don’t always to the same degree. Sometimes I’m trying to say, “You probably have psoriasis.” Other times I mean, “You most definitely have psoriasis.” I rarely use those terms though.
One 36-year-old man with a flaky scalp and scaly elbows wasn’t satisfied with my assessment. His dad has psoriasis. So does his older brother. He was in to see me to find out if he had psoriasis too. “Probably” was what I gave him. He pushed back, “What percent chance?” That’s a good question — must be an engineer. I’m unsure.
Kaiser Permanente
Dr. Jeffrey Benabio
With the exception of the poker players, our species is notoriously bad at probabilities. We’re wired to notice the significance of events, but terrible at understanding their likelihood. This is salient in lottery ticket holders and some NFL offensive coordinators who persist despite very long odds of things working out. It’s also reflected in the language we use. Rarely do we say, there’s a sixty percent chance something will happen. Rather, we say, “it’s likely.” There are two problems here. One, we often misjudge the actual probability of something occurring and two, the terms we use are subjective and differences in interpretation can lead to misunderstandings.
Let’s take a look. A 55-year-old man with a chronic eczematous rash on his trunk and extremities is getting worse despite dupilumab. He recently had night sweats. Do you think he has atopic dermatitis or cutaneous T-cell lymphoma? If you had to place a $100 bet, would you change your answer? Immanuel Kant thinks you would. In his “Critique of Pure Reason,” the German philosopher proposes that betting helps clarify the mind, an antidote to brashness. The example Kant uses is of a physician who observes a patient and concludes he has phthisis (tuberculosis), but we really don’t know if the physician is confident. Kant proposes that if he had to bet on his conclusion, then we’d have insight into just how convinced he is of phthisis. So, what’s your bet?
If you’re a bad poker player, then you might bet he has cutaneous T-cell lymphoma. However, not having any additional information, the smart call is atopic dermatitis, which has a base rate 1000-fold higher than CTCL. It is therefore more probable to be eczema even in a case that worsens despite dupilumab or with recent night sweats, both of which could be a result of common variables such as weather and COVID. Failure to account for the base rate is a mistake we physicians sometimes make. Economists rarely do. Try to think like one before answering a likelihood question.
If you think about it, “probably” means something different even to me, depending on the situation. I might say I’ll probably go to Montana this summer and I’ll probably retire at 65. The actual likelihoods might be 95% and 70%. That’s a big difference. What about between probably and likely? Or possibly and maybe? Do they mean the same to you as to the person you’re speaking with? For much of the work we do, precise likelihoods aren’t critical. Yet, it can be important in decision making and in discussing probabilities, such as the risk of hepatitis on terbinafine or of melanoma recurrence after Mohs.
I told my patient “I say about a 70% chance you have psoriasis. I could do a biopsy today to confirm.” He thought for a second and asked, “What is the chance it’s psoriasis if the biopsy shows it?” “Eighty six percent,” I replied.
Seemed like a good bet to me.
Dr. Benabio is director of Healthcare Transformation and chief of dermatology at Kaiser Permanente San Diego. The opinions expressed in this column are his own and do not represent those of Kaiser Permanente. Dr. Benabio is @Dermdoc on X. Write to him at dermnews@mdedge.com.