What Weight Should Now Be Given to Synovial Fluid Leucocyte Count When Septic Arthritis Is Suspected? A Narrative Review

Synovial fluid leucocyte count has long been regarded as a cornerstone in the diagnosis of paediatric septic arthritis, with a threshold of 50,000 cells/mm3 widely adopted in clinical practice. This narrative review critically appraised the available evidence regarding its diagnostic utility and interpretation in paediatric septic arthritis. To enhance the transparency and representativeness of the narrative synthesis, rather than to achieve the exhaustive and reproducible study retrieval required for a systematic review, relevant studies were identified through a structured literature search. Fourteen paediatric studies evaluating synovial fluid leucocyte count in the diagnostic assessment of septic arthritis were identified. Whilst several studies reported markedly elevated leucocyte counts in children with septic arthritis, others demonstrated microbiologically confirmed infections below the traditional cut-off, with substantial overlap observed with alternative diagnoses, particularly Lyme arthritis and juvenile idiopathic arthritis. Pathogen characteristics, host inflammatory responses, and prior antibiotic exposure were identified as important factors influencing synovial fluid findings. Current evidence suggests that synovial leucocyte count is best regarded as a marker of intra-articular inflammation rather than a definitive diagnostic test for septic arthritis. Its value lies in contributing to an integrated diagnostic assessment incorporating clinical presentation, imaging findings, microbiological investigations, and emerging molecular diagnostic techniques.