Investigations
1st investigations to order
FBC
Test
WBC count should be <12,000 cells/mm³, because one of the four primary predictors of septic arthritis is WBC count >12,000 cells/mm³.
Result
WBC count within normal range but may be slightly elevated
erythrocyte sedimentation rate
Test
Should be <40 mm/hour, because one of the four primary predictors of septic arthritis is a rate of >40 mm/hour.
Result
within normal range but may be slightly elevated
CRP
x-ray
Test
Used primarily to rule out other disorders, such as Legg-Calvé-Perthes' disease, fractures, and tumours, and for ensuring that the metaphyseal bone of the femur and acetabulum do not have signs of other processes such as osteomyelitis.[13][14][Figure caption and citation for the preceding image starts]: X-ray of a 5-year-old child with an irritable right hip showing subtle signs early in the disease process, such as capsular distension, joint space widening, and diminution of the definition of soft-tissue planes around the hip joint or slight demineralisation of the bone of the proximal femurFrom the collection of J. McCarthy [Citation ends].
Result
typically normal; however, x-ray images may reveal subtle signs early in the disease process, such as capsular distension, joint space widening, diminution of the definition of soft tissue planes around the hip joint, or slight de-mineralisation of the bone of the proximal femur
Investigations to consider
ultrasound
Test
Not typically used for the diagnosis of transient synovitis, although effusions are a common finding.[15] Ultrasound can be useful for guiding needle aspiration if septic arthritis has to be excluded.[13] If septic arthritis is being considered as a differential diagnosis, joint aspiration should be performed and fluid sent for cell count and culture/Gram stain.[16][Figure caption and citation for the preceding image starts]: Ultrasound of a hip documenting a hip effusionFrom the collection of J. McCarthy [Citation ends].
Result
increased fluid in joint
MRI with contrast
Test
Not typically used in the diagnosis of transient synovitis but rather to evaluate for other disorders, such as osteomyelitis of the proximal femur or pelvis.[13]
Result
typically normal but may see fluid in the hip joint if effusion present, as noted by increased signal intensity on T2 images
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