Introduction/Background Analysis of sentinel lymph nodes (SLN) by means of One-Step Nucleic Acid Amplification (OSNA) is a quick and accurate method. This assay detects the expression level of cytokeratin 19 (CK19) which is present in 57.9% of vulvar carcinoma with different pattern of expression. We compare CK19 expression in vulvar carcinoma biopsies and corresponding SLN metastases, in order to detect false negatives in OSNA analysis and determine a cut-off point

Methodology Ck19 was studied in paraffin tissues of SLN with metastatic infiltration of squamous carcinomas and in their corresponding vulvar biopsies between 2016 and 2022 in Donostia Hospital.

SLNB was carried out in 21 patients, 6 of them tested positive for metastatic squamous cell carcinoma. 1 of these cases, is also included in the OSNA validation study on vulvar carcinomas. In one case, It has not been possible to study the percentage of CK19 in the biopsy, neither in two SLNs.

Results CK19 expression found in biopsies range between negative to 95% of positivity, not exceeding 20% in 4 of them. In SLNs they range between negative and 95%, being half of positivities greater than 30% (40–95%).

Correlation has been found in 2 cases: One with low expression in biopsy as in the SLN metastasis and another with high expression in biopsy and SLN metastasis. In a third one, there is greater positivity for CK19 in SLNs than in biopsy.

Conclusion Although it might seem there is correlation between biopsy and SLN, the study alone of CK19 in SLN under diagnose the tumor in most cases, except in diffuse positivity. In those cases where expression is maintained in SLN, correlation with OSNA technique was demonstrated.

Diffuse expression in biopsy would be required in order to perform SLN study with OSNA to avoid under diagnosed metastases, especially micro metastases. More cases are needed to obtain more accurate data.

Disclosures NONE