Abstract Funding Acknowledgements Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Statutory grant for Department of Heart Diseases, Wroclaw Medical University, Poland. Background/Introduction Serum osmolarity seems to be an important factor regulating the control of volume status and diuretic response in patients with Acute Heart Failure (AHF). Nevertheless, data on it’s clinical significance in this population is limited. Purpose We aim to evaluate serum osmolarity associations with decongestion rate and clinical outcomes in patients with AHF. Methods Serum osmolarity was determined in Arterial Blood Gases at admission in patients with AHF hospitalized in a single cardiology center. The endpoints of the study are: decongestion rate and one-year mortality. Decongestion rate was defined as body weight change between admission and discharge (kg) divided by number of days of hospital stay. Study group was divided into tertiles according to serum osmolarity – A: <277 mmol/kg; B: 277-285 mmol/kg; C: >285 mmol/kg. Results Study group consisted of 319 patients, average age 68±13 years, 240 (75%) men. Mean left ventricle ejection fraction (LVEF) was 34±14%. Patients with lowest osmolarity (tertile A) were younger and had lower EF when compared to the other tertiles (B and C) (65±15 vs. 70±13 vs. 71±12 years, respectively, p=0.028) and (30±14 vs. 36±14 vs. 36±13; p=0.001), respectively. Patients with higher osmolarity had greater baseline systolic and diastolic blood pressure values (114±24/71±13 vs 133±26/78±14 vs 140±35/121±16 mmHg; p<0.001/p<0.001). There was U-shape relationship between serum lactate concentration at admission and osmolarity tertiles: (2.0 [1.6-2.7] vs 1.6 [1.4-2.2] vs 2.1 [1.6-2.7] mmol/l, respectively; p<0.001). NT-proBNP at discharge was higher in group A (4082 [2405-8324] vs 2827 [1519-5271] vs 2392 [1386-5631] pg/ml, p=0.001). During hospitalization tertile A more often required inotropic support vs tertile B and C (29% vs 4% vs 4%; p<0.001). There was a difference of body weight change from admission till discharge between the tertiles, with tertile B having the highest weight loss when compared to the other tertiles: (3 [1-5] vs. 4 [2-6] vs. 2 [0-4] kg; p=0.004). Tertile A was characterized by the longest hospital stay in comparison the rest of the population (7 [6-13] vs. 7 [5-9] vs. 7 [5-8] days; p=0.043). There was a U-shape relation between serum osmolarity and decongestion rate. The rate of decongestion were significantly slower in patients with the lowest and the highest osmolarity (tertile A and C): (0.26 [0.08-0.50] vs 0.57 [0.17-1.0] vs 0.25 [0.04-0.53] kg/day; p=0.002), respectively. The tertile A had the highest one-year mortality, when compared to the other groups (53% vs. 33% vs. 28%; p<0.001). Moreover, the serum osmolarity itself as a continuous value had prognostic significance HR (95% confidence interval): 0.96 (0.94-0.97), p<0.001. Conclusions Serum osmolarity assessed at admission to the hospital is associate with the decongestion rate and one-year mortality in AHF patients and thus may help identify high risk population.