Blood Tests for Respiratory Symptoms, Fever, and Abdominal Discomfort
For patients presenting with respiratory symptoms, fever, and abdominal discomfort requiring hospital evaluation, order: complete blood count (CBC), urea/creatinine and electrolytes, liver function tests, and C-reactive protein (CRP) if bacterial superinfection is suspected. 1
Core Laboratory Panel
The following blood tests should be obtained in all patients admitted to hospital with this presentation 1:
1. Complete Blood Count (CBC)
- Leucocytosis with left shift may indicate primary viral pneumonia, mixed viral-bacterial pneumonia, or secondary bacterial pneumonia
- Lymphopenia has been noted in severe viral infections (including avian H5N1 influenza and COVID-19) 1, 2
- Neutropenia or marked neutrophilia may have prognostic value 3
2. Urea, Creatinine, and Electrolytes
- Reveals hypo- or hypernatremia
- Identifies renal impairment
- Urea >7 mmol/L is part of severity scoring (CURB-65) 1
- Hypoalbuminemia correlates with bacteremia 3
3. Liver Function Tests
- Usually normal in uncomplicated respiratory infections 1
- Elevated AST/ALT occurs in 15% of COVID-19 patients and may indicate viral involvement 4
- Elevated in patients with severe disease and abdominal symptoms 2, 5
Additional Tests Based on Clinical Context
4. C-Reactive Protein (CRP)
- CRP ≥50 mg/L has 98.5% sensitivity and 75% specificity for identifying sepsis 3
- CRP ≥30 mg/L plus suggestive symptoms increases likelihood of pneumonia 6
- Most useful when bacterial superinfection is suspected 1
- Superior to routine use in viral infections alone 1
5. Procalcitonin (PCT)
- PCT ≥1.5 ng/ml has 100% sensitivity and 72% specificity for sepsis 3
- Consider in critically ill patients with low-to-intermediate probability of bacterial infection 7
- Not recommended routinely in outpatient settings 6
6. Creatine Kinase
- Order if myositis or severe myalgia is suspected 1
7. Inflammatory Markers in COVID-19 Context
If COVID-19 is suspected based on epidemiology 2:
- Erythrocyte sedimentation rate
- Lactate dehydrogenase (often elevated)
- D-dimer (elevated, indicates hypercoagulability)
- Troponin I (if myocarditis suspected)
Blood Cultures
Obtain blood cultures when:
- Fever with chills, hypothermia, or hemodynamic compromise 3
- Development of renal failure
- Signs of septic shock
- Results will change clinical management 6
Timing matters: Draw cultures as soon as possible after fever onset, ideally immediately before scheduled antibiotic dose if patient already on therapy 3
Common Pitfalls to Avoid
Don't rely on fever alone - elderly and immunocompromised patients may not mount fever despite serious infection 8
CRP/PCT cannot differentiate infection from SIRS alone - use as part of systematic evaluation, not in isolation 3
Sequential measurements are more valuable than single values for inflammatory markers 3
Leukocyte count alone is insensitive and nonspecific for bacterial pneumonia requiring antibiotics 9
Don't order routine microbiological testing unless results will change therapy 6
Algorithmic Approach
Step 1: Order core panel (CBC, electrolytes/renal function, LFTs) for all hospitalized patients 1
Step 2: Add CRP if:
- Clinical suspicion of bacterial pneumonia
- Considering antibiotic therapy
- CRP <10 mg/L makes pneumonia unlikely; >50 mg/L supports diagnosis 6, 9
Step 3: Add PCT if:
Step 4: Add blood cultures if:
- Fever with chills or hemodynamic instability 3
- Suspected bacteremia or sepsis
Step 5: Add creatine kinase only if severe myalgia present 1
Step 6: In COVID-19 pandemic context, add inflammatory panel (ESR, LDH, D-dimer, troponin) based on disease severity 2
This structured approach prioritizes tests that will actually change management while avoiding unnecessary testing that increases costs without improving outcomes.