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Knee red flags

What you cannot afford to miss in knee pain, before any special test.

Most knee pain is musculoskeletal. These are the conditions that are not, or that need referral before treatment, with the features that point to each. Every row carries its source, listed at the foot of the page.

Use with your local protocols. Referral pathways, and how urgently to act, differ between health services and countries: where your local protocol or current guideline differs, follow it. Never delay emergency care to check a protocol.

Can’t missWhat points to it
Fracture after injury (Ottawa knee rule)1,2An X-ray is indicated if any one of the following applies: age 55 or over, tenderness isolated to the patella, tenderness at the fibular head, inability to flex to 90°, or inability to take four steps both immediately after the injury and at the time of examination. In data pooled from six studies in adults, the rule missed very few fractures (sensitivity 98.5%, LR− 0.05)1; a 2020 review of eight studies reported sensitivity 99%, LR− 0.073. In older adults, particularly women over 70, a simple low-energy fall can cause a tibial plateau fracture.
Septic arthritis4A hot, swollen, painful knee. Fever is often absent in confirmed cases, so a normal temperature does not rule out infection. Same-day medical assessment is required, because the diagnosis is made by aspiration rather than by examination. Gout and pseudogout can mimic infection; joint fluid analysis distinguishes them.
Knee dislocation and vascular injury5Any injury with gross laxity in more than one direction should be regarded as a possible dislocation, even if the knee has reduced spontaneously. Popliteal artery injury is common after dislocation, and a palpable foot pulse does not exclude it. Emergency referral is required. Pulses, colour, temperature, sensation and ankle and toe dorsiflexion should be recorded. After a dislocation or tibial plateau fracture, pain out of proportion to the injury that is worse when the leg muscles are passively stretched (for example, by moving the toes) may indicate compartment syndrome: send the patient to an emergency department now6,7.
Deep vein thrombosis (DVT) versus ruptured Baker's cyst8,9Calf pain and swelling, especially after surgery, immobilisation or travel. A ruptured popliteal cyst can closely mimic a DVT, and the two can coexist. The presentation should be managed as a possible DVT, with same-day medical assessment using a structured pre-test score (Wells) and D-dimer or ultrasound. The symptoms should not be attributed to the cyst until a clot has been excluded.
Bone tumour in a young person10,11Constant or night pain not eased by rest or position, a swelling, or progressive pain requires prompt imaging arranged through a medical practitioner. The pain should not be labelled growing pains or Osgood–Schlatter without an examination for it.
Metastatic disease11,12A history of cancer, unexplained weight loss or systemic symptoms, with pain that is not mechanical. Referral for urgent (not same-day) medical assessment and imaging is indicated.
Swelling without injury, or on anticoagulants13A tense effusion with no clear trauma, or in a patient on anticoagulants or with a known bleeding disorder, may indicate a haemarthrosis. Same-day medical review is required.
Extensor mechanism rupture14,15,16Inability to hold the knee straight against gravity after a jump, landing or stumble. A gap above the patella suggests rupture of the quadriceps tendon (usually in a man over 40); a gap below it with a high-riding patella suggests rupture of the patellar tendon. In an adolescent, a tibial tubercle avulsion should be considered. Urgent referral for imaging and a surgical opinion is required.
Hip pain presenting at the knee17,18Knee or thigh pain with a normal knee examination warrants examination of the hip. In an adolescent with a limp and knee pain, slipped capital femoral epiphysis should be considered. The patient should stop weight-bearing (using crutches), and same-day medical assessment should be arranged for X-rays and orthopaedic referral.

A red flag raises suspicion; it does not make the diagnosis, and its absence does not rule the condition out. The table is not a complete list of every serious condition, and it is not a clinical protocol. When a red flag is present, act on the referral advice in the row, in line with your local protocols. This page is for registered health practitioners and students and is not clinical advice for patients; see the terms of use.

Last checked against the cited sources: 15 October 2026.

Then the tests: Knee special tests, with sensitivity, specificity and likelihood ratios.

