Laboratory Findings for Polymyalgia Rheumatica
The key laboratory findings in PMR include elevated inflammatory markers (ESR and/or CRP in >90% of cases), with ESR >40 mm/hr being particularly significant for predicting relapse risk, though normal inflammatory markers do not exclude the diagnosis. 1, 2
Core Laboratory Panel
Essential inflammatory markers:
- ESR is elevated (>40 mm/1st hour) in approximately 91.5% of patients and is the preferred marker for long-term monitoring and predicting relapse risk 2, 3
- CRP is elevated in 98.9% of patients at diagnosis and may be more sensitive than ESR 3
- Both ESR and CRP should be measured as part of the basic laboratory dataset 1
Baseline laboratory tests to exclude mimicking conditions:
- Rheumatoid factor and/or anti-cyclic citrullinated peptide antibodies (ACPA) to exclude rheumatoid arthritis 1, 4
- Complete blood count to assess for anemia and other causes of inflammation 1
- Glucose, creatinine, and liver function tests to establish baseline before glucocorticoid therapy 1
- Bone profile (calcium, alkaline phosphatase) to assess bone health before steroid initiation 1
- Urinalysis to exclude other inflammatory or infectious conditions 1
Additional Recommended Tests
Secondary screening tests:
- Protein electrophoresis to exclude paraproteinemia 1
- Thyroid stimulating hormone (TSH) to rule out thyroid disorders 1
- Creatine kinase to exclude myopathies and myositis 1
- Vitamin D level before glucocorticoid therapy 1
Special Considerations for Normal Inflammatory Markers
PMR with normal ESR/CRP occurs in 7-22% of cases and has distinct characteristics:
- Younger age at diagnosis 5, 6
- Longer symptom duration before diagnosis 7, 5
- Lower frequency of constitutional symptoms (fever, weight loss) 6
- Lower comorbidity rates 5
When ESR and CRP are normal but clinical suspicion is high:
- Measure serum amyloid A (SAA), which may be elevated when ESR and CRP are normal 5
- Consider ultrasonography to demonstrate bursitis 5
- Normal inflammatory markers warrant specialist referral according to EULAR/ACR guidelines 2
Prognostic Laboratory Markers
High ESR (>40 mm/hr) at diagnosis is associated with:
- Higher relapse rates requiring prolonged therapy 8, 2
- May guide decision for higher initial glucocorticoid doses 2
Persistently elevated inflammatory markers during treatment:
- Persistently elevated CRP and IL-6 during the first year of therapy predict higher risk of relapse/recurrence 3
- IL-6 levels remain elevated in 37.2% of patients after 4 weeks of prednisone therapy 3
- Patients with persistently elevated IL-6 during the first year have the highest relative risk of relapse 3
Common Pitfalls
Do not exclude PMR based solely on normal ESR or CRP - approximately 14% of patients have normal values at diagnosis, and clinical response to low-dose glucocorticoids (12.5-25 mg prednisone) plus a drop in inflammatory markers after therapy can confirm the diagnosis 7, 5, 6