Sensitivity, Specificity, and Positive Predictive Value

The interpretation of a screening examination ultimately falls into one of two categories: normal or abnormal. These judgments, in turn, ultimately are divided among four categories based on either the determination or estimation of the underlying reality of having or not having breast cancer at the time of screening. A normal interpretation may be a true-negative or a false-negative.

A positive interpretation may be a true-positive or a false-positive. As noted in the introduction, screening tests for breast cancer divide women into those who are likely to have the disease and those who are unlikely to have the disease. By convention, each of these initial findings is given a summary determination on the basis of the patient's status at 1 year after the screening examination and are defined as follows: true-positive (TP) - cancer diagnosed within 1 year after biopsy recommendation based on an abnormal mammogram; true-negative (TN) - no known cancer diagnosed within 1 year of a normal mammogram; false-negative (FN) - cancer diagnosed within 1 year of a normal mammogram; false-positive (FP) - no known cancer diagnosed within 1 year of an abnormal mammogram. There are three definitions of false-positives: FP1 refers to a case recalled for additional imaging evaluation of an abnormal finding on a screening mammogram in which no cancer was found within 1 year or a recalled case was not shown to be malignant within 1 year.

The additional imaging may take place at the same time as the screening examination or at a later date. FP2 refers to a case in which no known cancer was diagnosed within 1 year after an abnormal mammogram and recommendation for biopsy or surgical consultation. This definition is based on the recommendation for biopsy alone - a biopsy may or may not be done. FP3 refers to a case in which benign disease is found at biopsy within 1 year after an abnormal mammogram and biopsy. This last definition of FP is similar to the definition proposed by the American College of Radiology (ACR) breast imaging reporting and data system, described later in the section on Quality Assurance.

These summary categories are the basis elements for measuring the performance of screening programs. Because the large majority of women who undergo mammography examinations do not have breast cancer, nearly all normal interpretations (true-negatives) are accurate. True-positives are obviously measured in the near term by biopsy results. False-negatives or false-positives are based on the assumption that cancer would have been detected, or was not present, on the basis of the presence or absence of histologic confirmation of disease within 1 year.

Sensitivity is a measure of the probability of detecting breast cancer when it is truly present. It is the proportion of patients found to have breast cancer within 1 year of screening who were identified as having an abnormality at the time of screening [sensitivity = TP/(TP + FN)]. Another method of measuring sensitivity has been outlined by Day and Walter and is based on the ratio of observed (invited group) to expected (control group) rates after omitting results from the first screening round as a basis for estimating the proportion of cases that would be expected to appear as clinical cancers.