Infectious Arthritis
Rheumatology · Infectious Arthritis 🎯 Practice these cards
In acute bacterial arthritis, synovial fluid leukocyte counts are typically greater than ____ per microliter and often exceed ____, with a ____ predominance.
Bacterial synovial fluid is cloudy and low-viscosity, with leukocyte counts typically >50,000/µL and often >100,000/µL (mean ~100,000, >90% neutrophils). Lower counts occur with gonococcal, mycobacterial, and fungal infection, in people who inject drugs, and in immunocompromised hosts. Crystal arthropathies can mimic these counts, and crystals do not exclude coexisting infection.
A positive synovial fluid Gram stain is considered ____, but its sensitivity is only about ____; synovial fluid cultures are positive in more than ____ of nongonococcal bacterial arthritis cases unless antibiotics were given first.
A positive synovial fluid Gram stain is essentially definitive, but sensitivity is only ~30–50% overall (up to ~75% for staphylococci and streptococci) — a negative smear never rules out infection. Synovial fluid cultures are positive in >90% of nongonococcal bacterial cases unless antibiotics preceded aspiration; inoculating fluid into blood-culture bottles improves yield. Blood cultures are positive in 50–70% of S. aureus joint infections.
Approximately ____ of nongonococcal infectious arthritis cases in adults are caused by gram-positive cocci, and the most frequent microorganism in both native and prosthetic joints is ____.
About 75% of nongonococcal infectious arthritis in adults is caused by gram-positive cocci. S. aureus (including MRSA) is the most frequent organism in both native and prosthetic joints; S. epidermidis and other coagulase-negative staphylococci favor prosthetic joints. In young adults N. gonorrhoeae predominates; in older/comorbid adults, gram-negative bacilli, pneumococci, and β-hemolytic streptococci account for up to a third of cases.
Disseminated gonococcal infection presents either as the ____ syndrome (triad of tenosynovitis, dermatitis, and polyarthralgia) or as ____ arthritis; in the first, ____ cultures are more often positive, while in the second, ____ cultures are.
DGI presents as (1) the arthritis–dermatitis syndrome — tenosynovitis, painless pustular dermatitis, and migratory polyarthralgia (immune-complex mediated: synovial cultures consistently negative, blood cultures positive in <45%); or (2) purulent mono/oligoarthritis without skin lesions or fever (synovial cultures positive in <40%, blood cultures nearly always negative). Diagnose via NAAT/culture of mucosal sites (urogenital, rectal, pharyngeal).
Recurrent disseminated gonococcal infection should prompt evaluation for deficiency of the ____ complement components; the monoclonal antibody ____, which blocks C5, carries the same risk.
DGI risk factors: female sex (2–3×), recent menstruation and pregnancy, HIV infection, immunoglobulin deficiency, and — classically — terminal complement (C5–C9) deficiency, which causes recurrent gonococcemia. The C5 inhibitor eculizumab carries the same risk. Meningococcemia can mimic the DGI dermatitis–arthritis syndrome.
Lyme arthritis is a ____-stage manifestation most commonly affecting the ____; diagnosis relies primarily on ____, and antibiotic-refractory cases (≤10%) are treated with ____ or synovectomy.
Lyme arthritis is a late manifestation of B. burgdorferi infection (up to 60% of untreated patients), typically an inflammatory monoarthritis of the knee. Diagnosis is by two-tier serology (ELISA + Western blot; IgG positive in >90%), preferred over PCR/culture of blood or fluid. ~90% respond to 28 days of oral doxycycline, amoxicillin, or cefuroxime; incomplete responders get IV ceftriaxone 2–4 weeks (no benefit beyond 1 month). Antibiotic-refractory disease (≤10%) is autoimmune (HLA-DR4, OspA) → DMARDs or synovectomy.
Tuberculous arthritis typically presents as chronic pain in a single ____ joint; concurrent pulmonary tuberculosis is present in only a ____ of patients, and diagnosis usually requires ____.
Peripheral TB arthritis is a chronic, slowly progressive monoarthritis of weight-bearing joints (hip, knee, ankle) evolving over months to years; systemic symptoms occur in ~50% and concurrent pulmonary TB in only a minority. Synovial fluid averages ~20,000 cells/µL; AFB smear is positive in <1/3, fluid culture in ~80%, and synovial biopsy culture in ~90% (granulomatous) — biopsy is usually required. Treat as pulmonary TB: multidrug therapy for 6–9 months.
Prosthetic joint infections are classified as early onset (<____ months), delayed (3–24 months), and late onset (>____ months); early and delayed infections usually stem from ____, whereas late infections result from ____.
PJI complicates 1–3% of knee/hip replacements. Early (<3 months) and delayed (3–24 months) infections stem from surgical contamination at implantation — delayed infections are indolent (coagulase-negative staphylococci, Cutibacterium) and may present only as chronic unexplained pain or radiographic loosening. Late (>24 months) infections result from hematogenous seeding and present acutely with virulent organisms.
Empiric therapy for suspected bacterial arthritis: gram-positive cocci on smear → ____; gram-negative bacilli → ____; suspected Pseudomonas → ____.
After cultures are obtained, start empiric IV bactericidal therapy: gram-positive cocci → vancomycin (narrow to nafcillin/oxacillin/cefazolin for MSSA); gram-negative bacilli → third-generation cephalosporin (ceftriaxone/cefotaxime); Pseudomonas risk → ceftazidime or cefepime; no organism on smear → vancomycin plus a third-generation cephalosporin (add antipseudomonal coverage if immunocompromised). Total duration usually 3–6 weeks.
Gonococcal arthritis is treated with IV/IM ____ (1 g every 24 hours) for at least 7 days, plus ____ for 7 days if chlamydial infection has not been excluded.
Ceftriaxone 1 g IV or IM every 24 hours for ≥7 days, plus doxycycline 100 mg PO twice daily for 7 days if chlamydial infection has not been excluded. No empiric oral step-down (rising resistance); current CDC guidance has dropped routine azithromycin dual therapy in favor of higher-dose ceftriaxone monotherapy. Test and presumptively treat sexual partners.
In adults, ____ virus causes acute symmetric polyarthritis of the small hand joints (diagnosed by IgM); ____ virus arthritis is prodromal and resolves when jaundice appears; chronic ____ infection may produce mixed cryoglobulinemia (purpura, weakness, arthralgia).
Parvovirus B19: up to 60% of adults, acute symmetric polyarthritis of the small hand joints (RA mimic), diagnosed by IgM, self-limited (weeks–months). HBV: self-limited symmetric arthritis in up to 25%, pre-icteric prodrome, resolves with jaundice. HCV: arthralgia/arthritis in ~1/3, mixed cryoglobulinemia (purpura, weakness, arthralgia); both HBV and HCV may be RF-positive. Chikungunya: prominent synovitis/tenosynovitis that can persist for months–years, mimicking seronegative RA.
An infected joint must be drained: readily accessible joints by ____ (usually daily) while an effusion persists, while joints such as the ____ require surgical drainage.
Antibiotics alone are insufficient: drain the infected joint. Readily accessible joints (e.g., knee) by repeated needle aspiration, usually daily, while an effusion persists (ultrasound guidance helps). Surgical drainage is required for inaccessible joints (hip, shoulder, sternoclavicular), soft-tissue extension, or inadequate response; hip septic arthritis is best managed by arthrotomy (especially in children — femoral head viability). EBJIS 2023 favors arthroscopic lavage ± synovectomy over routine open surgery when surgery is needed.
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