Investigations
1st investigations to order
erythrocyte sedimentation rate (ESR)
Test
There is non-specific elevation of the ESR, particularly in the acute stage. Sensitivity and specificity of this test are low.
Result
elevated
C-reactive protein (CRP)
Test
There is non-specific elevation of CRP, particularly in the acute stage. Sensitivity and specificity of this test are low.
Result
elevated
antinuclear antibody (ANA)
Test
To rule out other forms of arthritis or other conditions that cause similar symptoms.
Result
negative
rheumatoid factor
Test
To rule out other forms of arthritis or other conditions that cause similar symptoms.
Result
negative
urogenital and stool cultures
Test
Urogenital and stool cultures are very helpful at the time of the inciting infection; however, these cultures are usually negative after the onset of arthritis.
Result
negative unless patients are tested very early after onset of infection
plain x-rays
Test
Typically, there is evidence of asymmetrical sacroiliitis. There is also evidence of enthesitis, especially of the Achilles' tendon.
These findings are unlikely to appear in the acute stage and are more commonly seen in chronic ReA. Sensitivity and specificity of imaging are low.
Result
sacroiliitis or enthesopathy
arthrocentesis with synovial fluid analysis
Test
To rule out crystal-induced arthritis such as gout.
Could also be considered as a subsequent test with future flares.
Result
negative
Investigations to consider
HLA-B27
Test
Can establish the presence or absence of HLA-B27. Sensitivity and specificity of this test are low.
Result
positive or negative
nucleic acid amplification tests
MRI
Test
Analysis of the sacroiliac joints can be useful in any patient with low back pain or buttock pain.
MRI may reveal erosive changes or ankylosis of the sacroiliac joints, and bone marrow changes around the sacroiliac joints indicative of active sacroiliitis.
Sensitivity and specificity of imaging are low.
Result
sacroiliitis or enthesopathy
ultrasound
Test
Ultrasound of extremities may complement x-ray.[64]
European guidelines suggest that ultrasound might provide additional information to conventional radiography when monitoring structural changes in peripheral spondyloarthritis.[65]
Result
synovial hypertrophy; increased vascularity (apparent on Doppler imaging)
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