In a patient suspected of ankylosing spondylitis, which blood test is more important: erythrocyte sedimentation rate (ESR) or high‑sensitivity C‑reactive protein (hs‑CRP)?

Neither ESR nor hs-CRP is Superior for Diagnosing Ankylosing Spondylitis

Neither ESR nor high-sensitivity CRP should be used as primary screening tests for ankylosing spondylitis, as both have poor sensitivity (~50%) and are not suitable screening parameters. 1 Instead, clinical features (inflammatory back pain) and HLA-B27 testing are the recommended screening tools in primary care. 1

Why Acute Phase Reactants Are Poor Screening Tests

Limited Diagnostic Performance

• ESR and CRP have only 50% sensitivity in patients with AS and early AS, meaning half of patients with the disease will have normal values 1
• The post-test probability with elevated ESR/CRP is only about 11-12%, making them unsuitable for screening 1
• Both markers received an overall rating of "0" (not suitable) as primary screening parameters due to low sensitivity 1

Lack of Superiority Between the Two Tests

• Multiple studies demonstrate that neither CRP nor ESR is superior to the other for assessing disease activity in AS 2, 3
• Correlation coefficients between ESR/CRP and disease activity measures (BASDAI, physician assessment) range from only 0.06-0.48, indicating poor to moderate correlation 3
• In a study of 191 AS patients, sensitivity ranged from 44-100% and specificity from 44-84% for both markers, with no clear advantage of one over the other 3

When to Use Each Marker (If Ordered)

Slight Advantage of CRP in Specific Contexts

• CRP may be marginally more sensitive and specific than ESR for detecting active disease episodes 4, 5
• One older study found CRP was significantly elevated in active AS while ESR values were not statistically different between active and inactive disease 5
• High-sensitivity CRP (hs-CRP) correlates better than routine CRP with clinical disease activity parameters in established AS, particularly in patients with negative routine CRP (<6 mg/L) 6

ESR Considerations

• ESR correlates with CRP (correlation coefficient ~0.30-0.50) but adds little independent information 2, 3
• ESR is affected by confounding factors like anemia and azotemia that artificially elevate values 7, 8

Recommended Diagnostic Approach

Primary Screening Parameters (Not Laboratory Tests)

For patients with chronic low back pain >3 months with onset <45 years, use these screening criteria: 1

1. Inflammatory back pain features (sensitivity 75%, post-test probability 14%):

   • Morning stiffness >30 minutes
   • Pain at night/early morning
   • Improvement with exercise 1

2. HLA-B27 testing (sensitivity 90%, post-test probability 32%):

   • Only 3 HLA-B27 positive patients need referral to diagnose 1 case of axial SpA
   • This is the ideal screening test with a likelihood ratio of 9 1

Role of Imaging Over Laboratory Tests

• MRI of sacroiliac joints has 90% sensitivity and 90% specificity, far superior to ESR/CRP 1
• Sacroiliitis on imaging (X-ray or MRI) in a young patient with chronic back pain is a valid referral criterion 1

Clinical Pitfall to Avoid

Do not rely on normal ESR or CRP to exclude ankylosing spondylitis. With only 50% sensitivity, half of patients with active AS will have normal acute phase reactants. 1 If clinical suspicion is high based on inflammatory back pain features or HLA-B27 positivity, refer to rheumatology regardless of ESR/CRP values. 1

Monitoring vs. Diagnosis Distinction

While ESR and CRP are poor for diagnosis, they may have limited utility for monitoring disease activity in established AS:

• Multiple studies show modest correlations (0.32-0.68) between CRP and disease activity scores in longitudinal monitoring 1
• However, many studies also show no significant correlation (NS) between inflammatory markers and clinical measures 1