Synopsis
Treat an effusion as a diagnostic sign, distinguish a benign popliteal cyst from septic, crystal, haemorrhagic and thromboembolic disease, select aspiration and imaging by urgency, and manage the intra-articular driver rather than the cyst alone.
- A Baker cyst is usually a posteromedial fluid distension driven by an intra-articular knee problem; examine and treat the joint rather than viewing the lump in isolation.
- Characterise every effusion by onset, trauma, fever, crystals, anticoagulation, inflammatory disease, mechanical symptoms and ability to bear weight.
- A hot unexplained effusion needs urgent sterile aspiration for cell differential, Gram stain, aerobic and anaerobic culture and crystals; crystals and negative Gram stain do not rule out infection.
Key red flags
Fever, systemic illness, intense warmth, severe passive pain or immunosuppression with effusion requires urgent septic-arthritis aspiration and source-control assessment.
Investigation priorities
Diagnose infection and crystal disease in a hot effusion.
Management branches
The effusion is acutely hot, severely painful or accompanied by systemic infection risk.
- Assess sepsis, obtain blood cultures when indicated and aspirate urgently before antibiotics if stable.
- Give intravenous therapy after samples or immediately after rapid blood cultures if unstable.
Key medicines
Topical NSAID for osteoarthritis-related symptomsApply a licensed topical NSAID to the painful knee at the product's adult frequency, commonly three or four times daily depending on formulation, for a time-limited trial alongside strengthening and activity management.
Interim therapeutic anticoagulation when the NICE DVT pathway indicatesIf proximal-leg-vein ultrasound cannot be obtained within the guideline timeframe for a person with likely DVT, give the exact weight- and renal-adjusted interim therapeutic anticoagulant specified by the local NICE-aligned venous-thromboembolism protocol while definitive testing is arranged.