A case control study of women with carcinoma in situ (CINIII) was unde
rtaken comparing Papanicolaou smears for which false negative reports
had been issued with slides for which true positive reports had been m
ade. The number of abnormal cells was the strongest differentiating fa
ctor. Where there were less than 50 abnormal cells on the slide, the o
dds of a false negative report being issued was 23.7 times greater (95
% confidence interval 3.7-150) than when there were 200 or more abnorm
al cells. In false negative slides, the abnormal cells were likely to
be not represented throughout the slide, present only as single cells
rather than as groups, small in size and with finely granular normochr
omatic nuclei. We conclude that there are intrinsic differences betwee
n true positive and false negative slides. Given these characteristics
, rapid rescreening of slides that are considered negative may not be
an effective method of reducing the false negative rate.