Assessing Risk Factors with Bayes’ Theorem

A really good clinician not only embraces Bayes’ Theorem, they live and die by Bayes’ Theorem. Any veteran, battle-tested PA or NP makes decisions based on Bayes’ Theorem, even though they may not know what it is.

What is Bayes’ Theorem? It is the concept of pre-test probability. How sure am I that a patient’s sick before I even run the tests?

I want you to think about this scenario: A 56-year-old man comes in with a one-hour history of chest pain. That’s all you know. What is the chance, based on this scenario, that he’s having a heart attack? What percent?

There’s no right or wrong answer here: 60%, 50%, 20%, 58%.

Now let’s say this man is a diabetic smoker. How does that change your percentage? Clearly, it ratchets it way up. If you don’t embrace risk factors into your calculation of pre-test probability, you’ll get burned.

If a diabetic smoker comes in with chest pain, I won’t send him home. Even with a completely normal work up and two negative sets of troponin. This patient is going to be admitted and I’m going to observe them or I’m going to transfer them. If they leave, they’re signing an AMA because I’ve been burnt too many times.

Cardiac Risk Factors

What are the cardiac risk factors we have to talk about? If you look at the national registry, 86% of people had one of these risk factors when they had their first MI. The mnemonic I teach to remember the big six arterial risk factors is SAD CHF.

• Smoking
• Age
• Diabetes
• Cholesterol
• Hypertension
• Family history

When I assess anybody with chest pain or shortness of breath, I’m looking at these risk factors. Which one is the worst? Diabetes, hands down.

Diagnosing Diabetics

Diabetics are sneaky. Diabetics were put on this earth for one reason and one reason only: to burn us in medicine. They love to have silent MIs. They like to have a perforated bowel with no abnormal bowel findings. Remember, you have three physical exam findings of the bowel: guarding, rigid and rebound. Diabetics don’t have that, even if they have a perforated bowel, because the nerves are shot.

If you don’t know that and you go to see a diabetic’s belly, they’re going to have a soft belly. You think, “Maybe it’s nothing. I’m not going to scan them. I’m not going to run a lactate level.” Next thing you know, you’re getting burned.

Venous Risk Factors

Venous risk factors are Virchow’s triad. Use “DSH” to remember this: damage, stasis and hypercoagulable state. There are only three hypercoagulable states. High-estrogen states, including pregnancy and birth control pills, cancers and genetics. Look for those venous risk factors when making pre-test assessments.