Test | Diagnostic value of test |
---|---|
Erythrocyte sedimentation rate (ESR) | ESR becomes elevated 24–48 h after the start of the inflammatory process but is normal in up to 25% of septic arthritis cases;8 ESR of >40 mm/h is a risk factor for septic arthritis.9 The sensitivity of an elevated ESR on admission to detect osteoarticular infection is 94%.10 |
C reactive protein (CRP) | CRP becomes elevated earlier than ESR (within 6 h of the inflammatory process). A CRP >10 mg/l is generally accepted as a risk factor for septic arthritis.11 12 Sensitivity of elevated CRP on admission is 95%.10 |
Full blood count | A normal white cell count (WCC) count is present in 25%–74% of septic arthritis cases.8 Neutrophilia is suggestive of septic arthritis. A WCC of >12×10−9 is generally accepted as a risk factor for septic arthritis.9 11 12 Sensitivity and specificity of elevated WCC for septic arthritis are 75% and 55%, respectively.13 |
Blood film | A normal blood film does not exclude leukaemia or other malignancy. A bone marrow aspirate may be required and specialist opinion is required where there is clinical concern. |
Blood culture | Blood cultures are positive in 46%–80% of patients with osteomyelitis14 15 and 22%–50% of patients with septic arthritis.16 17 |
Antistreptolysin O titre (ASOT)/anti-DNAse-B* | Raised ASOT suggests current or recent streptococcal infection and is present in up to 80% of patients with acute rheumatic fever. Sensitivity can be further increased by testing for additional antibodies such as anti-DNAse-B.8 Throat swab also indicated but often negative. |
Lactate dehyrogenase (LDH)* | Raised levels can suggest malignancy (especially lymphoma) but sensitivity and specificity low. LDH is often raised in other conditions, for example, haemolysis, meningitis, encephalitis and pancreatitis. |
Plain radiography | Diagnostic yield is low in young children (1–5 years) who have an otherwise normal examination and look well.18 May be normal even with significant pathology (eg, sepsis, early Perthes’ disease, transient synovitis, malignancy, juvenile idiopathic arthritis). |
Repeat radiographs after a period of review may be useful (eg, detecting periosteal reaction in Toddler's fracture or evolving Perthes’ disease). Anterior–posterior and ‘frog leg lateral’ hip x-rays should be undertaken in all children to detect early slipped upper femoral epiphysis (SUFE). Caution is required in conditions of the hip where bilateral changes may occur (eg, SUFE, hip dyplasia). | |
Ultrasonography | Very sensitive in detecting hip and joint effusions. |
Operator-dependent. Absence of effusion on hip USS makes septic arthritis very unlikely.19 Ultrasound findings will not differentiate among infection, blood or reactive fluid. Does not exclude osteomyelitis but may show periosteal reaction suggestive of osteomyelitis. | |
MRI* | Very sensitive in identifying early sepsis, Perthes’ disease, inflammatory disease and tumours when the pathological area is localised on clinical examination. May not always be able to differentiate infection from inflammation. Gadolinium enhancement can be used to improve detection of infection, synovitis and tumours. May need sedation/anaesthesia for younger children. |
Bone scan* | Very sensitive in identifying early osteomyelitis when an obvious focus of infection cannot be localised. Particularly useful when infection affects the pelvis or spine. May also detect early Perthes’ disease, tumours and stress fractures such as toddler fractures and particularly when the history is vague. |
CT* | Useful to detect early bone changes of sepsis and tumours and may detect occult fractures, but significant exposure to ionising radiation. |
*Often omitted at acute presentation, but may be useful where the diagnosis remains unclear, not localised and the limp persists.