What bloodwork should be ordered for a patient presenting with polyarticular pain?

Bloodwork for Polyarticular Pain

Order inflammatory markers (ESR and CRP), complete blood count with differential, rheumatoid factor (RF), anti-citrullinated protein antibodies (ACPA), and antinuclear antibody (ANA) as your initial laboratory panel for polyarticular pain. 1, 2

First-Line Essential Laboratory Tests

Inflammatory Markers

• ESR and CRP should be obtained immediately to assess for inflammation and help distinguish inflammatory from noninflammatory arthritis 1, 2, 3
• Significantly elevated markers indicate inflammatory arthritis or infection, while normal or mildly elevated markers suggest non-inflammatory causes such as osteoarthritis 3
• Important caveat: Anemia and azotemia can artificially elevate ESR independent of inflammatory activity 2
• Normal ESR does not exclude inflammatory arthritis; CRP may be more sensitive in acute crystal arthropathy 2

Complete Blood Count with Differential

• CBC with differential is mandatory to evaluate for infection, inflammation, or malignancy 1
• Helps characterize anemia, which commonly accompanies rheumatoid arthritis as anemia of chronic disease and correlates with disease activity 2
• Assesses for leukocytosis that may indicate infection 2

Autoimmune Serologic Testing

Rheumatoid Arthritis Markers

• RF and ACPA should be tested immediately when inflammatory polyarthritis is suspected 1, 2
• These antibodies are highly predictive of RA diagnosis and prognosis 2
• Critical pitfall: Negative RF and ACPA do not exclude progression to RA 2

Connective Tissue Disease Screening

• ANA testing should be performed if connective tissue disease is suspected based on clinical features 1

Additional Testing Based on Clinical Context

Metabolic Panel

• Comprehensive metabolic panel (blood glucose, renal function, liver function) should be obtained 4
• Blood glucose has the highest diagnostic yield among screening tests, as diabetes mellitus is the most common cause of polyneuropathy that may present with joint symptoms 4

Vitamin B12 Assessment

• Serum B12 should be checked, as deficiency was found in approximately 3.6% of patients with polyneuropathy 4
• Consider methylmalonic acid and homocysteine if B12 is in the low-normal range (200-500 pg/dL), as these metabolites detect deficiency in 5-10% of patients with normal-range B12 4

Infectious Disease Screening

• Blood cultures should be performed if fever is present or if bloodstream infection is suspected 1
• Consider testing for viral hepatitis B, C, and tuberculosis, especially before starting immunosuppressive therapy 1

When to Perform Joint Aspiration

Arthrocentesis must be performed urgently if septic arthritis or crystal-induced arthritis cannot be excluded clinically 1, 3

Synovial Fluid Analysis Should Include:

• Total cell count and differential leukocyte count 1
• Crystal analysis for gout or pseudogout 1
• Gram stain and culture for infectious causes 1
• Important: Withhold antimicrobials for at least 2 weeks prior to aspiration if the patient is medically stable, to increase organism recovery 3

Common Pitfalls to Avoid

• Do not rely on serum uric acid during acute presentations, as it may be normal during acute gout flare 2
• Do not assume normal inflammatory markers exclude inflammatory arthritis 2
• Do not skip joint aspiration when septic arthritis is a consideration—this is a critical rule-out that requires urgent evaluation 2
• Laboratory test results must be interpreted in the context of clinical information, as the etiologic yield of laboratory testing alone is limited by low specificity 4