Objective We aim to study association between neutrophile to lymphocyte (NLR) and platelet to lymphocyte (PLR) ratios and disease activity, and their value to predict bDMARD response. Methods A set of spondylarthritis (SpA) patients under bDMARD registered in the Reuma.pt registry was studied. Sociodemographic, clinical and laboratorial variables were assessed on bDMARD initiation, 6, 12, 18 and 24 months (M) thereafter. Univariable and multivariable generalized estimation equations models assessed associations with disease activity. The NLR and PLR predictive value was assessed using univariable and multivariable logistic regression models. Results A total of 170 patients were included. Most were male (54.7%), with a predominantly axial phenotype (84.7%). Significant associations were observed between NLR [B=1.55, 95% confidence interval (CI) = (1.38; 1.74)] and PLR [(B=1.16, 95% CI = (1.09; 1.24)] with ASDAS-CRP (p < 0.001). Both baseline ratios predicted ∆ ASDAS-CRP ≥ 1.1 at 6 months [OR = 2.20, 95% CI = (1.21, 4.00) for NLR; OR = 1.02, 95% CI = (1.01, 1.04) for PLR, p < 0.01)]. PLR was a significant predictor of ∆ ASDAS-CRP ≥ 1.1 in all timepoints [OR (12 M) = 1.02, 95% CI = (1.00, 1.03), p < 0.05; OR (18M) = 1.02, 95% CI = (1.01, 1.03), p < 0.001; OR (24M) = 1.01, 95% CI = (1.01, 1.02), p < 0.01]. Conclusion NLR and PLR were associated with disease activity during the follow up of these patients. They seem to be significant predictors of therapeutic response to bDMARD in naïve SpA patients.