Likelihood ratios for clinical examination, mammography, ultrasound and fine needle biopsy in women with breast problems

Instead of the conventionally used sensitivity and specificity, we present likelihood ratios (LRs) as a better measure of the accuracy of clinical examination, mammography, ultrasound and fine needle aspiration cytology in women presenting with breast problems. In addition, we point out that previous studies are prone to verification bias in the selection of subjects, and demonstrate methods of avoiding this problem. Both issues are illustrated using a dataset consisting of 7259 women who attended the Sydney-Square Breast Clinic consecutively in 1994, of whom 375 had surgical biopsy and 145 were found to have malignancy. Information on the non-cancer group was based on all women with a benign outcome on surgical biopsy and, to avoid verification bias, a sample of 232 subjects chosen randomly from consecutive attenders. LRs for five levels of each test, categorized from normal to malignant, ranged from 0.43 to 46.41 for clinical examination, from 0.18 to infinity for mammography, from 0.17 to infinity for ultrasound, and from 0.009 to infinity for cytology. LRs calculated using only subjects with a benign outcome on histology differed appreciably from those calculated using the more appropriate random sample of all non-cancers. Future researchers need to avoid verification bias in studies of test accuracy by including a random sample of consecutive subjects rather than only those who proceed to histology. LRs have an advantage over sensitivity and specificity because they provide a measure of test accuracy which allows test results to be reported as several categories. In clinical practice, these LRs are easy to use to calculate the probability of cancer.