RheumatologyTier 1Approach to a presentation

Monoarthritis, acute

Red flags

  • *Acute monoarthritis is septic arthritis until proven otherwise* - the joint is destroyed within days
  • Fever, rigors, systemic sepsis (absent in ~40-50% of septic arthritis - its absence proves nothing)
  • Inability to weight-bear or move the joint at all
  • Prosthetic joint - lower threshold, different organisms, orthopaedic emergency
  • Immunosuppression, diabetes, IV drug use, recent joint injection or surgery, overlying cellulitis
  • Known RA - septic arthritis is commoner, more often polyarticular, and more often missed as "a flare"
  • Sexually active young adult - disseminated gonococcal infection
  • Trauma with haemarthrosis - fracture, ligament rupture; haemophilia or anticoagulation
  • Back pain + fever + monoarthritis - consider endocarditis and discitis

Differential by mechanism1 exam ›

Infection
  • Septic arthritis - **Staphylococcus aureus is the commonest organism at all ages**; streptococci; Gram negatives in the elderly, IVDU and immunosuppressed
  • Disseminated gonococcal infection - young, sexually active; migratory polyarthralgia -> tenosynovitis -> pustular rash -> monoarthritis
  • Mycobacterial and fungal - indolent, monoarticular, immunosuppressed or endemic exposure
  • Lyme disease (not endemic in Australia - travel history), brucellosis
  • Prosthetic joint infection - coagulase-negative staphylococci, Cutibacterium acnes
Crystal
  • Gout - 1st MTP (podagra), midfoot, ankle, knee; peaks within 12-24 h
  • CPPD (pseudogout) - *the commonest cause of acute monoarthritis in the elderly*
    • Knee > wrist > shoulder > ankle > elbow
    • *CPPD in a younger patient -> screen for haemochromatosis, hyperparathyroidism, hypothyroidism, hypomagnesaemia, hypophosphatasia*
  • Basic calcium phosphate - calcific periarthritis, Milwaukee shoulder
Inflammatory
  • First presentation of a spondyloarthritis - reactive, psoriatic, enteropathic, AS
  • Monoarticular onset of RA or juvenile idiopathic arthritis
  • Sarcoidosis, Behcet disease, familial Mediterranean fever
Structural / other
  • Haemarthrosis - trauma, anticoagulation, haemophilia, pigmented villonodular synovitis
  • Internal derangement, meniscal tear, osteochondral fracture
  • Avascular necrosis
  • Acute flare of osteoarthritis (a diagnosis of exclusion)
  • Charcot (neuropathic) joint
  • Foreign body synovitis (plant thorn)
  • Tumour - osteosarcoma, synovial sarcoma, metastasis
Periarticular mimics
  • Bursitis, tendinitis, cellulitis, gouty tophus rupture, fracture
  • Distinguish by pain on ACTIVE but not passive movement, focal tenderness, and preserved passive range

Focused history

  • Speed of onset - hours = crystal or sepsis; days-weeks = inflammatory, indolent infection, tumour
  • Which joint - 1st MTP (gout), knee (CPPD, sepsis, reactive), hip (sepsis, AVN)
  • Previous identical episodes with complete resolution - crystal disease
  • Fever, rigors, sweats
  • Trauma, recent joint injection or surgery, prosthesis
  • Skin breach, cellulitis, ulcer, IV drug use, indwelling line
  • Sexual history and urethral/vaginal discharge - gonococcal
  • Diarrhoea, urethritis or conjunctivitis 1-4 weeks earlier - reactive arthritis
  • Psoriasis, IBD, uveitis, back pain - spondyloarthritis
  • Drugs: diuretics, low-dose aspirin, ciclosporin (gout); anticoagulants (haemarthrosis); starting or stopping urate-lowering therapy
  • Alcohol, purine-rich intake, dehydration, recent surgery or acute illness
  • Immunosuppression, diabetes, CKD, malignancy, HIV
  • Travel and tick exposure

