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Introduction: MYC rearrangements (MYCr) occur in 5 to 15% of diffuse large B-cell lymphomas (DLBCL) and 20 to 35% of high-grade B-cell lymphoma, NOS (HGBL-NOS), are a defining criterion of the category HGBL with rearrangements of MYC and/or BCL2/BCL6 (HGBL, with MYCr and BCL2/BCL6), and may be present in 90% of Burkitt lymphoma. The current WHO classification considers cytogenetic techniques as the appropriate tool to detect MYCr but does not define how to approach to the identification of such alteration. As the global incidence of MYCr in large B-cell lymphomas (LBCL) is low, it is necessary to clarity whether FISH or other cytogenetic methods have to be applied to all LBCL or only in selected cases. We previously identified LMO2 as a potential surrogate marker of MYCr in LBCL (Colomo L, Am J Surg Pathol 2017). Our aim with this study is to confirm this observation and evaluate the clinical impact of this marker in the survival of patients with LBCL. Methods: We have prospectively studied between September 2014 and July 2019 a new series of 180 LBCL including patients with DLBCL, HGBL, with HGBL, with MYCr and BCL2/BCL6, HGBL-NOS and transformed low-grade lymphomas into DLBCL (tDLBCL) diagnosed according to WHO criteria. LMO2 (clone 1A9-1), MYC (clone Y69) and a common immunohistochemistry (IHC) panel of B and T-cell markers have been used for the histological categorization of the cases, using whole tissue sections. The cutoff for LMO2 and MYC were 30% and 40%, respectively. MYC and BCL6 genes were studied using break apart probes, and BCL2 gene using dual-color dual-fusion probes (IGH/BCL2), all from Vysis-Abbott. We have statistically correlated the loss of expression of LMO2 and the overexpression of MYC with the presence or absence of MYCr. Moreover, we performed survival analyses assessing the clinical impact of LMO2 in a series of 162 LBCL patients (112 DLBCL, 20 HGBL, with MYCr and BCL2/BCL6, 4 HGBL-NOS and 26 tDLBCL). The survival series included cases diagnosed before 2014 with IHC and FISH data. Results: The prospective series included 132 patients with DLBCL (78M/52F; median age 67 years, range 35-95), 9 HGBL, with MYCr and BCL2/BCL6 (5M/4F; median age 67 years, range 42-85), 4 HGBL-NOS (2M/2F; median age 58 years, range 42-89), and 35 tDLBCL (31 transformed follicular lymphomas, 3 marginal zone lymphoma and 1 lymphoplasmacytic lymphoma; 23M/20F; median age 64 years, range 40-82). LMO2 and MYC were expressed as follows, respectively: 84/130 (65%) and 46/132 (35%) in DLBCL; 1/9 (11%) and 8/9 (89%) in HGBL, with MYCr and BCL2/BCL6; 0/4 and 3/4 (75%) HGBL-NOS; 25/34 (73%) and 7/33 (21%) tDLBCL. MYCr were identified in 9/132 (7%) DLBCL; all HGBL, with MYCr and BCL2/BCL6; 4/4 HGBL-NOS; 7/35 (20%) tDLBCL. The table shows the comparisons between LMO2 and MYC protein expression for the identification of the presence of MYCr in the series of LBCL. Whereas in the whole series LMO2 and MYC had similar results, among CD10-positive cases, LMO2 had better results than MYC and identified better the presence of MYCr than MYC protein expression. The 5-year progression-free survival (PFS) according the diagnostic categories was 59% for DLBCL, 28% for HGBL, with MYCr and BCL2/BCL6, 25% for HGBL-NOS and 22% for tDLBCL (P=0.015). In addition, PFS was significantly lower for the presence of MYCr (26% vs 53%, P=0.02) and MYC IHC expression (35% vs 53%, P=0.005), and showed a positive trend for LMO2 loss of expression (39% vs 52%, P=0.1). The 5-year overall survival (OS) according the diagnostic categories was 67% for DLBCL, 23% for HGBL, with MYCr and BCL2/BCL6, 50% for HGBL-NOS and 77% for tDLBCL (P<0.001). In addition, OS was significantly shorter for the presence of MYCr (37% vs 71%, P=0.002), MYC protein expression (46% vs 75%, P=0.001), and for LMO2 loss of expression (46% vs 74%, P=0.005). In a Cox regression survival analysis including IPI and LMO2 for the 68 CD10-positive cases, IPI (HR: 1.61 P=0.03) was the most important variable for predicting OS, and LMO2 showed a significant trend (HR: 0.44 P=0.06). However, the addition of MYC IHC and MYCr did not add predictive accuracy to IPI score (HR: 1.6 P=0.31; HR: 1.8 P=0.19, respectively). Conclusions: LMO2 detection by IHC is a useful tool to detect MYCr in aggressive LBCL, particularly in CD10-positive cases. Moreover, LMO2 protein expression captures the prognostic significance of the different diagnostic histological categories and the presence of MYCr in this group of lymphomas. Disclosures Sanchez-Gonzalez: Alexion: Consultancy, Honoraria; Gilead: Consultancy, Honoraria; Shire: Consultancy, Honoraria; Amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Takeda: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees. Salar:Celgene: Consultancy; Gilead: Consultancy, Speakers Bureau; Janssen: Consultancy, Speakers Bureau.

