Reasoning GP · Algorithm · 1-page summary
Knee pain
For: adults and young people with knee pain in primary care. Not for: the limping child (limping-child pathway); long-term OA, gout or arthritis care (full algorithm).
Core rule: a hot, swollen knee is septic until proven otherwise — normal temperature, CRP or WCC and finding crystals do not exclude it. A tense effusion within about 2 hours of injury is an ACL tear or fracture until proven otherwise. Always examine the hip.
0Stop — call 999 now — do not delay transfer for imaging
- Pale, cold, pulseless leg; knee dislocation (palpable pulse does not exclude artery injury); deformity with absent pulse or paraesthesia; compartment syndrome; major trauma → 999
- Spreading redness + fever + disproportionate pain (necrotising fasciitis); septic arthritis with shock or rapid deterioration → 999
- While waiting: analgesia; do not inject the joint; no delay for imaging. Never withhold antibiotics to get an aspirate if septic, unstable or transfer is significantly delayed (blood cultures × 2 first, discuss with on-call).
1Triage by age, timing and site
| Filter | Think of |
|---|---|
| <40 | Trauma, patellofemoral, inflammatory, osteochondritis dissecans, Osgood-Schlatter |
| 40–60 | Meniscal tear, early OA, pes anserine bursitis |
| >60 | OA most likely; PMR, gout |
| Effusion | ≤2 h after injury = haemarthrosis (ACL, fracture, patellar dislocation) · hours–24 h = meniscal · insidious = OA, inflammatory, crystal, infection, tumour |
| Site | Medial = meniscus, MCL, OA, pes anserine · anterior = patellofemoral · posterior = Baker's, DVT, popliteal artery · check hip, spine |
2Screen — one row is enough
- Septic arthritis — hot, swollen, painful knee, restricted movement; immunosuppressed, prosthetic joint → Immediate hospital assessment and aspiration.
- Haemarthrosis — tense effusion ≤2 h after injury → Same-day assessment, X-ray per Ottawa, acute knee pathway.
- True locking (fixed loss of extension) or cannot straight-leg-raise after injury → Same-day orthopaedics; do not force.
- Calf or posterior knee swelling → Same-day Wells: ≥2 → ultrasound in 4 h (or D-dimer, interim anticoagulation, scan in 24 h); ≤1 → D-dimer.
- New unexplained effusion; swollen on anticoagulants; tibial plateau or pathological fracture → Same-day
- Persistent unexplained bone pain, swelling, night pain → X-ray; suggests sarcoma → consider suspected cancer pathway. Child or young person: consider 48 h X-ray and referral.
3Assess — only once blocks 0–2 are answered
☐ Gait ☐ Effusion ☐ Joint line, tibial flare ☐ Extension, SLR ☐ Lachman, collaterals, sag ☐ Hip, spine ☐ Pulses, calf
Ottawa: X-ray if ≥55, isolated patellar or fibular head tenderness, cannot flex to 90° or take 4 steps; not reassuring with deformity, dislocation or high energy. Not routine: X-ray, bloods or MRI for typical OA.
4Classify — only if the screen is negative
- ACL — twist, pop, immediate swelling, giving way → Urgent acute knee pathway; rehab meanwhile. Meniscal — joint-line pain, delayed effusion → rehab.
- OA — 45+, activity pain, stiffness ≤30 min. Anterior pain on stairs or sitting (patellofemoral), tendinopathy, ITB → physio.
- Medial tibial flare tenderness — pes anserine bursitis, not OA. Painful, limited hip rotation — referred hip OA.
- Inflammatory or crystal — stiffness >30 min, psoriasis, IBD; gout/CPPD: aspirate if uncertain. GCA symptoms → Same-day.
- Uncertain — say so; set a review, re-examine hip and spine.
5Treat today — doses: NICE/SPC, BNF, local formulary
- OA: exercise and weight loss are core. Offer topical NSAID first; oral NSAID only if suitable, shortest time, PPI where indicated (check renal, BP, CV, GI). Consider steroid injection (2–10 weeks' relief). Not: routine paracetamol or weak opioids, strong opioids, glucosamine, hyaluronan, lavage.
- Soft tissue: protection, early movement, physio.
- Gout/CPPD flare (sepsis excluded): NSAID, colchicine or short oral steroid by renal function and interactions. Gout: discuss urate-lowering therapy; target <360 μmol/L (consider <300 if tophi).
6Refer routinely
- Orthopaedics: OA with substantial impact, non-surgical care ineffective (no score; not excluded by age, smoking, obesity); persistent mechanical symptoms or failed rehab; recurrent patellar dislocation; loose body.
- Rheumatology: persistent synovitis — do not wait for RF, anti-CCP or CRP; Urgent if small or multiple joints or prolonged. Atypical PMR.
- Physio first-line (self-referral): OA, patellofemoral, tendinopathy, degenerative meniscal tear. MSK/pain: OA not for surgery.
7Never reassure by
- Normal temperature, CRP or WCC in a hot, swollen knee.
- Finding crystals — infection can coexist.
- A palpable pulse after a knee dislocation.
- A negative Lachman in a painful, swollen knee.
- Calling calf pain a Baker's cyst before the DVT pathway is negative.
- Treating medial knee pain as knee OA without examining the hip.
8Safety-net — say it, then document it
999 “Call 999 for sudden severe knee pain with a cold, pale leg; a deformed knee after injury, or a numb, weak or cold foot; severe pain with a tight, swollen leg; spreading redness with fever and feeling very unwell; or a hot swollen knee with fever and you feel very unwell, faint or confused.”
Same day: “A hot, swollen, painful knee, with or without fever; a swollen knee on blood thinners; calf or back-of-knee swelling; the knee suddenly won't straighten, or you can't lift the straight leg after an injury; new pain, swelling, redness or fever in a knee replacement — contact us today.” Return if not improving by 6 weeks, new locking or giving way, or weight loss.
✓Safe to leave primary care only if — all five, documented
No block 0–2 row applies; septic joint considered
Ottawa applied after trauma; extension and SLR checked
Hip, spine and pulses examined
One diagnosis fits, or uncertainty and review stated
NSAID risks checked; safety-net and review date given
Summary of the full Reasoning GP knee pain algorithm, which holds the history, examination tests, investigations, treatment ladders, follow-up schedule and evidence. Sources: NICE NG226, NG38, NG12 (updated April 2026), NG219, NG158, NG100, NG253; BOA/BASK ACL best practice. Clinical reference — follow local pathways and check doses: NICE/SPC, BNF.