To calculate the probability that the chest-pain patient is having a myocardial infarction (MI), we must know three things:

• The pretest probability
• The 2 x 2 contingency table of the clinical finding for the disease in question (MI, in this case)
• The result of this test (in this case, the patient is positive for chest pain)

Because the presence or absence of other findings is not yet known in the patient, we can take the pretest probability to be the baseline prevalence of MI in the population. Let's pretend that in your clinic's region, the baseline prevalence of MI in any given year is 5% for a 55-year-old person. Therefore, the pretest probability of MI in this patient is 5%. We will see later that the post-test probability of disease in this patient is the pretest probability multiplied by the likelihood ratio for positive chest pain (LR+). To get LR+, we need the 2 x 2 contingency table.