Introduction: On a routine basis, a haemodialysis treatment is only feasible, if the propensity for blood to coagulate when in contact with a foreign surface is counteracted. In fact, for a proper anticoagulation in HD, it is necessary a very delicate equilibrium, that ideally should be targeted to prevent coagulation in the extracorporeal circuit (ECC) during treatment and to prevent bleeding in the patient. According to the EBPG, the initial loading dose of heparin should be 50IU/kg followed by a maintenance dose of 800-1500IU/hr in patients without haemorrhagic complications. Online Haemodiafiltration is today considered by many the most advanced treatment modality. However it is demonstrated in several papers that the true benefits derive from a proper convection volume. In post-dilution online haemodiafiltration, a very thin balance occurs when maximizing substitution rate and filtration factor vs dialyser status, especially concerning anticoagulation dose. Objectives: • Assess the correlation between the prescribed anticoagulation’s dose and the classification of the dialyzer’s aspect, dry weight, substitution volume (SubsVol) and spKt/V. • Evaluate the correlation between dry weight and the classification of the dialyzer’s aspect. • Assess whether there are differences between the different categories of the dialyzer’s aspect, and spKt/V and SubsVol. Methods: It is a multicentre, quantitative, descriptive-correlational, study involving all active patients during one month, in the period from 01-04-2016 to 04-30-2016 according to inclusion criteria. Previous training was done to the Nursing staff in order to evaluate and classify both the dialyser's and the venous chamber's appearance of the ECC venous line. The dialyzer was classified into five categories and the venous drip chamber into three categories. Registration was performed immediately after the patient disconnection. Results: 2,829 patients were enrolled, mean age was 68.96 SD=13.75 years, 60.8% were men. The average haematocrit was 33.91 SD=3.45%. The average dry weight was 68.53 SD=13.27kg (10% were above 86Kg). Mean heparin dose was 58.13UI/Kg. Only 32.4% of the patients had a clean dialyzer at the end of treatment. 19.4% of patients finished the treatment with more than 10% of clotted fibres. Patients with no residual blood, (clean) 32.4% presented a higher heparin dose (66.32IU/kg) comparing with overall average dose. The simple linear regression indicated that a 10.9% response’s variation in the dialyzer aspect can be explained by heparin dose. Patients with dry weights ? 69Kg presented an inferior heparin dose comparing with overall average dose and as dry weight increases, it also increases the possibility of having more clotted dialyzer fibres. The relationship between heparin dose and dialyzer aspect is weak and negative (r=-0.330) but with statistical significance (p<0.001) According Pot Hoc tests in ANOVA (table 1 and table 2) we concluded that there were significant differences in average of spKt/V and SubsVol between "Clean” group and the other categories of dialyzer’s aspect classification. Conclusions: Patients with higher dry weights presented a lower heparin dose and have a higher tendency for clotted dialyzer fibres. Patients with lower heparin dose presented a higher tendency for clotted dialyzer fibbers. Dialyzers with “Clean” presented an average heparin dose of 66.32UI/kg. There were significant differences in spKt/V and SubsVol between "Clean” group and the other categories of dialyzer’s aspect classification. Only the "Clean” group is the more significant to achieve the target.

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