Septic arthritis
A hot, swollen, painful joint is septic until proven otherwise. Aspirate before antibiotics whenever it's safe.
A 66-year-old diabetic man has a hot, swollen right knee for 2 days. He had "an injection in the knee" from a local clinic a week ago. Temperature 38.4 °C. He cannot bend the knee at all.
What must happen before the first antibiotic dose?
- Treat as septic until proven otherwiseOne hot, swollen joint with painful, limited movement.
- Aspirate before antibioticsUnless the patient is septic: then cultures and antibiotics now.
- Send the fluidGram stain, culture, cell count, crystals.
- Blood cultures, CBC, CRP, creatinine, sugarESR too in children.
- Empirical IV antibioticStraight after aspiration.
- Orthopaedics for washoutMost need it, especially hip and prosthetic joints.
- Rest, splint, pain reliefThen early physiotherapy.
- Prosthetic joint
- Child with a hot hip or refusing to walk
- Septic patient
- Hip, shoulder or spine involvement
- Frank pus on aspiration
- Gout or pseudogout (can coexist)
- Reactive arthritis
- Gonococcal arthritis in young adults
- TB or brucella (chronic, Pakistan)
- Cellulitis over a joint
- Osteomyelitis
| Synovial white cells | Above 50,000/µL strongly suggests infection, lower counts do not exclude it |
| Crystals | Finding gout or pseudogout crystals does not rule out infection |
| Prosthetic joint | Orthopaedics before antibiotics if the patient is stable |
| Kocher criteria (child's hip) | Not weight-bearing, fever above 38.5 °C, ESR above 40 mm/h, white cells above 12,000/µL |
| Kocher score | The more criteria present, the higher the chance of septic hip, 3–4 is high risk |
Check every dose against a current drug reference and your hospital protocol.
On this page
Why aspirate first?
Bacteria and pus destroy joint cartilage quickly. Culture of joint fluid is the best way to find the organism, and the first antibiotic dose can make it grow nothing. Aspirate first, unless the patient is septic or aspiration will be delayed for hours.
- Do not aspirate through infected skin.
- Prosthetic joints: aspiration by orthopaedics in theatre or clinic, ideally before antibiotics if stable.
- Hip: usually ultrasound-guided.
Reading the joint fluid
| Finding | Suggests |
|---|---|
| Cloudy or frank pus | Infection |
| White cells above 50,000/µL, mostly neutrophils | Infection likely |
| Gram stain shows organisms | Infection (but a negative stain does not exclude it) |
| Needle-shaped, negatively birefringent crystals | Gout |
| Rhomboid, positively birefringent crystals | Pseudogout |
| Blood-stained | Trauma, haemophilia, anticoagulants |
Who gets it?
- Diabetes, rheumatoid arthritis, older age, prosthetic joints, IV drug use, immunosuppression.
- Unsterile joint injections: in Pakistan, steroid injections given in clinics without proper asepsis are a common route.
- Young sexually active adults: gonococcal arthritis (often with skin pustules and tenosynovitis).
- Children: usually spread from blood; hip and knee most common.
Empirical antibiotics
- Duration: usually about 2 weeks IV then oral, for 4–6 weeks in total; guided by the microbiologist and response.
- Children: weight-based doses per the paediatric policy.
Washout
- Arthroscopic or open washout, or repeated needle aspiration, removes pus and protects cartilage.
- The hip, shoulder and prosthetic joints almost always need surgery.
The child's hip
- Transient synovitis more likely
- Rest, NSAID, review in 24–48 hours
- Return if fever or worse
- Ultrasound for effusion
- Aspiration by orthopaedics
- Treat as septic hip
Many centres add CRP above 20 mg/L as a fifth predictor. Whatever the score, an unwell child with a hot joint needs orthopaedics today.
Frequently asked questions
What are the Kocher criteria?
Four predictors of septic arthritis of the hip in children: not bearing weight, fever above 38.5 °C, ESR above 40 mm/h and white cells above 12,000/µL. The more present, the higher the risk.
What is the treatment of septic arthritis?
Aspirate the joint, then start empirical IV antibiotics (usually flucloxacillin; vancomycin if MRSA risk), and arrange washout. Total treatment is usually 4–6 weeks.
Can gout and septic arthritis occur together?
Yes. Finding crystals does not rule out infection. Always send the fluid for Gram stain and culture.
Test yourself
Exam-style questions on this guide, from our two question banks. Choose an answer to see why each option is right or wrong.
1. A stable patient has a hot, swollen knee. Joint aspiration can be done in 30 minutes. When should antibiotics start?
Answer: Straight after aspiration. Aspirate first in a stable patient, then start empirical IV antibiotics immediately.
- After culture results in 48 hours: Too late.
- Only if fever: Fever may be absent.
- Before aspiration: Antibiotics can make the culture negative.
- Only if crystals are absent: Crystals don't exclude infection.
2. Joint fluid shows urate crystals and 80,000 white cells/µL. What is the best management?
Answer: Treat as septic arthritis. Gout and septic arthritis can coexist; a very high cell count needs treatment for infection.
- Steroid injection into the joint: Dangerous if infected.
