Investigations:
- Leucocytosis, high ESR and CRP are features of septic arthritis.
- Synovial fluid from the infected joint should send for WBC counts, gram stain, and culture before starting antibiotics.
- In septic arthritis synovial WBC counts are more 50000 cells /mm3.
- Blood cultures should be obtained for all suspected cases of septic arthritis before starting antibiotics.
- Radiology of infected joint in early stages will show periarticular soft tissue swelling, fat pad oedema with normal periarticular bone. As the infection worsens loss of joint space, periarticular osteoporosis and periosteal reaction will happen. CT and MRI are very sensitive techniques to pick up early changes of septic arthritis.
- Ultrasound is not only useful for assessing the amount of joint effusion but also to guide synovial fluid aspiration.