Focused examination

  • Vital signs - fever, tachycardia, sepsis screen
  • Is it the joint or around it?
    • True arthritis: pain on ACTIVE AND PASSIVE movement in all planes, effusion, global restriction
    • Periarticular: focal tenderness, pain on active movement only, preserved passive range
  • Effusion, warmth, erythema, tenderness, range of movement
  • Overlying skin - cellulitis, portal of entry, ulcer, tophus
  • Look for the diagnosis elsewhere
    • Tophi - ear helix, olecranon, Achilles, finger pads
    • Psoriasis - scalp, umbilicus, natal cleft, nails
    • Pustular rash and tenosynovitis - gonococcal
    • Conjunctivitis, urethritis, keratoderma blennorrhagicum, circinate balanitis - reactive arthritis
    • Murmur - endocarditis
    • Other joints - subclinical synovitis suggests a polyarticular disease presenting monoarticularly
  • Chondrocalcinosis on an old radiograph does not prove the current episode is CPPD

Investigation strategy

Joint aspiration - the investigation
  • *Aspirate every acute monoarthritis before giving antibiotics or steroids, unless it means an unacceptable delay in a septic patient*
    • Do NOT aspirate through overlying cellulitis - seed a sterile joint
    • Prosthetic joint: aspirate in theatre or under orthopaedic supervision
  • Send: cell count and differential, Gram stain, culture (including blood-culture bottles), polarised light microscopy
    • Add gonococcal and mycobacterial NAAT/culture if indicated
Synovial WCC (x10^6/L)Interpretation
<2,000Non-inflammatory - OA, trauma, AVN
2,000-50,000Inflammatory - crystal, spondyloarthritis, RA; and early or partially treated sepsis
>50,000, neutrophils >90%Septic until proven otherwise
  • *No cell count reliably excludes sepsis* - gout regularly exceeds 50,000 and septic arthritis can present below 20,000, particularly in the immunosuppressed
  • Gram stain is positive in only ~30-50% of culture-proven septic arthritis
  • Crystals do NOT exclude infection - the two coexist
  • Bloody aspirate -> trauma, coagulopathy, PVNS, Charcot, tumour; lipid droplets = intra-articular fracture
Bloods
  • Blood cultures x2 BEFORE antibiotics (positive in ~50% of septic arthritis)
  • FBE, CRP, ESR, UEC, LFT, coagulation
  • Serum urate - ~49% normal during an acute gout flare; a normal level does not exclude gout
  • CPPD screen if young or recurrent: calcium, PTH, magnesium, ferritin and transferrin saturation, ALP, TSH
  • STI NAAT (urine, urethral, cervical, pharyngeal, rectal) if gonococcal disease is possible
Imaging
  • X-ray - fracture, chondrocalcinosis, erosions, AVN, tumour, prosthetic loosening
    • Usually normal early in sepsis - a normal film does not reassure
  • Ultrasound - confirms effusion, guides aspiration, detects tenosynovitis and enthesitis
  • MRI - osteomyelitis, AVN, tumour, occult fracture, PVNS
  • CT/DECT - urate deposition, complex anatomy (SI joint, sternoclavicular)
  • Echocardiogram if bacteraemia, murmur or IVDU

