Example 1. ECG for Chest Pain.

Consider the ambulatory patient with chest pain. (This example is from page 82-83 of Sackett DL, Haynes RB, and Tugwell P, Clinical Epidemiology: A Basic Science for Clinical Medicine (First Edition). Boston: Little, Brown, and Company, 1985. The data are from a study by Bartel HG, Behar VS, Peter RH, Orgain ES, and Kong Y. Graded exercise stress tests in angiographically documented coronary heart disease. Circulation 49: 348, 1974.)

Patients in the Bartel et al study had various types of chest pain and were tested using both exercise electrocardiography and coronary arteriography. Let us assume the coronary arteriography indicates the patient's true disease status: whether there is coronary artery stenosis of at least 70%. The study provides the number of patients who had the disease and had a positive exercise ECG result (137 True Positives), who had the disease but had a negative ECG (90 False Negatives), who did not have the disease and had a positive ECG (11 False Positives), and who did not have the disease and had a negative ECG (112 True Negatives). What does this allow us to say about the accuracy of the Exercise ECG?

Count of Cases
This study provides you with the counts of cases in the cells of the table, so you can use the 2x2probs calculator. The True Positives go in the upper left cell of the 2 by 2 table. The False Positives are in the upper right. The False Negatives are in the lower left, and the True Negatives in the lower right cell. After you have hit the "return" button, your table should look like this:

Resulting Statistics
Scroll below and see the statistics which can be calculated from this table. You can see that the sensitivity of Exercise ECG (the probability the test will be positive, if the patient has the disease) is .60 (60%), and the specifity (the probability the test will be negative, if the patient does not have the disease) is .91. In this study, the proportion of patients with disease was .65.

If a patient with chest pain, like those in this study, has a positive Exercise ECG, the probability of significant coronary artery stenosis goes from the .65 before the test to .93 (the predictive value of a positive test). If the Exercise ECG is negative, the probability decreases from .65 to .55 (the predictive value of a negative test).