Interpreting data from audits when screening and diagnostic mammography outcomes are combined

OBJECTIVE: The objective of this study was to use mathematic models to aid mammography practices in interpreting outcomes data derived from a combination of screening and diagnostic examinations, and in interpreting diagnostic mammography outcomes data that are not segregated by indication for examination. MATERIALS AND METHODS: We analyzed outcomes from 51,805 consecutive mammography examinations. Screening and diagnostic examinations were audited separately. Diagnostic examinations were audited by indication for examination. Extrapolating from our known mix of screening (79%) and diagnostic (21%) examinations, we determined expected combined outcomes for various mixes that might be encountered in clinical practice. Similarly, we determined the expected overall diagnostic mammography outcomes for various clinically relevant mixes of indications for examination. RESULTS: Outcomes vary substantially depending on the mix of screening and diagnostic examinations performed. For example, expected outcomes for practices with screening-diagnostic mixes of 90-10% and 50-50% are, respectively: rate of abnormal findings, 6% versus 11%; rate of positive biopsy findings, 38% versus 42%; cancer detection rate, 10 per 1,000 versus 30 per 1,000; mean invasive cancer size, 14.4 mm versus 16.0 mm; nodal metastasis rate, 8% versus 11%; and rate of stage 0 and stage I cancers, 87% versus 82%. Diagnostic outcomes also vary substantially according to indication for examination, with a higher rate of abnormal findings, a higher rate of positive biopsy findings, and a larger mean invasive cancer size expected for mixes involving a high percentage of workups for palpable lesions. CONCLUSION: When screening and diagnostic mammography outcomes are not segregated during auditing, and when diagnostic outcomes are not segregated by indication for examination, analysis of combined audit data should be based on extrapolations from known outcomes.