Characteristics of false-negative smears tested in the normal screening situation.

It is common practice to rescreen false-negative (FN) smears. However, it is inevitable that this is done with some foreknowledge; at least it is known that the test smears contain one or more special cases. Therefore, we decided to test smears in the normal screening situation, when cytotechnologists are completely unaware of being tested. This experiment was done with five FNs and five true-positive (TP) smears. In a third experiment the FNs were tested with the cytotechnologists aware of their presence. Finally, 10 qualities of FNs and TPs were analyzed. In the normal screening situation, only in 1 of 25 tests was the FN recognized as malignant, while all the TPs were detected at the first testing. However, when the cytotechnologists were aware of being tested, the FN was detected in seven of eight tests. The FNs differed from the TPs in 5 of the 10 analyzed characteristics. FNs contained few (if any) large neoplastic epithelial fragments. Detached malignant cells were nearly lacking. The cancer cells had small nuclei that presented little anisokaryosis. The FNs possessed generally hypochromatic nuclei. These features explain why the malignant cells were almost never identified in the normal screening situation. We conclude that FNs and TPs differ and that it seems virtually impossible to avoid all false-negative diagnoses.