What tests will most likely confirm the diagnosis of a swollen knee in a sexually active young woman with fever, joint aches, and vesiculopustular skin lesions?

Diagnostic Confirmation of Disseminated Gonococcal Infection

The diagnosis will most likely be confirmed by nucleic acid amplification testing (NAAT) of urogenital specimens (cervical or urine) combined with blood and synovial fluid cultures, as urogenital cultures are positive in 86% of cases while synovial fluid cultures are positive in only 44% of disseminated gonococcal infection cases. 1

Clinical Presentation Strongly Suggests Disseminated Gonococcal Infection (DGI)

This patient's presentation is classic for the arthritis-dermatitis syndrome of DGI:

• Sexually active young woman (most common demographic) 1, 2
• Migratory polyarthralgias affecting hands and wrists initially, now localized to one joint (knee) 1, 2
• Vesiculopustular skin lesions (pathognomonic cutaneous manifestation) 2, 3
• Fever and constitutional symptoms 1, 2

Notably, only 16% of DGI patients have genital symptoms, so their absence does not exclude the diagnosis 4

Optimal Diagnostic Testing Strategy

First-Line Testing (Highest Yield)

• Urogenital NAAT (cervical swab or urine gonococcal probe) should be obtained immediately, as this has the highest positivity rate at 86% 1, 2
• Blood cultures (at least 2 sets) should be drawn before antibiotics, though only positive in 12% of cases 1, 2
• Synovial fluid aspiration of the affected knee for:
  • Culture (positive in 44% of cases) 1, 4
  • PCR/NAAT (positive in 33% when culture is negative, increasing overall diagnostic yield to 91%) 4
  • Cell count with differential (expect >20,000 WBC/µL with purulent appearance) 1, 4

Additional Testing Sites

• Rectal and pharyngeal cultures/NAAT should be obtained, as these are positive in 39% and 7% of cases respectively when urogenital sites are negative 1

Critical Diagnostic Pitfalls to Avoid

• Do not wait for culture results to initiate treatment, as joint destruction can occur rapidly and lead to irreversible neurological complications if treatment is delayed 5
• Do not rely solely on synovial fluid culture, as it is positive in less than half of cases; urogenital testing has nearly double the yield 1, 4
• PCR/NAAT of synovial fluid is essential when culture is negative, as it increases diagnostic yield from 58% to 91% 4
• Test all mucosal sites (cervical, urethral, rectal, pharyngeal) as gonorrhea can be isolated from non-genital sites even when urogenital cultures are negative 1

Confirmatory Laboratory Findings

When synovial fluid is obtained, expect:

• Purulent appearance with WBC count >20,000/µL in all culture-positive cases 1, 4
• Elevated inflammatory markers (ESR, CRP) though these are nonspecific 1
• Positive Gram stain showing gram-negative diplococci in some cases, though sensitivity is low 4

Treatment Should Begin Immediately

Once specimens are collected, empiric treatment with intravenous ceftriaxone should be initiated without waiting for culture confirmation, given the risk of permanent joint damage and the emergence of penicillin-resistant strains in at least 5% of cases 1, 2