ConditionBones, joints & muscles
Septic Arthritis
Septic arthritis is an infection inside a joint, usually caused by bacteria such as Staphylococcus aureus. It typically causes one hot, red, swollen and very painful joint, most often the knee or hip, with fever and difficulty moving it. It is a medical emergency: bacteria can destroy joint cartilage within days. Diagnosis needs a sample of joint fluid, and treatment is urgent drainage of the joint plus several weeks of antibiotics. Children who refuse to walk or move a limb with fever need same-day care.
Also called: Infectious arthritis, bacterial arthritis, pyogenic arthritis, joint infection
At a glance
- What it is
- An infection inside a joint, usually bacterial.
- Most common joints
- Knee (about half), hip, shoulder, ankle, wrist.
- Main germ
- Staphylococcus aureus, including MRSA; streptococci; gonorrhea in young sexually active adults.
- Symptoms
- One hot, swollen, very painful joint, unable to move or bear weight, fever.
- Diagnosis
- Joint aspiration (fluid test), blood tests and cultures, imaging.
- Treatment
- Urgent drainage (needle or surgery) and IV then oral antibiotics for weeks.
When to get help
Call 911 or go to the ER
- A suddenly hot, red, swollen, very painful joint with fever, especially in a child, an older adult, or someone with a joint replacement, diabetes or a weak immune system
- A child with fever who refuses to walk, bear weight or move an arm or leg
- Joint pain with fever, chills, confusion or a racing heart (possible sepsis)
See a doctor within a day or two
- A single swollen, painful joint without an obvious injury, even without fever
- New pain, swelling or wound drainage around a joint replacement
- Joint pain with a skin rash and genital discharge or a new sexual partner
Book a routine appointment
- Ongoing joint stiffness or pain after treatment for a joint infection
Why septic arthritis is an emergency
Bacteria and the body's inflammatory response release enzymes that break down cartilage within 24 to 48 hours. Delayed treatment can lead to permanent joint damage, arthritis, bone infection and sepsis. Mortality in adults is significant, especially in older people and those with other illnesses. Prompt drainage and antibiotics greatly improve outcomes.
Causes
Bacteria reach the joint by:
- the bloodstream, from an infection elsewhere such as the skin, urinary tract or lungs, the most common route
- direct entry, through a wound, animal bite, injection or surgery
- spread from nearby infection, such as bone infection (osteomyelitis)
Common organisms:
| Organism | Typical setting |
|---|---|
| Staphylococcus aureus (including MRSA) | Most common at all ages |
| Streptococci | Adults and children |
| Neisseria gonorrhoeae | Sexually active young adults; often several joints, skin rash and tendon pain (gonorrhea) |
| Kingella kingae | Children under 4 |
| Gram-negative bacteria | Older adults, injection drug use, weakened immunity |
| Coagulase-negative staph | Joint replacements (prosthetic joint infection) |
Risk factors
- age over 80, or young children
- existing joint disease, such as rheumatoid arthritis, osteoarthritis or gout
- joint replacement or recent joint surgery or injection
- diabetes, kidney or liver disease
- immune-suppressing medicines or conditions
- skin infections and skin ulcers
- injection drug use
- sexually transmitted infections
Symptoms
- sudden pain in one joint, severe and worse with any movement
- swelling, warmth and often redness
- inability to move the joint or bear weight
- fever and chills, though older adults and people on steroids may have little fever
- feeling generally unwell
In children
- limping or refusing to walk
- holding a limb still (pseudoparalysis)
- crying when a diaper is changed, if the hip is affected
- fever and irritability
Septic arthritis vs other causes of a hot, swollen joint
| Condition | Clues |
|---|---|
| Septic arthritis | Fever, very painful, unable to move joint; infection in joint fluid |
| Gout | Often big toe; previous attacks; crystals in fluid; can coexist with infection |
| Pseudogout | Often knee or wrist in older adults; calcium crystals |
| Reactive arthritis | After gut or genital infection; several joints, eye and skin symptoms |
| Lyme arthritis | Knee swelling, less pain, history of tick exposure |
| Transient synovitis (children) | Hip pain after a virus, child less unwell, low inflammation markers |
Because these look alike, any hot swollen joint with uncertain cause should be aspirated.
Diagnosis
- Joint aspiration: a needle removes fluid for white cell count, Gram stain, culture and crystal examination. A very high white cell count (often over 50,000 per microliter) suggests infection
- Blood tests: white cell count, CRP and ESR, blood cultures
- Imaging: X-ray as a baseline; ultrasound to detect fluid, especially in the hip; MRI if bone infection is suspected
- Swabs for gonorrhea when relevant
Treatment
Drainage
The infected fluid must be removed:
- repeated needle aspiration, or
- arthroscopic (keyhole) washout, or
- open surgery, especially for the hip, prosthetic joints or when other methods fail
Antibiotics
- IV antibiotics started right after joint fluid is taken, chosen to cover likely organisms (often including MRSA), then adjusted to culture results
- switch to oral antibiotics once improving
- total duration usually 2 to 6 weeks; children with uncomplicated infection may need shorter courses
Supportive care
- pain relief
- splinting briefly, then early physical therapy to keep joint movement
Prosthetic joint infection
Infections of joint replacements often need surgery to clean or replace the implant and longer antibiotic courses.
Complications
- cartilage destruction and early arthritis
- joint stiffness or fusion
- bone infection
- growth plate damage in children
- sepsis and death
Frequently asked questions
What are the symptoms of septic arthritis?
The main symptoms are sudden severe pain in one joint, swelling, warmth, redness, inability to move the joint, and fever. Children may limp, refuse to walk or keep a limb still. Older adults may have little fever.
How is septic arthritis diagnosed?
A doctor removes fluid from the joint with a needle and tests it for white cells, bacteria and crystals. Blood tests, blood cultures and imaging such as ultrasound or MRI support the diagnosis.
How is septic arthritis treated?
It needs urgent drainage of the infected joint, by needle, keyhole surgery or open surgery, plus antibiotics, usually given through a vein at first and then by mouth for a total of several weeks.
Can septic arthritis cause permanent damage?
Yes. Bacteria can destroy joint cartilage within days, leading to arthritis, stiffness or deformity. Fast diagnosis and treatment greatly reduce this risk.
How do you tell septic arthritis from gout?
They can look identical: both cause a hot, swollen, painful joint. Only joint fluid testing reliably tells them apart, showing crystals in gout and bacteria or very high white cell counts in infection. They can also occur together.
Sources
This guide was written from current patient and clinical guidance, including:
- American Academy of Orthopaedic Surgeons (OrthoInfo): Septic Arthritis
- American Academy of Family Physicians: Septic Arthritis in Adults
- Pediatric Infectious Diseases Society and IDSA: Clinical Practice Guideline on Acute Hematogenous Osteomyelitis and septic arthritis in children (2023)
- MedlinePlus: Bacterial (septic) arthritis
- NICE Clinical Knowledge Summaries: Septic arthritis