Management

1. Suspected septic arthritis - a joint-threatening emergency
  • Aspirate -> blood cultures -> empirical IV antibiotics immediately (do not wait for results)
    • Flucloxacillin IV, with vancomycin if MRSA risk, severe sepsis, or penicillin allergy
    • Add Gram-negative cover in the elderly, immunosuppressed or in IVDU
    • Ceftriaxone if gonococcal disease suspected (plus doxycycline/azithromycin for chlamydia, and contact tracing)
  • Urgent orthopaedic referral for joint washout or repeated aspiration - antibiotics alone are insufficient in a purulent native joint
  • Prosthetic joint infection: orthopaedic surgery decides DAIR vs one- or two-stage revision - never treat with antibiotics alone
  • Rationalise antibiotics on culture; typically 2 weeks IV then 4 weeks oral (longer if prosthetic or osteomyelitis)
  • Immobilise briefly, then early mobilisation and physiotherapy
2. Crystal arthritis - once sepsis is excluded
  • Any of: NSAID, colchicine, or corticosteroid - choose by comorbidity
    • Intra-articular corticosteroid is ideal for a single accessible joint (only after sepsis is excluded)
    • Prednisolone 15-30 mg for ~5 days if NSAIDs and colchicine are contraindicated - the usual choice in CKD
    • Colchicine 1 mg then 0.5 mg one hour later, then 0.5 mg 2-3x daily
  • *Do not start, stop or change urate-lowering therapy during the flare* - continue existing ULT
  • Plan urate-lowering therapy afterward if indicated
  • CPPD: symptomatic treatment only, plus screen for the metabolic cause
3. Inflammatory monoarthritis without sepsis or crystals
  • Intra-articular corticosteroid; NSAID
  • Look for the underlying spondyloarthritis - HLA-B27, skin, bowel, eye, back, enthesitis, STI screen
  • Persistent monoarthritis >6 weeks -> rheumatology referral and consider synovial biopsy (TB, sarcoid, PVNS, tumour)
4. Haemarthrosis
  • Reverse or manage anticoagulation; factor replacement in haemophilia
  • Imaging for fracture or internal derangement; orthopaedic review
5. In all cases
  • Analgesia, rest then early mobilisation, physiotherapy
  • Reassess in 48-72 h if the diagnosis was not established at the outset

Traps

  • *"It looks just like his usual gout" - and it is septic.* Crystal disease and infection coexist, especially in older and immunosuppressed patients
  • *A normal temperature does not exclude septic arthritis* - fever is absent in up to half
  • A normal serum urate does not exclude gout (~49% normal during a flare); a raised urate does not diagnose it
  • Chondrocalcinosis on X-ray does not prove the current attack is CPPD
  • Giving steroid or antibiotics before aspirating destroys the diagnostic yield
  • Aspirating through cellulitis converts a soft-tissue infection into a septic joint
  • A synovial WCC below 50,000 does not exclude sepsis; Gram stain is negative in half
  • Septic arthritis in RA is often polyarticular and is dismissed as a flare - a disproportionately inflamed single joint in RA is sepsis until aspirated
  • Missing disseminated gonococcal infection - ask about sexual history; the joint fluid is often culture-negative, so send NAATs from mucosal sites
  • The hip and sacroiliac joint - deep, hard to examine, cannot be aspirated at the bedside; MRI and image-guided aspiration
  • Forgetting the prosthesis - a prosthetic joint changes both the organism list and the management pathway entirely

Talk track

1. Sepsis first, always

  • "My working diagnosis for any acute monoarthritis is septic arthritis, because that is the diagnosis that destroys the joint within days if I get it wrong."

2. Aspirate before anything else

  • "I would aspirate the joint before giving antibiotics or steroid - cell count and differential, Gram stain, culture in blood culture bottles, and polarised light microscopy - along with two sets of blood cultures."

3. Interpret the fluid carefully

  • "A count over fifty thousand with more than ninety per cent neutrophils is septic until proven otherwise, but no count excludes it, and the presence of crystals does not either - gout and sepsis coexist."

4. Treat and refer

  • "If sepsis is likely I start empirical flucloxacillin, adding vancomycin if there is MRSA risk, and I refer urgently for washout - antibiotics alone do not clear a purulent native joint."

5. Then work backwards to the cause

  • "Once infection is excluded, I look for the crystal, the spondyloarthritis or the structural lesion - and in an older patient with a knee, I am thinking pseudogout, with a metabolic screen if they are under fifty-five."

8 of 8 sections written · drafted 2026-09-04