Inflammatory low back pain: high negative predictive value of contrast-enhanced color Doppler ultrasound in the detection of inflamed sacroiliac joints

OBJECTIVE: To determine the value of microbubble contrast agents for color Doppler ultrasound (CDUS) compared with magnetic resonance imaging (MRI) in the detection of active sacroiliitis. METHODS: An observational case-control study of 103 consecutive patients (206 sacroiliac SI] joints) with inflammatory low back pain according to the Calin criteria and 30 controls (60 SI joints) without low back pain was conducted at the University Hospital of Innsbruck. All patients and controls underwent unenhanced and contrast-enhanced CDUS and MRI of the SI joints. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of unenhanced and contrast-enhanced CDUS were evaluated. RESULTS: Forty-three patients (41%) with 70 of 206 SI joints (34%) and none of the controls nor the 60 control SI joints demonstrated active sacroiliitis on MRI. Unenhanced CDUS showed a sensitivity of 17%, a specificity of 96%, a PPV of 65%, and an NPV of 72%; contrast-enhanced CDUS showed a sensitivity of 94%, a specificity of 86%, a PPV of 78%, and an NPV of 97%. Detection of vascularity in the SI joint was increased by contrast administration (P < 0.0001). Clustered receiver operating curve analysis demonstrated that enhanced CDUS (A(z) = 0.89) was significantly better than unenhanced CDUS (A(z) = 0.61) for the diagnosis of active sacroiliitis verified by MRI (P < 0.0001; 2-sided test). CONCLUSION: Microbubble contrast-enhanced CDUS is a sensitive technique with a high NPV for detection of active sacroiliitis compared with MRI.