Detection of Crystals in Gram-Stained Synovial Fluid

Gout and calcium pyrophosphate deposition (CPPD) disease are common crystalline arthropathies. Identification of crystals in synovial fluid using polarized light microscopy (PLM) aids diagnosis. In routine clinical practice, the volume of synovial fluid available for PLM is frequently insufficient, especially when fluid is also needed for other analyses, such as Gram stain and culture. Few studies have investigated the reliability of microscopic examination for crystals, with or without polarization, using Gram-stained synovial fluid specimens. This study estimated the sensitivity and specificity of microscopy with and without polarization on Gram-stained specimens for identification of crystals compared with standard PLM on unstained samples.

In total, 105 unstained synovial and bursal fluid samples were processed and examined by laboratory personnel using routine PLM procedures. Gram-stained samples of the same fluid were independently interpreted by microbiologists trained in examining fluid for crystals using standard light microscopy, followed by at least 2 pathologists using PLM.

Compared with standard PLM on unstained samples, the sensitivity and specificity of crystal examination by microbiologists on Gram-stained samples were 16% and 98%, respectively. Pathologists, using PLM on these Gram-stained samples, achieved slightly better sensitivity (24.5% and 96%, respectively). Concordance regarding crystal type was 100% and unaffected by Gram stain.

Crystal examination of Gram-stained synovial and bursal fluid by trained personnel has low sensitivity but high specificity compared with standard evaluation of unstained samples. Examination of Gram-stained samples with PLM may be helpful when sample volumes are limited and, when positive, has sufficient specificity to support crystalline arthropathy diagnoses.