Aim: To compare breast fine needle aspiration (FNA) specimens prepared by conventional smearing (CS) versus monolayer preparation (MP), with respect to the conclusiveness of the cytopathological diagnosis.

Methods: From 1992 to 1996, aspirators prepared aspirates themselves by direct smearing onto 2–4 slides. From 1999 to 2003, aspirate preparation was performed in the laboratory, creating a MP, using a Hettich cytocentrifuge. FNA diagnoses were categorised into inadequate (C1), benign (C2), atypical (C3), suspicious for malignancy (C4) and malignant (C5). The reference standard constituted histological follow-up. A conclusive FNA diagnosis was defined as C2 in lesions benign on follow-up and C5 in lesions malignant on histology.

Results: From 1992 to 1996, 692 aspirates were processed by CS, whereas from 1999 to 2003, 1301 aspirates were processed by MP. More FNA were ultrasound-guided in the MP group (85.6% versus 21.5%, p<0.001). When compared with CS, MP-prepared FNA had conclusive diagnoses significantly more often (72.8% versus 58.5%, p<0.001). This effect remained significant when corrected for the difference in ultrasound guidance (adjusted odds ratio 1.7, 95% confidence interval 1.3 to 2.2, p<0.001), and was larger for malignant lesions than for benign lesions (51.7% versus 79.9%, p<0.001).

Conclusion: Patients presenting with breast lesions can more often be offered a same-day, conclusive cytopathological diagnosis when FNA are prepared by a manual MP processing technique.