Get the book for how to do and read each knee test with published figures, and which ones to trust for the ACL, meniscus and kneecap

This table opens the chapter ‘The knee’ in Rule It In, Rule It Out. The book then works through the questions that matter in this region, with the published figures for each test and how to read them.

Coming in late 2026 as a PDF and a paperback. Join the list to hear when it's out

Sources

  1. Bachmann LM, Haberzeth S, Steurer J, ter Riet G. The accuracy of the Ottawa knee rule to rule out knee fractures: a systematic review. Ann Intern Med. 2004;140(2):121–124. Link
  2. Malik S, Herron T, Mabrouk A, et al. Tibial Plateau Fractures. StatPearls Publishing; updated April 22, 2023. Link
  3. Sims JI, Chau MT, Davies JR. Diagnostic accuracy of the Ottawa Knee Rule in adult acute knee injuries: a systematic review and meta-analysis. Eur Radiol. 2020;30(8):4438–4446. Link
  4. Margaretten ME, Kohlwes J, Moore D, Bent S. Does this adult patient have septic arthritis? JAMA. 2007;297(13):1478–1488. Link
  5. Mohseni M, Mabrouk A, Simon LV. Knee Dislocation. StatPearls Publishing; updated February 27, 2024. Link
  6. Gamulin A, Lübbeke A, Belinga P, Hoffmeyer P, Perneger TV, Zingg M, Cunningham G. Clinical and radiographic predictors of acute compartment syndrome in the treatment of tibial plateau fractures: a retrospective cohort study. BMC Musculoskelet Disord. 2017;18:307. Link
  7. British Orthopaedic Association. BOAST: Diagnosis and management of compartment syndrome of the extremities. London: BOA; 2014, updated July 2025. Link
  8. Wells PS, Anderson DR, Rodger M, et al. Evaluation of D-dimer in the diagnosis of suspected deep-vein thrombosis. N Engl J Med. 2003;349(13):1227–1235. Link
  9. Frush TJ, Noyes FR. Baker's Cyst: Diagnostic and Surgical Considerations. Sports Health. 2015;7(4):359–365. Link
  10. Grimer RJ, Briggs TWR. Earlier diagnosis of bone and soft-tissue tumours. J Bone Joint Surg Br. 2010;92-B(11):1489–1492. Link
  11. Storari L, Piai J, Zitti M, et al. Standardized Definition of Red Flags in Musculoskeletal Care: A Comprehensive Review of Clinical Practice Guidelines. Medicina. 2025;61(6):1002. Link
  12. Coleman RE. Clinical features of metastatic bone disease and risk of skeletal morbidity. Clin Cancer Res. 2006;12(20):6243s–6249s. Link
  13. Johnson MW. Acute Knee Effusions: A Systematic Approach to Diagnosis. Am Fam Physician. 2000;61(8):2391–2400. Link
  14. Pope JD, El Bitar Y, Mabrouk A, Plexousakis MP. Quadriceps Tendon Rupture. StatPearls Publishing; updated April 22, 2023. Link
  15. Brinkman JC, Reeson E, Chhabra A. Acute Patellar Tendon Ruptures: An Update on Management. J Am Acad Orthop Surg Glob Res Rev. 2024;8(4):e24.00060. Link
  16. Zhao Z, Zhang H, Zhou F, Yang Y, Liu X. An update on tibial tubercle avulsion fractures: a narrative review. Ann Med Surg (Lond). 2026;88(2):1567–1573. Link
  17. Lesher JM, Dreyfuss P, Hager N, Kaplan M, Furman M. Hip joint pain referral patterns: a descriptive study. Pain Med. 2008;9(1):22–25. Link
  18. Webb CW, Liu R, Bouchereau-Lal N. Slipped Capital Femoral Epiphysis: Rapid Evidence Review. Am Fam Physician. 2025;112(4):414–423. Link

Red flags for other regions: Neck · Shoulder · Elbow · Wrist and hand · Thoracic spine and chest wall · Low back and pelvis · Hip and groin · Ankle and foot

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