Diagnostic Accuracy of Ultrasonography in the Detection of Subacromial Bursitis Using Magnetic Resonance Imaging as the Gold Standard
Background: Subacromial bursitis is a frequent cause of shoulder pain and functional disability and is commonly associated with rotator cuff pathology. Accurate imaging is essential for diagnosis. Ultrasonography is widely used for its accessibility and cost-effectiveness compared with magnetic resonance imaging (MRI), which is considered the gold standard for detailed soft tissue assessment. Aim: This study aimed to determine the diagnostic accuracy of ultrasonography in detecting subacromial bursitis using MRI as the reference standard and to assess its association with demographic and clinical factors. Methods: This cross-sectional diagnostic accuracy study was conducted in the Department of Diagnostic Radiology, Mayo Hospital, Lahore, Pakistan, from January 2025 to January 2026. A total of 134 patients aged 20-60 years with clinically suspected subacromial bursitis were included through non-probability consecutive sampling. All patients underwent ultrasonographic examination followed by MRI of the shoulder on the same day. The sample size was calculated based on an anticipated specificity of 77.8%, an expected disease prevalence of 70%, a 95% confidence level, and a 13% absolute precision. Diagnostic performance parameters, including sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and overall diagnostic accuracy, were calculated using MRI as the reference standard. Statistical analysis also included odds ratio, stratification, normality testing, and chi-squared test for association. Results: Ultrasonography demonstrated a sensitivity of 76.3%, a specificity of 82.9%, a PPV of 91%, an NPV of 60.7%, and an overall diagnostic accuracy of 78.4% in detecting subacromial bursitis. A strong association was observed between ultrasound and MRI findings for subacromial bursitis (OR=15.68; 95% CI: 6.10-40.28; p<0.001), with moderate agreement between both modalities (Cohen's kappa=0.54). The prevalence and severity of subacromial bursitis and associated rotator cuff pathologies increased with age. Stratified analysis showed higher ultrasound sensitivity in younger patients (≤40 years), whereas specificity was higher in older patients (>40 years). Conclusion: Ultrasonography demonstrated good diagnostic accuracy in detecting subacromial bursitis when compared with MRI. Its high PPV and moderate agreement with MRI findings support its use as an initial imaging modality for the evaluation of suspected subacromial bursitis. An increase in subacromial and associated rotator cuff abnormalities was observed with age. Ultrasonography provides a practical, accessible, and cost-effective imaging option, particularly in resource-limited settings. However, MRI should be considered in equivocal cases or when clinical suspicion persists despite a negative ultrasound examination.