Simple Equations Help Monitor Mammography False-Positive Rates

Before mammography practices can attempt to reduce the rate of false-positive interpretations, they must first determine how to measure and monitor their false-positive rates.

In both screening and diagnostic mammography, a false-positive assessment is defined as one of the following:

• no known tissue diagnosis of cancer within one year after a positive screening examination;
• no known tissue diagnosis of cancer within one year after a recommendation for biopsy or surgical consultation on the basis of a positive exam; and
• benign tissue diagnosis within one year after recommendation for biopsy on the basis of a positive exam.

Before mammography practices can attempt to reduce the rate of false-positive assessments, they must first determine how to measure and monitor their false-positive rates.

Screening Mammography
With screening mammography, the recall rate can be measured to determine whether the false-positive rate is being impacted by some intervention. The recall rate is defined as the percentage of recalled cases due to screening mammography divided by the total number of exams. “Recalled cases” include the following categories in the Breast Imaging Reporting and Data System (BI-RADS) cases: BI-RADS categories 0, 4, and 5.

Another way to define the recall rate is the number of cases that are truly cancer divided by the number of recalls. Both definitions give insight into false-positive rates in the screening setting.

Diagnostic Mammography
In diagnostic mammography, the tissue diagnosis rate and/or the biopsy rate can be monitored to determine an intervention’s impact on the false-positives. The tissue diagnosis rate is defined as the number of positive tissue diagnoses divided by the number of exams. The biopsy rate is defined as the number of cancers divided by the number of biopsies.

Audits
In common practice, most audits measure the recall rate for screening mammography and the number of cancers divided by the number of biopsies for diagnostic mammography. If you prefer, you could measure the other approaches equally as well.

Reference
Edward Sickles, MD:
Measuring the False-Positive Rates for Screening and Diagnostic Mammography.