The operating characteristics of tests are often summarized as sensitivity (ie, the proportion of patients with disease who have a positive test result, or the true-positive rate) and specificity (ie, the proportion of patients without disease who have a negative result, or the true-negative rate). Using sensitivity and specificity to describe the utility of a diagnostic test has some limitations, since they indicate the probability that the test result will be positive if the patient has the disease, and the probability that the test will be negative if the patient does not have the disease. Clinicians cannot apply these numbers directly, because they do not know whether or not the patient has the disease. What the physician wants to know is conditional, the probability that the patient has the disease if the test is positive or negative (ie, the positive and negative predictive values of the test, respectively). Sensitivity and specificity can be determined by analyzing the columns in a 2 × 2 table (Table 1
), while the positive and negative predictive values are obtained by analyzing the rows. By convention, the reference standard is at the top and the new diagnostic test that is being compared to it is on the side. For sleep apnea, the reference standard is a definition of sleep apnea based on the AHI (the most common cutoffs used are 10 or 15), and the diagnostic test values are the result of the portable monitor. Sensitivity, specificity, prevalence (or pretest probability), and predictive values provide valuable information about a diagnostic test. However, it can be a challenge to interpret several different numbers that all describe the operating characteristics and outcomes of a test.