Methods Unsatisfactory/insufficient/inadequate or equivalent breast aspirates were retrieved from the involved institutions, and those with a repeat aspiration performed within 365 days were included. Clinical and radiological information were retrieved. Available cytological slides were reviewed.

Results Totally 539 paired aspirates were retrieved, with 61.2% (n=330/539) and 10.9% (n=59/539) cytological diagnosis being informative (not insufficient) and actionable (not insufficient nor benign) on repeat aspiration. Younger age (p=0.005) was associated with an informative diagnosis and prior radiotherapy (p=0.097) and insufficient aspirates performed under free-hand (p=0.097) trended with an actionable diagnosis. Radiological findings of calcification (p=0.026) and hyperechogenicity (p=0.045), a small lesion size on initial (p=0.037) and repeat (p=0.059) radiological assessment and interval size increment (p=0.019) correlated with informative/actionable diagnoses. Cytomorphological parameters, except for a trend with crushing artefact (p=0.063), do not correlate with the cytologic diagnosis of the repeat aspirate.

Conclusions Repeating breast FNAC on patients after an insufficient diagnosis yields an informative (‘sufficient’) result in over 60% of cases. Small lesions with calcification, hyperechogenicity and/or interval size increment are more likely to yield diagnostic results on repeat aspiration and indicate select patients suitable for repeat FNAC over more invasive procedures. The lack of associations with cytomorphological parameters cautions against overinterpretation of insufficient breast aspirates.