Lotan - Figure 13

What do Clinicians Want? Predictive Value

FIG. 13:  What clinicians really want to know is what to do if a test result is positive or negative, and therefore what they are really interested in is the positive or negative predictive value of a test. 

Positive predictive value is the probability that a positive test result reflects the underlying condition being tested.  In other words, if a marker is positive, does a patient actually have bladder cancer?  If the positive predictive value is very high, then the marker is actionable. 

Negative predictive value is the proportion of patients with a negative test who are correctly diagnosed.  What that means is that if the test has a very high negative predictive value, when the test is negative, the patient does not have cancer, and one could consider not doing any additional testing, such as cystoscopy. 

The challenge for physicians is that both positive and negative predictive values depend on the underlying disease prevalence.