Here's a general example of how pretest probability works...Imagine you are a physician seeing a male patient. Before any tests are done: what is the likelihood (the pretest probability) that the male patient you are seeing is pregnant? Obviously, the likelihood is zero, the patient is male. Now, imagine that a pregnancy test is done and it is positive. Now what is the likelihood that the male patient is pregnant? Barring some miracle, you'd probably say its still zero. You might decide that the test was a "false positive" (no test is perfect), that you got a faulty test, that samples were been mixed up, or something similar. Do you see how your pretest probability changed how you interpret a test result? Of course, this is just a simplified example but you can see how pretest probability assessment might be important when interpreting medical tests for PE.