Bacterial infection of a joint — orthopedic emergency requiring urgent arthrocentesis and drainage.
Also known as: septic arthritis, pyogenic arthritis, bacterial arthritis
Overview
Pyogenic infection of a joint space, most commonly bacterial. Untreated, it leads to rapid cartilage destruction within days. Native joint and prosthetic joint infections share principles but differ in microbiology and surgical management.
Epidemiology
Annual incidence ~4-10 per 100,000; higher in patients with RA, prosthetic joints, diabetes, or immunosuppression. Knee is the most commonly involved joint in adults; hip is more common in young children.
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Question 1MusculoskeletalMedium
A 65-year-old man presents with a 2-day history of a hot, swollen, exquisitely painful right knee and inability to bear weight. He is febrile to 38.4°C. Arthrocentesis yields cloudy synovial fluid with a white blood cell count of 75,000 cells/mm³ (90% neutrophils) and a synovial glucose of 15 mg/dL with a simultaneous serum glucose of 95 mg/dL. Gram stain is negative. Which of the following is the most appropriate management?
AJoint aspiration alone
BOral antibiotics only
CEmpirical IV antibiotics
DLow-dose oral colchicine
EIntra-articular steroids
Reveal answer & full explanation
Correct answer: C — Empirical IV antibiotics
AJoint aspiration alone
BOral antibiotics only
CEmpirical IV antibiotics✓
DLow-dose oral colchicine
EIntra-articular steroids
Why Empirical IV antibiotics is correct
A synovial WBC above 50,000 cells/mm³ with neutrophil predominance, a markedly low synovial glucose, and fever indicates bacterial septic arthritis until proven otherwise, even with a negative Gram stain (Gram stain is only about 50% sensitive)
Prompt empirical intravenous antibiotics (e.g., vancomycin plus ceftriaxone to cover MRSA and gram-negatives/gonococcus) are started immediately after fluid is obtained, before cultures return
Joint decompression by serial aspiration or surgical lavage accompanies antibiotics, but antimicrobial therapy is the indispensable step that separates adequate from inadequate management here
Non-gonococcal septic arthritis is treated for roughly 2–4 weeks
Why the others are wrong
Intra-articular steroids — would suppress local immunity and worsen a joint infection; this is the anchoring trap for someone who mistakes the inflammatory fluid for a crystal or inflammatory arthritis
Oral antibiotics only — inadequate initial coverage for a closed-space joint infection, which requires IV therapy; the trap is under-treating a surgical-urgency infection
Joint aspiration alone — drainage is necessary but insufficient without antimicrobials; this is the right-action-but-incomplete trap that omits the antibiotics that treat the infection
Low-dose oral colchicine — an older man with an acutely hot, swollen knee fits a gout or pseudogout flare, and crystal arthritis can also raise the synovial WBC very high, but fever with a synovial glucose far below serum glucose must be treated as septic arthritis until cultures return, and finding crystals would not exclude coexisting infection; the trap is anchoring on the more common crystal diagnosis
Question 2MusculoskeletalMedium
A 65-year-old woman with rheumatoid arthritis on methotrexate has acute-onset severe right knee pain and swelling with a temperature of 38.8°C. Arthrocentesis yields synovial fluid with a white blood cell count of 68,000/mm³ (92% neutrophils) and a glucose of 18 mg/dL; Gram stain is negative. Which of the following is the most appropriate next step in management?
AIV antibiotics and joint drainage
BColchicine and high-dose NSAIDs
COral prednisone and rheumatology referral
DAwait culture results before antibiotics
EOral levofloxacin with next-day recheck
Reveal answer & full explanation
Correct answer: A — IV antibiotics and joint drainage
AIV antibiotics and joint drainage✓
BColchicine and high-dose NSAIDs
COral prednisone and rheumatology referral
DAwait culture results before antibiotics
EOral levofloxacin with next-day recheck
Why IV antibiotics and joint drainage is correct
A synovial white blood cell count of 68,000/mm³ with 92% neutrophils, fever, and immunosuppression from methotrexate is septic arthritis until proven otherwise.
A negative Gram stain does not exclude infection (sensitivity only about 50%), so empiric therapy cannot wait for culture results.
Start empiric IV antibiotics immediately — vancomycin for MRSA plus ceftriaxone for gram-negative organisms — paired with joint drainage by serial aspiration or surgical washout.
Most non-gonococcal septic arthritis is caused by Staphylococcus aureus, and any delay in antibiotics or drainage rapidly destroys cartilage and raises mortality.
