In a sexually active 17-year-old with a generalized rash and an edematous, painful, erythematous left ankle, what findings are expected on synovial fluid analysis?

Disseminated Gonococcal Infection with Septic Arthritis

The correct answer is B: WBC 75,000 with neutrophils 95%. This sexually active adolescent presenting with a generalized rash and monoarticular arthritis has disseminated gonococcal infection (DGI) with septic arthritis, which characteristically produces a markedly elevated synovial fluid WBC count (typically 50,000-100,000 cells/μL) with neutrophil predominance (>90%) 1, 2.

Clinical Reasoning

This presentation is classic for DGI, which frequently manifests with:

• Petechial or pustular skin lesions (the generalized rash)
• Asymmetric arthritis or tenosynovitis
• Occurs in sexually active young adults 1, 3

The combination of sexual activity, rash, and acute monoarthritis in a 17-year-old should immediately raise suspicion for gonococcal septic arthritis.

Expected Synovial Fluid Findings

Septic arthritis produces inflammatory synovial fluid with:

• WBC count: 50,000-100,000 cells/μL (threshold for septic arthritis diagnosis) 2
• Neutrophil predominance: >90% 2, 4
• These two parameters are the most diagnostically useful tests, with high sensitivity (0.84 for WBC count, 0.75 for neutrophil percentage) and specificity (0.84 and 0.92 respectively) 4

Why Other Options Are Incorrect:

• Option A (RBC 100,000): Suggests hemarthrosis from trauma, not infection
• Option C (WBC 100,000, lymphocytes 80%): Lymphocyte predominance suggests chronic inflammatory conditions like tuberculosis or fungal arthritis, not acute bacterial infection
• Options D & E (WBC 250): These low counts indicate noninflammatory arthropathy (osteoarthritis, trauma), not septic arthritis

Critical Management Points

Hospitalization is recommended for DGI, especially in adolescents where compliance may be uncertain 1. The patient requires:

1. Immediate arthrocentesis with Gram stain and culture (though Gram stain sensitivity is limited in synovial fluid)
2. Presumptive treatment for concurrent chlamydia unless excluded by testing 1
3. Ceftriaxone 1g IM/IV every 24 hours as first-line therapy 1

Common Pitfall to Avoid:

If initial blind tap is negative, consider ultrasound-guided aspiration of smaller joints (subtalar, ankle compartments) as gonococcal arthritis can affect these sites and be missed by blind aspiration 5. Don't dismiss the diagnosis based on one negative tap if clinical suspicion remains high.

The synovial fluid WBC count and neutrophil percentage provide independent diagnostic information and should both be ordered; chemistry studies (glucose, protein, LDH) are not recommended as they provide misleading or redundant information 4.