Red Flag = Serious Pathology

Why is it important to screen for Red Flags in Knee / Lower limb.

It is so because these red flags may require immediate referral.

  • Fracture | Acute Trauma
  • Infection (septic arthritis, osteomyelitis)
  • Tumour
  • Inflammatory Arthritis
  • Deep Vein Thrombosis (DVT)
  • Peripheral Vascular Disease

Fracture | Acute Trauma

Urgent referral is required for the following

  • Suspected Fracture
  • Suspected Quadriceps or Patellar tendon rupture
  • First patellar dislocation
  • Recurrent patellar dislocation with severe swelling
  • Soft tissue injury with gross instability
  • Neurovascular damage
  • Suspected Slipped Capital Femoral Epiphyses
  • Suspected osteonecrosis – SONK (spontaneous osteonecrosis of knee), also called subchondral insufficiency fracture


Urgent referral is required for suspected infection


Urgent referral is required for suspected tumour. This could be

  • Sarcoma (bone or soft tissue)
  • Bone metastases

Inflammatory Arthritis / Polyarthritis

Urgent referral to rheumatology is required for suspected inflammatory polyarthritis.

Deep Vein Thrombosis (DVT)

Timely detection of DVT can avoid Pulmonary Embolism (PE).

DVT Red flags in patient history

DVT Physical Exam

  • Patient can be asymptomatic
  • Symptomatic patients can have discoloration, pain, warmth, swelling, and tenderness of the affected extremity.
  • Homan’s sign – pain and tenderness in the calf on passive dorsiflexion of the ankle with the knee extended. This test has poor diagnostic value.
  • Wells Score – validated for use in outpatient setting for suspicion of DVT.
    Score < = 1 – DVT unlikely
    Score > = 2 – DVT likely

Wells Score for Diagnosing Suspected DVT

DVT Wells Score

Peripheral Arterial Disease (PAD)

PAD = narrowing of arteries

Clinical feature – Intermittent claudication. This is pain and cramping in legs typically in calf muscles with exercise which eases with rest.

This is not an immediate emergency but timely recognition can avoid ischaemia related complications.

PAD Red Flags in Patient History

Red Flags Peripheral Arterial Disease

PAD Physical Exam

6 minute walk test

Ankle Brachial Index (ABI) –

Measurement of ABI = Highest ankle systolic blood pressure / Highest brachial systolic blood pressure. (measure BP at both arms and both ankles and take the highest reading for comparison)

  • ABI values greater than 1.4 indicates calcification / vessel hardening. Refer to vascular specialist
  • ABI Value 1 – 1.4 is considered normal.
  • ABI Value 0.9 – 1 is acceptable/borderline.
  • ABI Value 0.8 - 0.9 suggests some arterial disease. Treat risk factors. 
  • ABI Value 0.4 - 0.8 suggests moderate arterial disease. Refer to vascular specialist.
  • ABI Value less than 0.4 suggests severe arterial disease. Refer to vascular specialist.

PAD Assessment 

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