Accuracy in detecting knee effusion with clinical examination and the effect of effusion, the patient's body mass index, and the clinician's experience

The purpose of this study was to determine the accuracy of detecting knee effusion with clinical examination and to evaluate whether the amount of effusion, patient obesity, and the clinicians' experience affect the clinicians' decisions in patients with knee osteoarthritis. Patients presenting with knee pain were examined by two residents with different levels of experience and underwent ultrasonographic examination, including measurement of effusion in the medial, mid, and lateral aspects of the suprapatellar bursa. One hundred seventy-two knees of 86 patients were examined. Of the knees investigated, 127 (73.8 %) had effusion. The consistency between ultrasonographic and resident examination were weak (kappa = 0.193, p = 0.007 and kappa = 0.349, p < 0.001), although the more experienced senior resident had a stronger agreement. The overall inter-rater agreement between the two residents was low (kappa = 0.254). The senior resident had a significantly higher accuracy ratio (p = 0.036). In the knees without effusion, the two examiners had no agreement (kappa = -0.028, p = 0.856); however, the ratios of the true decisions were similar (p = 1.0). The accuracy of the less experienced resident's decisions was affected by effusion depth (p = 0.005). Clinicians' decisions and their accuracy in detecting knee effusion during clinical examination were different, especially in the absence of effusion. The consistency between ultrasonography and residents was low. The accuracy of clinical examination was affected by effusion depth and experience, but not by patient obesity.