 By re-testing negative results the pathologists, in fact, tried to determine negative predictive value (NPV) of the test (i.e derive how many negative test results were true negative against reference laboratory).NPV is not a fixed or stable parameter of the test, as it will change with prevalence (interested readers can be referred to Bayesian methods in diagnostic testing).By making simple NPV formula calculations using the above listed test parameters one can derive negative predictive value of only about 60 - 65% (for a 90% sensitive and 90% specific test under condition of 80% prevalence of ER positive breast cancers in North American population).