Why Septic Arthritis Is Examined
Septic arthritis is an orthopaedic emergency — bacteria within a joint space destroy articular cartilage within days. It requires same-day diagnosis and treatment. OSCEs examine it in clinical assessment stations (acutely swollen red knee — what is your approach?), procedural stations (joint aspiration), and clinical reasoning. The key skill is never assuming it is gout or pseudogout without excluding infection first.
⚠️ Red Flag
Never assume a hot swollen joint is crystal arthropathy without excluding septic arthritis. Both conditions can coexist. Joint aspiration is the only definitive way to differentiate them — it is always indicated in an acutely hot monoarthritic joint.
Differential Diagnosis of the Acute Hot Joint
| Diagnosis | Features favouring |
|---|---|
| Septic arthritis | Fever, raised CRP/WCC, single joint, immunocompromised, skin breach, prosthetic joint |
| Gout | First MTP joint (podagra), uric acid above 360, tophi, alcohol excess, diuretics |
| Pseudogout | Elderly, knee or wrist, chondrocalcinosis on X-ray, hyper/hypoparathyroidism |
| Reactive arthritis | Recent GI or STI, asymmetric large joint oligoarthritis, preceding diarrhoea or urethritis |
| Haemarthrosis | Recent trauma, anticoagulated patient, bleeding disorder |
| Acute flare of OA/RA | Known diagnosis, bilateral or polyarticular, no fever, normal WCC |
Kocher Criteria (Paediatric Septic Arthritis)
Four clinical predictors for septic arthritis of the hip in children:
| Criterion | Points |
|---|---|
| Fever (above 38.5 degrees C) | 1 |
| Non-weight-bearing | 1 |
| ESR above 40 mm/hr | 1 |
| WCC above 12 x 10⁹/L | 1 |
| Score | Probability of septic arthritis |
|---|---|
| 0 | Under 0.2% |
| 1 | 3% |
| 2 | 40% |
| 3 | 93% |
| 4 | 99% |
Adult criteria are less well validated — any hot joint with systemic features warrants aspiration.
Assessment — Systematic Approach
History Key Points
- Duration and onset of swelling (hours = septic arthritis or crystal; days to weeks = inflammatory arthritis)
- Fever, rigors, systemic illness
- Recent trauma, joint injections, procedures, skin cuts
- Sexual history (gonococcal arthritis — commonest in sexually active young adults)
- Travel history (TB arthritis, reactive arthritis)
- Recent GI illness (reactive arthritis — Salmonella, Campylobacter, Yersinia, Shigella)
- Past medical history: diabetes, RA (predisposes to infection), prosthetic joint, immunosuppression
Examination
- Temperature, HR, BP (systemic sepsis)
- Inspect joint: swelling, erythema, warmth
- Assess range of movement — passive movement extremely painful in septic arthritis; crystal arthritis causes pain through range but often allows passive movement better than active
- Examine for skin breach, nail changes, tophi, psoriatic plaques
- Check other joints — monoarthritis vs oligoarthritis vs polyarthritis
Investigations
| Investigation | Finding |
|---|---|
| FBC | Raised WCC (neutrophilia in infection) |
| CRP, ESR | Markedly raised in septic arthritis |
| Blood cultures | Positive in 50% of septic arthritis — take before antibiotics |
| Uric acid | Raised in gout (note: can be normal during acute attack) |
| X-ray of joint | Usually normal early; prosthetic loosening; chondrocalcinosis (pseudogout) |
| Joint aspiration (synovial fluid) | Essential — see below |
Joint Aspiration and Synovial Fluid Analysis
Synovial fluid appearance and interpretation:
| Finding | Normal | Septic arthritis | Crystal arthropathy | Haemarthrosis |
|---|---|---|---|---|
| Appearance | Clear, straw | Turbid, purulent | Turbid | Bloody/xanthochromic |
| White cells | Below 200/mm3 | Above 50,000/mm3 (neutrophils) | 2,000-50,000/mm3 | Variable |
| Gram stain | Negative | Positive in 60-80% | Negative | Negative |
| Culture | Negative | Positive in 80-90% | Negative | Negative |
| Crystals | None | None | Present (see below) | None |
Crystal identification under polarised light microscopy:
| Crystal | Appearance | Condition |
|---|---|---|
| Monosodium urate (MSU) | Needle-shaped, negatively birefringent | Gout |
| Calcium pyrophosphate (CPP) | Rhomboid-shaped, positively birefringent | Pseudogout (CPPD) |
💎 Clinical Pearl
Always send synovial fluid for MC&S, cell count, and crystal analysis simultaneously. A crystal-positive result does not exclude septic arthritis — they can coexist, especially in patients with underlying crystal arthropathy.
