Lactate levels are rapidly available and may potentially aid in diagnosing septic arthritis in a native joint. This study will determine the utility of synovial lactate in discriminating septic from aseptic 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".

The most recent pertinent study was published in Acta Orthopædica Belgica 2014. This was a small retrospective study comparing septic arthritis to aseptic arthritis in samples taken from the emergency department between 2006 and 2013. In the final analysis, lactate was identified as the most accurate early marker in predicting a positive synovial fluid culture. The reported sensitivity was 74.5% with a specificity of 87.2%. This study is limited by its retrospective nature, small sample sizes, and lack of information regarding the prevalence of disease in the local population.

A metanalysis in the Journal of the American Medical Association 2007 cited two studies that reported a prevalence of septic joint in the study population of 8% and 27%. The prevalence of septic joint in our local population is unknown. This study will reveal the incidence of septic joints in the emergency department population.

Review of data from the Khoo Teck Puat Hospital electronic medical records from six months in 2018 reveal 91 synovial joint samples tested for gram stain, culture, or cell count. The investigators expect it will take one year to obtain 200 samples.

Inclusion criteria:

• Age 21 and older
• Suspected infected joint

Exclusion criteria:

• Age under 21
• Prosthetic joint affected
• Overlying cellulitis

Patients 21 years or older with tender swollen joints undergoing arthrocentesis will be enrolled in the study. Exclusion criteria will be overlying cellulitis, affected prosthetic joints and age under 21.