Multivariate logistic regression with stepwise selection of variables statistically correlated in univariate analysis (Table 4) showed that for low BMD (any site), male sex (OR 3.87, 95% CI 1.21–7.36, P = 0.023), high ASDAS-CRP score (OR 2.83, 95% CI 1.36–4.76, P = 0.015), the presence of BME on sacroiliac MRI (OR 2.83, 95% CI 1.77–6.23, P = 0.000) and spinal MRI (OR 4.06, 95% CI 1.96–8.46, P = 0.000), and high grade of sacroiliitis (OR 2.93, 95% CI 1.82–4.45, P = 0.002) were risk factors, while high BMI (OR 0.91, 95% CI 0.71–0.97, P = 0.011) was a protective factor. The gene discussed is CRP; the disease is Sacroiliac arthritis.