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61 Background: Recent trends in surgical practice advocate selective use of axillary nodal clearance (ANC) following sentinel lymph node biopsy (SNB) in the treatment of breast cancer. We aimed to determine the effectiveness of one-step nucleic acid amplification (OSNA) using CK19 mRNA copy number and tumor-related factors in predicting non-sentinel axillary nodal involvement, in order to formulate local guidelines for ANC. Methods: Patients due to have SNB at our institution for invasive breast cancer as well as selected patients with high grade ductal carcinoma in situ with negative assessment of the axilla on pre-operative imaging were included. Alternate slices of each node were sent for assessment by either OSNA or Histopathology. Immediate ANC was performed if OSNA was positive. The CK19 mRNA copy number, the total tumor load (summation of m RNA copy number of positive nodes,TTL), the total nodal status at ANC and tumor characteristics including grade, tumor size and lymphovascular invasion (LVI) for each patient were determined. A model of risk probability was constructed using TTL and tumor related factors. Results: 664 nodes were examined from 425 patients who had SNB performed between 2011 and 2014. After excluding 8 patients who did not meet the study criteria, 648 nodes from 417 patients were included for analysis. The concordance between OSNA and histology was 91.4%; positive predictive value (PPV) and negative predictive value (NPV) was 77% and 97% respectively. Patients with TTL less than 1400 did not have additional non sentinel lymph node involvement. TTL (p

194 Background: One step nucleic acid (OSNA) is a recently developed molecular diagnostic assay to detect lymph node metastases. Aim of the present study was to assess OSNA for intra-operative detection of sentinel node metastases with use of standard histopathology as the “gold standard.” Moreover, discordant cases were further investigated to assess their impact on patients’ management. Methods: 157 breast cancer patients with clinically and ultrasonographically negative axilla underwent axillary staging with sentinel node (SLN) biopsy. 244 SLNs were evaluated by OSNA and standard histopathology (alternate slices were submitted to OSNA and standard test). The “turn-around time” taken for an intra-operative OSNA result was recorded. Results: Sensitivity and specificity of OSNA were 92.3% and 95.6% respectively with positive predictive value of 80% and negative predictive value of 98.4%. The average time taken for an intra-operative result in this group was 34 minutes. Concordance rate was 95% while 12/244 nodes showed discordant results between OSNA and Histology. The discordant cases were re-evaluated by histopathologists and characteristics of this subgroup analysed. Patient’s management was altered by OSNA alone in 5 patients as they underwent axillary lymph node dissection on the basis of a positive OSNA when histopathology did not show evidence of metastasis. Two patients with nodes negative on OSNA had histopathology results showing metastases requiring delayed axillary clearance. The rest of the discordant nodes belonged to patients who had other nodes biopsied concurrently that were concordant in OSNA and histology. Conclusions: OSNA is a fast, valuable and highly accurate standardised method for the intra-operative evaluation of axillary lymph node metastasis and is part of our routine intra-operative assessment tool for sentinel nodes in breast cancer.