Acute Single Joint Pain

Differential Diagnosis

Common Diagnoses

Occasional Diagnoses

  • Fracture
  • Reactive arthritis
  • Psoriatic Arthritis
  • Rheumatoid Arthritis (RA)
  • Patellar Tendinitis, Osgood–Schlatter’s Disease
  • Osgood–Schlatter’s Disease
  • Patellar Tendonitis

Rare Diagnoses

  • Septic Arthritis (SA)
  • Haemophilia
  • Local Tropical Infections (e.g. Madura Foot [Mycetoma Pedis], Filariasis)
  • Malignancy (Usually Secondary)
  • Avascular Necrosis
  • Recurrent Joint Subluxation

Ready Reckoner

Key distinguishing features of the most common diagnoses

OACP/Anterior Knee PainTraumatic SynovitisGout Traumatic Haemarthrosis
Sudden OnsetPossibleNoYesPossibleYes
History of Acute TraumaPossibleNoYesPossibleYes
Recurrent ProblemYesYesNoYesNo
Several Joints PainfulPossibleNoNoPossibleNo
Hot, Red JointNoNoNoYesPossible

Possible Investigations


Possible:FBC, ESR/CRP, uric acid, X-ray, joint aspiration (in monoarthritis of large joint).

Small Print:Rheumatoid factor/anti-CCP antibodies, clotting studies/factor VIII assay, arthroscopy.

  • FBC/ESR/CRP: WCC and ESR/CRP raised in infection, systemic inflammatory conditions; Hb may be reduced in the latter.
  • Uric acid: Once attack has subsided, useful to add weight to clinical diagnosis of gout (especially if considering treatment with allopurinol).
  • Rheumatoid factor may be useful if symptoms suggest possible RA (consider anti-CCP antibodies if rheumatoid factor is negative).
  • X-ray: Essential if fracture suspected. May also reveal OA, avascular necrosis, malignancy and help to distinguish between RA and psoriatic arthritis.
  • Sterile aspiration of joint fluid: To look for pus (septic arthritis), blood (haemarthrosis) and crystals (gout/pseudogout).
  • Clotting studies/factor VIII assay: If haemophilia a possibility.
  • Arthroscopy: May be required urgently in secondary care if trauma has resulted in a haemarthrosis.

Top Tips

  • Autoimmune blood tests can be misleading in possible arthritis. The diagnosis should be clinical; blood testing simply adds weight and prognostic information to the clinical assessment. Positive tests can be found in normal patients – beware of inappropriately labelling an insignificant problem as a significant arthritis on the basis of a blood test.
  • Gout is very painful, will limit movement and may cause a slight fever. Septic arthritis gives a similar picture but with marked restriction of movement and, usually, a high fever. If in doubt, arrange urgent assessment.
  • In obscure cases, question and examine the patient carefully. For example, in reactive arthritis, symptoms of urethritis or conjunctivitis may have been minimal or forgotten; in psoriatic arthritis, there may only be insignificant skin lesions.

Red Flags

  • If one joint is red, very hot, intensely painful with marked limitation of movement and systemic illness, septic arthritis must be excluded – admit.
  • Haemarthrosis usually develops rapidly after trauma and indicates significant damage requiring immediate referral; effusion due to synovitis usually takes a day or longer to accumulate and is less urgent.
  • Septic arthritis is notoriously easy to miss in a patient with coexisting RA. The systemic signs may be absent and the diagnosis may mistakenly be viewed as a flare-up of rheumatoid arthritis.
  • Consider reactive arthritis in a young adult male with a monoarthritis of the knee not caused by trauma.
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