BackgroundTo assess the diagnostic performance of clinical examination and ultrasound (US) assessment of knee effusion in primary knee osteoarthritis (KOA) patients. Furthermore, the success rate for effusion aspiration and the factors related to it were investigated.MethodsThis cross-sectional study included patients diagnosed with primary KOA-induced knee effusion clinically or sonographically. The affected knee of each patient was subjected to clinical examination and US assessment using the ZAGAZIG effusion and synovitis ultrasonographic score. Patients with confirmed effusion and consented to aspiration were prepared for direct US-guided aspiration under complete aseptic techniques.ResultsOne hundred and nine knees were examined. During visual inspection, swelling was detected in 80.7% of knees and effusion was confirmed by US in 67.8% of knees. Visual inspection was the most sensitive at 90.54% while bulge sign was the most specific at 65.71%. Only 48 patients (61 knees) consented to aspiration procedure; 47.5% had grade III effusion, and 45.9% had grade III synovitis. Successful aspiration was achieved in 77% of knees. Two needle types were used; a 22 gauge / 3.5-inch spinal needle in 44 knees and an 18 gauge/ 1.5-inch needle in 17 knees, with a success rate of 90.9% and 41.2%, respectively. Aspirated amount of synovial fluid correlated positively with effusion grade (rs=0.455,p< 0.001) and negatively with synovitis grade on US (rs= − 0.329,p= 0.01).ConclusionsThe superiority of the US over clinical examination in detecting knee effusion suggests that US should be used routinely to confirm the presence of effusion. Long needles (spinal needle) may have a higher success rate of aspiration than shorter needles.