What initial immunological labs are recommended for a patient presenting with recurring fever and abdominal pain?

Initial Immunological Laboratory Tests for Recurring Fever and Abdominal Pain

For patients presenting with recurring fever and abdominal pain, initial immunological laboratory testing should include complete blood count with differential, C-reactive protein, erythrocyte sedimentation rate, comprehensive metabolic panel, liver function tests, serum albumin, and fecal calprotectin when possible. 1

Core Laboratory Testing Algorithm

First-line Testing

• Complete blood count with differential - Evaluate for anemia, leukocytosis/leukopenia, thrombocytosis/thrombocytopenia
• C-reactive protein (CRP) - More sensitive than ESR for acute inflammation 1
• Erythrocyte sedimentation rate (ESR) - Assess overall inflammatory status
• Comprehensive metabolic panel - Assess electrolyte balance and organ function
• Liver function tests - Evaluate for hepatic involvement
• Serum albumin - Important marker of inflammation and nutritional status 1
• Fecal calprotectin - Sensitive marker for intestinal inflammation 1

Infectious Disease Workup

• Blood cultures - Minimum of two sets (60 mL total) from different anatomical sites 1
• Stool cultures - For bacterial pathogens including Salmonella, Shigella, Campylobacter, and Yersinia 1
• Clostridium difficile toxin testing - Especially if recent antibiotic use 1
• Stool testing for parasites - Particularly when symptoms persist >14 days 1

Specific Clinical Scenarios

When Inflammatory Bowel Disease (IBD) is Suspected

• All first-line tests above with emphasis on fecal calprotectin
• Additional testing for infectious causes to rule out IBD mimics 1
• Consider abdominal imaging if symptoms persist despite negative infectious workup

When Autoinflammatory Syndrome is Suspected

• Genetic testing for familial Mediterranean fever (FMF) - Particularly in patients of Mediterranean descent with recurrent fever and abdominal pain 2
• Serum amyloid A - If available, to assess risk of amyloidosis
• Genetic testing for other periodic fever syndromes - Based on clinical presentation 1

When Immunodeficiency is Suspected

• Immunoglobulin levels (IgG, IgA, IgM, IgE)
• Lymphocyte subset analysis - To evaluate T-cell, B-cell, and NK-cell populations
• Complement levels (C3, C4, CH50)
• Specific antibody responses to vaccines 1

When Hemophagocytic Lymphohistiocytosis (HLH) is Suspected

• Ferritin - Markedly elevated in HLH 3
• Triglycerides and fibrinogen
• Soluble IL-2 receptor (if available)
• EBV PCR - Common trigger for HLH 3

Important Considerations

1. Timing of symptoms affects test selection and interpretation:

   • Acute symptoms (<48 hours): CRP and WBC have limited diagnostic value 4
   • Persistent symptoms (>14 days): Consider expanded testing for parasites and unusual pathogens 1

2. Avoid common pitfalls:

   • Relying solely on CRP and WBC for triage decisions - these markers alone have insufficient diagnostic accuracy 4
   • Failing to rule out infectious causes before diagnosing functional or inflammatory disorders
   • Not considering ethnic background when evaluating recurrent fever with abdominal pain (e.g., FMF in Mediterranean populations) 2

3. Follow-up testing should be guided by initial results:

   • Persistent abnormalities despite treatment warrant further investigation
   • Normal initial tests with ongoing symptoms may require specialized testing

Remember that recurring fever with abdominal pain represents a broad differential diagnosis spanning infectious, inflammatory, autoimmune, and neoplastic etiologies. The laboratory evaluation should be comprehensive but targeted based on clinical presentation, patient demographics, and risk factors.