The highest risk probability of septic arthritis of the knee was 96% for the patients less than the age of 5 years, with CRP > 2, and %PMN ≥ 85%, and the lowest risk probability of septic arthritis was 11% when patients were ≥5 years of age, with CRP ≤ 2, and %PMN <85% (Figure 5). Here, CRP is linked to bacterial arthritis.