INTRODUCTION The diagnosis of CMML according to WHO 2017 requires the presence of ≥1x109/L and ≥10% of monocytes in peripheral blood (PB). Establishing an accurate diagnostic is difficult since many clinical situations present persistent monocytosis. The presence of dysplasia is frequent but not always present and cytogenetic aberrations are infrequent in this disease (20-25% of cases). Although 85-90% of CMML patients present ≥1 mutation in TET2, SRSF2 or ASXL1, the use of NGS panels is not widespread. The study of PB monocyte subsets by flow cytometry (FC) has gained interest for CMML diagnosis. The increase of classical monocytes (Mo1) upper 94% presents a high sensitivity (Sn) and specificity (Sp) for CMML diagnosis (Sn 90.6, Sp 95.1; Selimoglu-Buet et al, Blood 2015). The 94% threshold was validated in two studies (Talati C et al, Blood 2017; Tarfi S et al, Blood Cancer J 2018). However, some controversies have recently appeared in the literature. Picot T detected the 95% cutoff as the one with the best Sn (100%) and Sp (97%) (Picot T et al, Front Oncol 2018). Hudson CA found that the presence of < 1.13% (Sn 100, Sp 96) of non-classical monocytes (Mo3) was the best predictor for CMML diagnosis (Hudson CA et al, Am J Clin Pathol 2018). With the exception of the study of Tarfi S, based on 47 CMML, the rest presented a very low number of patients (Talati C: 29; Picot T: 15; Hudson CA: 16) and therefore a bias could be expected specially when studying the Sn of the proposed methods. Moreover, the different series assessing the "monocyte assay" have no molecular data and therefore this could diminish the accuracy of the results since some patients may have received misdiagnoses. The aim of our study was to assess the Sn and Sp of different thresholds of Mo1 and Mo3 in a large series with well-annotated clinical, cytogenetic and molecular data. Moreover, we assessed whether the study of CD2 and CD56 monocyte expression in combination with the %Mo1 >94 test improves the detection of the disease. METHODS 50 CMML, 12 MDS, 11 MPN with ≥1x109/L monocytes and 79 reactive monocytosis with ≥1x109/L monocytes (N = 152) were prospectively studied from 02/2016 to 07/2019. We studied PB monocyte subsets by FC: Mo1 (CD14bright/CD16-), Mo2 (CD14bright/CD16+) and Mo3 (CD14dim or -/CD16bright). In addition, we assessed the expression of CD56 and CD2 in monocytes (positivity ≥ 20%). Finally, targeted NGS of the entire exonic sequence of 25 genes recurrently mutated in myeloid malignancies was performed (VAF sensitivity: 2%). Chi-Square or Fisher exact tests were used as appropriate. ROC curves were developed to explore optimal cutoffs in terms of sensitivity (Sn) and specificity (Sp). Moreover, we plotted the AUC of the subset of Mo1 and Mo3. Finally, the Youden index (YI) was used to detect the threshold of Mo1 and Mo3 with the best balance between Sn and Sp. RESULTS AND DISCUSSION The Sn and Sp of the Mo1>94% test in our series were similar to those reported by the French group (GFM). Our Sn and Sp were 90% and 92% respectively with a YI of 82. The Sn and Sp of the Mo1>93% were 94% and 84% with a YI of 78. Finally, the 95% cutoff proposed by Picot T et al showed a Sn of 81% and a Sp of 96% with a YI of 77. Therefore, the 94% cutoff presented the best balance between Sn and SP of the different thresholds assessed. The Mo3 threshold of 1.13% proposed by Hudson CA et al showed a Sn of 67% and a Sp of 95% with a YI of 62. The best Mo3 cutoff in our series was established in 3.18% with a Sn of 90% and Sp of 83%. The YI of this threshold was 73. The AUC for the percentage (%) of Mo1 (0.937, IC 95%: 0.89-0.99) was better than the AUC of the % of Mo3 (0.924, IC 95%: 0.88-0.97) reinforcing the use of %Mo1 as the item with the best discriminative power for CMML diagnosis. The AUC of the percentage of Mo1 population was similar to that reported by the GFM (Figure 1). The Sn and Sp for CD56 expression in monocytes was 67% and 91% respectively, while CD2 expression showed a Sn of 38% and a Sp of 99%. Finally, the presence of at least one of the following: Mo1 >94%, CD56+ or CD2+ presented the highest Sn (98%) and a Sp of 84%. This method may be a very good screening test due to the low false negative rate expected. This combined approach showed the best balance between Sn and Sp (YI: 82). CONCLUSIONS Our study supports the utility of the Mo1 >94% test as the best flow cytometry assay for establishing accurate diagnoses in CMML. The combined assay of Mo1, CD56 and CD2 may be of high utility as a screening test. Figure 1 Disclosures Bellosillo: Qiagen: Consultancy, Speakers Bureau; TermoFisher Scientific: Consultancy, Speakers Bureau.