Clinical utility of the alizarin red S stain on permanent preparations to detect calcium-containing compounds in synovial fluid.

The alizarin red S stain for permanent cytologic preparations is a valuable test that is complementary to compensated polarized light microscopic examination to detect calcium crystals. Alizarin red S has the greatest sensitivity for detection of calcium pyrophosphate crystals because crystals are stained regardless of how weakly or strongly birefringent they may be. Alizarin red S stain does not distinguish between amorphous types of calcium compounds; therefore, the different types of calcium compounds can be distinguished only when typical crystal morphologic features are present. Diagnostic importance can be attached to intracellular material that is stainable. In contrast, the diagnostic value of stainable, amorphous, extracellular material is unreliable because it is difficult to distinguish this extracellular material from contaminants frequently found in clinical specimens. Alizarin red S does not stain monosodium urate or corticosteroid crystals. Air-dried cytospin smears are helpful because they may frequently demonstrate more crystals than the wet-mount preparation. Furthermore, special stains can be performed subsequently on air-dried cytospin smears if necessary.