What is the difference between complete blood count (CBC) in enteric fever and viral fever?

Differences Between Complete Blood Count in Enteric Fever and Viral Fever

In enteric fever, the complete blood count typically shows leukopenia (low white blood cell count) with relative lymphocytosis, while viral fevers generally present with normal to low white blood cell counts with lymphocytic predominance. 1

Enteric Fever CBC Findings

• White Blood Cell Count: Leukopenia (low WBC) is common in enteric fever, though the total white blood cell count may be normal or even elevated in some cases 1, 2
• Differential Count: Relative neutropenia with lymphocytic predominance is characteristic 2
• Monocyte Count: Often shows monocyte predominance, which may suggest the presence of an intracellular pathogen such as Salmonella 1
• Platelet Count: May be decreased (thrombocytopenia) 1
• Anemia: May develop as the disease progresses, particularly in severe or prolonged cases 2

Viral Fever CBC Findings

• White Blood Cell Count: Usually normal to low (leukopenia) 1, 3
• Differential Count: Typically shows lymphocytic predominance 1
• Neutrophil Count: Often decreased 3
• Platelet Count: Usually normal but can be decreased in some viral infections 2
• Eosinophil Count: Generally normal 1

Diagnostic Implications

For Enteric Fever:

• CBC alone is insufficient for diagnosis of enteric fever due to its non-specific nature 1
• Blood culture remains the gold standard diagnostic test with approximately 50% sensitivity 1
• Bone marrow culture has higher sensitivity (particularly valuable if antimicrobial agents have been administered) 1
• Additional cultures of stool, duodenal fluid, and urine may be beneficial for diagnosis 1
• Serologic tests (like Widal test) should not be used due to poor performance characteristics 1

For Viral Fever:

• CBC patterns can suggest viral etiology but are not definitive 3
• Molecular diagnostic tests are more reliable for specific viral diagnosis 1
• Clinical correlation is essential as CBC findings in viral infections can overlap with early bacterial infections 3, 2

Clinical Pearls and Pitfalls

• Pitfall: Relying solely on CBC for differentiating enteric from viral fever can lead to misdiagnosis as patterns may overlap 3
• Pitfall: Previous antimicrobial use can alter the typical CBC findings in enteric fever 4
• Pearl: Monocytosis is more suggestive of enteric fever than viral infection 1
• Pearl: Persistent fever with relative bradycardia and normal to low WBC count should raise suspicion for enteric fever in returning travelers from endemic areas 1
• Caveat: In early stages of enteric fever, CBC findings may be similar to viral infections, making differentiation challenging 2

Diagnostic Algorithm

1. For suspected enteric fever:

   • Obtain blood cultures (2-3 samples of 20mL each for adults) before antibiotic administration 1
   • Consider bone marrow culture if patient has received prior antibiotics 1
   • Perform culture-independent molecular testing from stool and blood specimens 1
   • Do not rely on serologic tests 1

2. For suspected viral fever:

   • Molecular diagnostic testing for specific viral pathogens is more reliable than CBC interpretation 1
   • Consider epidemiological factors and exposure history 2

Remember that while CBC findings can provide helpful clues, they should not be used in isolation to differentiate between enteric and viral fevers due to significant overlap in presentations 3, 2.