Which blood tests should be ordered to assess oxygenation, ventilation, cardiac involvement, possible pulmonary embolism, and infection in a patient with suspected respiratory compromise?

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Last updated: September 12, 2026 • View editorial policy

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Blood Tests for Suspected Respiratory Compromise

For patients with suspected respiratory compromise requiring assessment of oxygenation, ventilation, cardiac involvement, pulmonary embolism, and infection, obtain: arterial blood gas (ABG), complete blood count with differential, D-dimer (if PE suspected and hemodynamically stable), cardiac troponin, natriuretic peptides (NT-proBNP or BNP), comprehensive metabolic panel including electrolytes and renal function, and blood cultures (two sets) before antibiotics.

Core Laboratory Assessment

Arterial Blood Gas Analysis

ABG is essential for assessing both oxygenation (PaO2) and ventilation (PaCO2) status. Obtain ABG in any patient with severe illness or chronic lung disease to assess both oxygen levels and carbon dioxide retention 1. However, recognize that ABG has significant limitations for diagnosing PE specifically—even with normal PaO2 (≥80 mm Hg), normal PaCO2 (≥35 mm Hg), and normal alveolar-arterial gradient (≤20 mm Hg), PE cannot be excluded in 38% of patients without prior cardiopulmonary disease and 14% with prior cardiopulmonary disease 2. The key value of ABG is guiding oxygen therapy and ventilation management, not excluding PE 3.

Complete Blood Count and Chemistry

All admitted patients require CBC with differential and comprehensive metabolic panel (glucose, sodium, liver and renal function, electrolytes) 1. While these tests have limited value for determining pneumonia etiology, they provide crucial prognostic information and influence hospitalization decisions, particularly in patients ≥65 years or with coexisting illness 1.

Cardiac Biomarkers for PE Risk Stratification

Troponin and Natriuretic Peptides

When PE is suspected, measure cardiac troponin and NT-proBNP (or BNP) for risk stratification, even in hemodynamically stable patients 4. These biomarkers identify right ventricular dysfunction and help classify PE severity:

  • Elevated troponin or NT-proBNP in patients with low PESI/sPESI scores (otherwise "low-risk") indicates intermediate-low-risk PE with 4-fold increased early mortality risk (OR 4.19,95% CI 1.39-12.58) 4
  • NT-proBNP correlates significantly with pulmonary artery pressure (β = 0.002, P < 0.001) 5
  • These biomarkers should be assessed even when clinical prediction rules suggest low risk 4

D-Dimer Testing

D-dimer is indicated in hemodynamically stable patients with low or intermediate clinical probability of PE 6. Critical caveats:

  • High sensitivity but low specificity—useful only for exclusion, not confirmation 7
  • Specific assays matter: Vidas D-dimer can safely exclude PE with low-to-moderate pretest probability; SimpliRED only with low pretest probability 7
  • Know which assay your hospital uses, as results from different assays are not interchangeable 7

Infection Assessment

Blood Cultures

Draw two sets of blood cultures before initiating antibiotics 1. Yield is approximately 11% with S. pneumoniae most commonly identified 1. This is particularly important for identifying bacteremia and resistant pathogens that may alter antibiotic selection.

Algorithmic Approach by Clinical Scenario

Hemodynamically unstable patient (suspected high-risk PE):

  • ABG immediately
  • Cardiac troponin and NT-proBNP
  • CBC, comprehensive metabolic panel
  • Skip D-dimer (proceed directly to imaging)

Hemodynamically stable with suspected PE:

  • Calculate clinical probability score (Wells, Geneva)
  • If low/intermediate probability: D-dimer first
  • ABG for severity assessment
  • Cardiac biomarkers (troponin, NT-proBNP) for risk stratification
  • CBC, metabolic panel

Suspected pneumonia/infection:

  • ABG if severe illness or chronic lung disease
  • CBC with differential
  • Two sets of blood cultures before antibiotics
  • Comprehensive metabolic panel
  • Consider procalcitonin (though not specifically mentioned in guidelines, commonly used)

Critical Pitfalls

Do not rely on ABG to exclude PE—12-14% of confirmed PE patients have completely normal arterial blood gases 3, 2. ABG guides respiratory support but lacks diagnostic discriminant value for PE.

Do not skip cardiac biomarkers in "low-risk" PE—34% of patients with low PESI/sPESI scores have RV dysfunction on imaging, and elevated biomarkers identify a subset with significantly worse outcomes requiring reclassification to intermediate-risk 4.

Timing matters for blood cultures—obtain before antibiotics to maximize yield, though treatment should not be delayed if sampling is difficult 1.

References

7
Research

Laboratory tests in the diagnosis of pulmonary embolism.

Respiration; international review of thoracic diseases, 2003

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This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.