Why the others are wrong
Await culture results before antibiotics — the negative Gram stain is the bait: it misses roughly half of culture-proven septic arthritis, and cultures take 24-48 hours, during which an untreated purulent joint loses cartilage irreversibly (delay-of-therapy trap).
Colchicine and high-dose NSAIDs — treats crystal arthritis, but no crystals are reported and a septic-range count with fever in an immunosuppressed patient demands antibiotics; premature closure on a gout or pseudogout flare.
Oral levofloxacin with next-day recheck — oral monotherapy does not cover MRSA in a methotrexate-immunosuppressed patient and, more importantly, leaves the infected joint undrained; septic arthritis requires IV therapy plus source control, not outpatient follow-up.
Oral prednisone and rheumatology referral — rheumatoid effusions are inflammatory and can have a low glucose, so a flare of her rheumatoid arthritis is tempting, but a count of 68,000/mm³ with 92% neutrophils, fever, and methotrexate immunosuppression make this septic arthritis until proven otherwise; systemic steroids without antibiotics or drainage would blunt the signs of infection while the joint is destroyed (inflammatory-flare look-alike trap).
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Intravenous drug use (sternoclavicular, sacroiliac, vertebral involvement)
Recent joint surgery or intra-articular injection
Skin or soft tissue infection, bacteremia, endocarditis
Age <5 or >65
Sexually active young adult (gonococcal arthritis)
Pathophysiology
Bacteria enter the joint by hematogenous seeding (most common), direct inoculation (trauma, injection, surgery), or contiguous spread from adjacent osteomyelitis or soft tissue infection. Bacterial proliferation drives a brisk neutrophilic synovitis; bacterial enzymes and host inflammatory mediators rapidly destroy articular cartilage. Microbiology: Staphylococcus aureus is the most common cause overall (gram-positive cocci in clusters; MRSA increasingly), followed by streptococci; Neisseria gonorrhoeae is the classic cause in young sexually active adults (gram-negative intracellular diplococci; synovial culture often negative); gram-negative bacilli including Pseudomonas occur in IV drug users, the elderly, and the immunocompromised; Salmonella in sickle cell disease; Kingella kingae in children under 4.
Ultrasound — guides aspiration, especially for deep joints (hip)
MRI — if osteomyelitis suspected or to evaluate axial joints (SI, sternoclavicular)
Diagnostic algorithm
Synovial Fluid
Normal
Non-inflammatory (OA)
Inflammatory (RA, crystal)
Septic
WBC/mm3
<200
<2,000
2,000-50,000
>50,000 (often >100,000)
% PMNs
<25%
<25%
>50%
>75%
Color/clarity
Clear, pale yellow
Clear, yellow
Yellow, cloudy
Purulent
Crystals
None
None
Possible
May coexist
Culture
Negative
Negative
Negative
Often positive
Synovial fluid analysis — the cornerstone of joint diagnosis.
Complications
Rapid cartilage destruction with permanent joint damage if treatment delayed
Osteomyelitis of adjacent bone
Sepsis and septic shock
Chronic post-infectious arthritis
Avascular necrosis of femoral head (pediatric hip)
Recurrence, especially with prosthetic joints
PANCE pearls
Synovial fluid WBC >50,000 with PMN predominance strongly suggests infection, but no threshold rules it out — culture is definitive.
Gonococcal arthritis often has lower fluid WBC and culture sensitivity; suspect in young, sexually active patients with migratory polyarthralgia, tenosynovitis, and pustular rash.
Crystals and infection can coexist — never exclude septic arthritis just because crystals are found.
A prosthetic joint with persistent pain after the early post-op period is infected until proven otherwise — aspirate before starting antibiotics if at all possible.
Children with refusal to bear weight, fever, ESR/CRP elevation, and high WBC (Kocher criteria) should be evaluated for pediatric septic hip.
References
IDSA 2013 — IDSA Clinical Practice Guideline for the Diagnosis and Management of Prosthetic Joint Infection (Osmon et al., Clin Infect Dis 2013)
AAOS 2019 — AAOS Clinical Practice Guideline: Diagnosis and Prevention of Periprosthetic Joint Infections (2019; summary in Tubb et al., J Am Acad Orthop Surg 2020;28(8):e340-e348)
BSR 2006 — BSR/BHPR Guideline for Management of the Hot Swollen Joint (Coakley et al., Rheumatology 2006)
Educational use only. This outline is a study aid for PA students and is not medical advice or a substitute for clinical judgment. FirstPassPA is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCCPA or PAEA. PANCE® and PANRE® are registered trademarks of the National Commission on Certification of Physician Assistants; End of Rotation™ is a program of the Physician Assistant Education Association.