Prior Probability: Where Judgment Begins

It is one of the worst places you can find yourself as an emergency clinician: sitting right on the fence and feeling stuck between two choices you do not like.

Are you going to subject a patient to a large and unnecessary workup? Or are you going to ignore the voice in your head and hope you are not wrong in deferring testing?

A patient on the fence

A previously healthy 33-year-old walks into your ED. No history of migraines. No trauma. They tell you:

“This headache started during a stressful meeting a few hours ago. It is different from anything I have had before. It is just pounding, and my neck feels tight.”

They are sitting up, well appearing, with stable vital signs except for a slightly elevated blood pressure. There are no focal neurologic deficits. They are worried—but not panicked.

You find yourself wondering:

“Is this a tension headache—or the start of a nightmare?”

You think to yourself, “Well, there was no syncope—but it was sudden onset. Does a stressful meeting count as exertion?”

As you run through these risk factors in your head, you are performing a calculation that dates back to 1763, described by a quiet English minister named Thomas Bayes.

Bayes’ theorem in plain English

Your current suspicion, updated by the strength of the evidence, becomes your new suspicion.

More formally:

P(A) is your pre-test probability—your initial suspicion.

P(B | A) describes how likely you are to see the evidence if your suspicion is correct.

P(A | B) is your post-test probability—what you believe after considering the evidence.

You do not need to solve the equation in real time, and most of us do not. But it helps to know that this is the logic underneath our clinical decisions.

If you want to get a little sharper with it, the calculators on Bayes Razor can help you plug in your gestalt and see how it shifts with key findings, bringing structure to something we are already doing intuitively.

Back to our patient

As you walk into the darkened room, a very uncomfortable patient holding their forehead turns their head to the side to say hello when you greet them.

“Maybe—but probably not—a subarachnoid hemorrhage,” your gut tells you.

This is the point where you should ask yourself:

How do I turn this feeling into P(A)?

Establishing the prior

A good starting point is an established prevalence.

For example, suppose we use 0.3% as an illustrative reference estimate for SAH in a broad population of patients presenting to the emergency department with headache. The actual prevalence is difficult to reduce to a single number because it changes with the population being studied and how patients were selected.

If you believe there is a 10% chance that this patient has a bleed, you are saying that this particular patient is approximately 33 times more likely to have SAH than the average patient represented by that starting prevalence.

If that is what you think, great—start there.

If the estimate suddenly seems too high once you see the reference prevalence, adjust downward and refine your prior probability.

The purpose is not to create false precision. The purpose is to make the assumption in your head visible.

Adjusting a prior probability with testing and risk factors is covered in other lessons. For now, focus on establishing the best starting point possible.

Experience and bias

Your past experiences matter, and they absolutely influence how you build a prior.

Maybe you have had a patient who looked just like this one and turned out to have a devastating bleed. That memory sticks, and it may push your estimate higher.

Or maybe you read this lesson and then immediately walk in to see a patient with a headache. Availability bias may increase your suspicion without your realizing it.

Perhaps the triage nurse is daring you to succumb to anchoring bias by writing:

“Patient arrives with migraine × 2 hours.”

Biases can move your prior probability higher or lower than where you would place it more objectively.

The goal is not to eliminate intuition. It is to understand what is shaping it.

It is important to remain aware of these biases and continually work on debiasing strategies.

Where judgment begins

Every decision you make starts with a hunch.

Pre-test probability is not just academic jargon. It is the clinical scaffolding for everything that comes next.

The better your starting point, the better your judgment will hold up under pressure.

Trust your gut, but train it.

When it counts, take the extra second to ask:

“How likely do I really think this is?”

Then consider the prevalence, your past experience, and the biases that may be moving your estimate.

**1.** Thunderclap Headache Syndrome Presenting to the Emergency Department: An International Multicentre Observational Cohort Study.

Roberts T, Horner DE, Chu K, et al.

Emergency Medicine Journal : EMJ. 2022;39(11):803-809. doi:10.1136/emermed-2021-211370.