- Allopurinol now: Not in acute attack and doesn't address infection.
- Discharge with NSAIDs: Unsafe.
- Colchicine only: Infection can coexist with gout.
3. A 6-year-old won't bear weight, has a temperature of 39 °C, ESR 55 mm/h and white cells 15,000/µL. What is the likely diagnosis?
Answer: Septic arthritis of the hip. All four Kocher criteria are present: high risk of septic hip. Ultrasound, aspiration and IV antibiotics.
- Slipped upper femoral epiphysis: Wrong age and no fever.
- Transient synovitis: All four Kocher criteria make this less likely.
- Perthes disease: Not with fever and high markers.
- Growing pains: No.
4. A sexually active 22-year-old has fever, migratory polyarthralgia, tenosynovitis of the wrist and a few pustules on the hands. Which organism is most likely?
Answer: Neisseria gonorrhoeae. Disseminated gonococcal infection presents with the triad of tenosynovitis, dermatitis (pustules) and migratory polyarthralgia, or as purulent monoarthritis. Culture blood, joint, and genital/throat sites (NAAT). Treat with IV ceftriaxone. S. aureus is the commonest cause of septic arthritis overall; Salmonella causes osteomyelitis in sickle cell disease.
- Staphylococcus aureus: Staph aureus usually affects a single large joint.
- Streptococcus pyogenes: Strep pyogenes does not cause this triad.
- Mycobacterium tuberculosis: TB causes a chronic monoarthritis.
5. A 5-year-old has a painful limp. Temperature 39 °C, refuses to bear weight on the right leg, ESR 55 mm/h, WCC 15 × 10⁹/L. Ultrasound shows a hip effusion. What is the most likely diagnosis and next step?
Answer: Septic arthritis — urgent aspiration. Kocher’s criteria for septic arthritis of the hip: non-weight-bearing, fever above 38.5 °C, ESR above 40 mm/h and WCC above 12 × 10⁹/L. With all four, the probability of septic arthritis is very high. This is an emergency: aspirate (usually in theatre), washout and IV antibiotics. Transient synovitis usually follows a viral illness, with a well child and mild inflammatory markers.
- Transient synovitis — NSAIDs: Well, afebrile child with near-normal markers.
- Perthes disease — observation only: Perthes disease presents gradually in an afebrile child; high fever, raised ESR and WCC with an effusion suggest septic arthritis.
- SUFE — pinning: SUFE affects overweight adolescents and is not febrile; a febrile 5-year-old with an effusion needs aspiration.
6. Normal synovial fluid is clear and very viscous. Which synovial cells secrete the hyaluronic acid responsible?
Answer: Type B (fibroblast-like) synoviocytes. The synovial intima has two cell types without a basement membrane. Type A synoviocytes are macrophage-like and remove debris, while type B synoviocytes are fibroblast-like and secrete hyaluronic acid and lubricin. Hyaluronic acid gives synovial fluid its viscosity, and lubricin lubricates the cartilage surface. In inflammation such as septic or rheumatoid arthritis, hyaluronic acid is degraded and the fluid becomes less viscous.
- Type A (macrophage-like) synoviocytes: Phagocytic cells; they clear debris.
- Articular chondrocytes: Make cartilage matrix, not synovial fluid hyaluronan.
- Mast cells of the subintima: Not the source of hyaluronic acid.
- Endothelial cells of capillaries: Supply plasma filtrate, not hyaluronic acid.
7. A 25-year-old woman has a slowly enlarging deep mass near the knee. Biopsy shows a biphasic tumour of spindle cells and epithelial glands, and genetic testing shows a t(X;18) SS18-SSX fusion. What is the tumour?
Answer: Synovial sarcoma. Synovial sarcoma occurs in young adults, usually in deep soft tissue near large joints, especially the knee. Despite its name, it does not arise from synovium. It may be biphasic (spindle cells with glands) or monophasic, and it is defined by the t(X;18) translocation creating the SS18-SSX fusion gene. It often contains calcification on X-ray and can be slow-growing at first, which delays diagnosis.
- Liposarcoma: A fat-forming sarcoma of older adults, often retroperitoneal.
- Leiomyosarcoma: A smooth muscle sarcoma, often in the uterus or retroperitoneum.
- Pleomorphic sarcoma: High-grade, in older adults; no specific translocation.
- Rhabdomyosarcoma: Childhood muscle tumour; myogenin-positive.
References and dose sources
- NICE CKS: Septic arthritis
- NICE NG51: Sepsis
- BOAST — management of musculoskeletal soft tissue infections (native large joint infection)
- BSR/BHPR/BOA — management of the hot swollen joint in adults
- Flucloxacillin dosing: Drugs.com: Flucloxacillin (international) (see also our flucloxacillin dose page)
- Clindamycin dosing: Drugs.com: Clindamycin dosage (see also our clindamycin dose page)
- Vancomycin dosing: Drugs.com: Vancomycin dosage (see also our vancomycin dose page)
- Ceftriaxone dosing: Drugs.com: Ceftriaxone dosage (see also our ceftriaxone dose page)
International guidance is linked where it reflects good practice. Where your hospital’s protocol differs, follow your hospital.