Empirical Antibiotic Treatment (UK Guidelines)
While awaiting cultures:
| Patient group | Antibiotic | Route |
|---|---|---|
| Standard adult (native joint) | Flucloxacillin 2 g QDS | IV |
| Penicillin allergy | Clindamycin 900 mg TDS | IV |
| Suspected gonococcal (young adult) | Ceftriaxone 1 g OD | IV |
| Prosthetic joint infection | Vancomycin IV (cover MRSA) | IV |
| MRSA risk (previous MRSA, healthcare exposure) | Vancomycin | IV |
Duration: 2-6 weeks total (IV initially 2 weeks then PO), guided by microbiology.
Surgical washout (arthroscopic joint lavage) is required in most cases — antibiotics alone are insufficient to clear established joint infection.
Frequently Asked Questions
"What is the most common organism causing septic arthritis?"
Staphylococcus aureus is the most common cause in adults (50-70%), including MRSA in healthcare-associated infections. Streptococcal species (group A and B) are the next most common. Neisseria gonorrhoeae is the most common cause in sexually active young adults (age 15-40) — consider in any young adult with monoarthritis, skin pustules, or tenosynovitis. Gram-negative organisms (E. coli, Pseudomonas) occur in the elderly, immunocompromised, and IV drug users.
"How does gonococcal arthritis present differently from non-gonococcal?"
Gonococcal arthritis (disseminated gonococcal infection) typically presents as a migratory polyarthritis (moving from joint to joint), tenosynovitis, and a pustular skin rash, rather than the classic hot monoarthritis of non-gonococcal septic arthritis. The joint fluid culture is often negative as the bacteraemia is transient by the time arthritis develops. Blood cultures and genital, rectal, and pharyngeal swabs are more likely to be positive. Responds rapidly to IV ceftriaxone.
"What is the difference between gout and pseudogout?"
Gout is caused by monosodium urate crystal deposition from hyperuricaemia. It classically affects the first metatarsophalangeal joint (podagra) and is precipitated by alcohol, dietary purines, dehydration, and diuretics. Pseudogout (calcium pyrophosphate deposition disease) affects larger joints — particularly the knee and wrist — and is associated with ageing, hyperparathyroidism, haemochromatosis, and hypomagnesaemia. Both are diagnosed by crystal identification under polarised light microscopy. Treatment is similar: NSAIDs, colchicine, or intra-articular corticosteroids.
"How do you manage a hot prosthetic joint?"
Prosthetic joint infection (PJI) requires urgent orthopaedic review and admission. Do not give antibiotics before aspiration and cultures. Treatment involves: prolonged IV antibiotics (vancomycin +/- rifampicin), surgical debridement and retention (DAIR — debridement, antibiotics, implant retention) if early infection with intact implant, or two-stage revision (implant removal, spacer, re-implantation) for established or late infection. PJI is catastrophic — prevention with antibiotic prophylaxis at surgery and surgical site infection prevention is essential.
"What is reactive arthritis and how does it differ from septic arthritis?"
Reactive arthritis (formerly Reiter's syndrome) is a sterile inflammatory arthritis triggered by a remote infection — typically GI (Salmonella, Campylobacter, Yersinia) or urogenital (Chlamydia trachomatis). The joint itself is not infected — bacteria are not present in the synovial fluid. It typically presents 2-4 weeks after the triggering infection as an asymmetric large joint oligoarthritis (knee, ankle, hip). The classic triad is arthritis, urethritis, and uveitis ("can't see, can't pee, can't bend the knee"). Management: NSAIDs for joint symptoms; treat the underlying infection; corticosteroids if severe.
Related Posts
- Musculoskeletal History OSCE — systematic history for the patient with joint pain
- Knee Examination OSCE — examining the knee for septic arthritis and joint effusion
- Blood Results Interpretation OSCE — interpreting CRP, WCC, and uric acid in acute arthritis