How may synovial fluid WBC count be estimated by wet drop examination?
When microscopy is performed, the synovial fluid WBC count can be easily estimated. The finding of two or fewer WBCs per high-power field (40× high dry objective) confidently suggests a noninflammatory fluid (<2000 WBCs/mm 3 ). If greater than four WBCs per high-power field are seen, there is a significant chance that the synovial fluid is inflammatory, and formal determination of the WBC count should be ordered. The rule of thumb is that one WBC seen at 40× objective estimates 500 cells in total WBC count. For example, if you observe 10 WBCs in a 40× field, the estimated cell count would be 5000/mm 3 .
