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Synovial Fluid Analysis — SCE Rheumatology MCQ

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ModerateSynovial Fluid AnalysisSCE Rheumatology

A 64-year-old man with psoriatic arthritis treated with adalimumab presents with a 20-hour history of severe pain and swelling of his native right knee. He also has chronic kidney disease and takes bendroflumethiazide. His temperature is 38.1°C, blood pressure is 128/74 mmHg and NEWS2 is 1. The knee is warm with a large, tense effusion. C-reactive protein is 138 mg/L and peripheral white cell count is 13.2 × 10⁹/L. Urgent aspiration before antimicrobial treatment produces only 1.2 mL of turbid synovial fluid. The laboratory advises that the volume may be insufficient for all requested analyses. Blood cultures have not yet been taken. Which investigation and treatment sequence is most appropriate?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: D — Prioritise Gram stain and bacterial culture, then leukocyte count, then crystal microscopy; take blood cultures and start empiric antibiotics after aspiration

Explanation lettering: B = shown as A · E = shown as B · A = shown as C · C = shown as D · D = shown as E

This immunosuppressed patient has an acutely inflamed native joint with fever and substantial systemic inflammation. Crystal arthritis is plausible because of chronic kidney disease and thiazide exposure, but septic arthritis must receive diagnostic priority. When synovial fluid volume is limited, the SANJO guideline prioritises bacterial identification, followed by leukocyte count and then crystal analysis. Blood cultures should also be obtained, and because aspiration has already been completed, empiric antibiotics should now be started without awaiting laboratory results. A is incorrect because identifying crystals would not exclude concomitant infection and should not delay microbiological sampling or antibiotics. B is attractive because a synovial leukocyte count above 50,000/µL increases the probability of infection, but no leukocyte threshold reliably confirms or excludes septic arthritis; immunosuppression and prior treatment may further attenuate the count. D is incorrect because Gram stain has limited sensitivity: a negative result cannot justify omitting culture. E correctly prioritises culture but unnecessarily omits useful cell-count data and delays treatment until culture confirmation, which may take days or remain negative. Antibiotics should ideally follow aspiration and blood-culture collection in a haemodynamically stable patient, because pre-aspiration antibiotics reduce synovial leukocyte counts, neutrophil proportions and culture positivity.

Reference: Guideline for management of septic arthritis in native joints (SANJO) (12 January 2023) — https://pubmed.ncbi.nlm.nih.gov/36756304/ Ask an expert: Gout (8 January 2026) — https://www.bmj.com/content/392/bmj.r2248.full.pdf Effect of Antibiotic Administration Before Joint Aspiration on Synovial Fluid White Blood Cell Count in Native Joint Septic Arthritis (11 January 2024) — https://pubmed.ncbi.nlm.nih.gov/38221984/