Consider John Doe from Case 2. He has symptoms suggestive of an MI and an elevated troponin. Assume that his pretest probability is again 15.3% for having an MI. When you go to examine him, he reports his pain radiates to both arms. Interestingly, firm palpation reproduces his chest pain. How does this change his pre-test probability?

Pulling a few statistics from “The Rational Clinical Exam” by JAMA:

FeatureLR+ (95% CI)LR- (95% CI)
Prior abnormal stress test3.1 (2.0-4.7)0.92 (0.88-0.96)
Chest pain reproduced by palpation0.28 (0.14-0.54)​​1.2 (1.0-1.2)

To use these LR’s, we must first convert our pretest probability into odds.

Pretest Odds = 0.153/(1-0.153) = 1.180637544

Next we multiply by the appropriate likelihood ratios:

Odds X Likelihood ratios = (1.18)(3.1)(0.28) = 1.03

Now to get our post-test odds expressed as probability:

New probability = 1.03/(1.03+1) = 0.51

We can now say that, based on the new information provided, our patient has a 51% chance of having an acute MI without even needing to see the troponin yet.