Study of Synovial Lactate Level as Marker of Septic Arthritis

A painful, swollen joint can be caused by variety of conditions. Distinguishing bacterial arthritis and other causes of arthritis is essential to reducing significant morbidity and mortality. In most cases, septic arthritis is monoarticular and occurs most commonly in large peripheral joints, such as the knee.

Current recommended diagnostic tools include clinical suspicion after examination followed by testing of synovial fluid from the affected joint. The gold standard is often gram stain or synovial fluid culture. The early markers such as synovial WBC, polymorphonuclear white blood cells are not accurate in predicting the diagnosis of septic arthritis. This leads to diagnostic dilemmas, inappropriate admissions, and unnecessary costs pending the results of synovial fluid cultures.

The current emergency department protocol for the diagnosis of septic arthritis states that if none of the following criteria is met, it is unlikely to be septic arthritis:

1. synovial fluid white cell count more than 50 000/uL; or
2. synovial fluid neutrophil more than 75%; or
3. positive gram stain.

However, it has been noted that septic arthritis can have white cell counts lower than 50 000/uL which means that the current protocol cannot safely rule out septic arthritis. A neutrophil percentage of at least 90% has a reported specificity ranging from 68 to 83% while our current protocol uses a cut off of 75% which has a low specificity resulting in many unnecessary admissions. A high neutrophil percentage is also seen in inflammatory joint conditions, and therefore may not be adequate to distinguish infective from non-infective joint pain. A recent review of existing studies in 2011 stated "Future ED-based diagnostic trials are needed to evaluate the role of clinical gestalt and the efficacy of nontraditional synovial markers such as lactate".