White blood cell count and total protein concentration to predict the absence of microcrystals in synovial fluid

Synovial fluid analysis is essential for diagnosing crystal-induced arthritis. Monosodium urate monohydrate (MSU) crystals in gout and calcium pyrophosphate dihydrate (CPP) crystals in pseudogout trigger inflammatory reactions that result in white blood cell (WBC) activation. This study aimed to evaluate the predictive value of synovial fluid WBC count and total protein concentration for the absence of microcrystals.

This prospective study analyzed all synovial fluid samples collected at a single center in a 6-month period. The absolute WBC count and total protein concentration were recorded for each sample. A single expert used polarized light microscopy to detect microcrystals. Mann-Whitney U-tests was used to compare mean counts and concentrations in samples with and without crystals. Diagnostic performance was assessed through the area under the receiver-operating characteristic curve (AUC).

A total of 205 samples were included. The absolute WBC count was significantly higher in samples with crystals than in those without. No differences were found between MSU and CPP. The ROC curve showed an AUC 0.773, and an absolute WBC count <1650/mm³ yielded 95.7% sensitivity, 53.1% specificity, and 97.7% negative predictive value for predicting the absence of microcrystals. Total protein concentration was not significantly different between samples with and without crystals.

The WBC count is useful for screening for the absence of microcrystals in synovial fluid; the cutoff <1650 WBC/mm³ accurately predicts the absence of crystals, obviating the need for polarized light microscopy and thus simplifying and shortening laboratory analysis of synovial fluid, leading to a reduction in laboratory turnaround time.