When evaluating a urinary cytology specimen, one of the first aspects that need to be established is the means by which the specimen was collected: clean catch/voided vs. brushing/washing.
In voided urines, transitional cells are shed singly; in washings/brushings, the cells tend to be aggregated in sheets and clusters (image A) & (image B); however, if there are no fibrovascular cores, these sheets do not represent papillary clusters.
Cytologic features to look for:
Fibrovascular cores (discussed above).
Multinucleation of the cells may be seen as a reactive phenomenon.
Nuclear borders should remain smooth, and chromatin is finely dispersed, with occasional small nucleoli.
N:C ratio should be <1:2, and there should not be overlapping of nuclei (the cells show "respect" for each other).
Look at the background: is it "clean" (think benign), inflammatory (think reactive), or necrotic (think cancer).