What labs should be ordered for a patient presenting with syncope?

Laboratory Testing for Syncope Evaluation

Targeted blood tests are reasonable in the evaluation of selected patients with syncope based on clinical assessment from history, physical examination, and ECG, but routine comprehensive laboratory testing is not useful. 1

Initial Approach to Laboratory Testing

• Basic laboratory tests are only indicated if syncope may be due to loss of circulating volume or if a syncope-like disorder with a metabolic cause is suspected 1
• Routine and comprehensive laboratory testing has been shown to be not useful in the evaluation of patients with syncope 1
• Laboratory testing should be guided by findings from the initial evaluation, which includes history, physical examination, and ECG 2

Recommended Laboratory Tests Based on Clinical Suspicion

Volume Depletion/Blood Loss

• Complete blood count (to assess for anemia or blood loss) 2
• Hematocrit (included in San Francisco Syncope Rule as a risk factor when <30%) 1

Metabolic Disorders

• Electrolytes (sodium, potassium, calcium, magnesium) 2
• Blood glucose (to rule out hypoglycemia) 2
• Renal function tests (BUN, creatinine) when dehydration is suspected 1

Cardiac Causes

• Brain natriuretic peptide (BNP) and high-sensitivity troponin may be considered when cardiac cause is suspected, though their usefulness is uncertain 1
• These cardiac biomarkers should not be routinely ordered for all syncope patients 1

Risk Stratification and Laboratory Testing

• Laboratory testing should be more comprehensive in high-risk patients, including those with:

  • Abnormal ECG 1
  • History of cardiovascular disease 1
  • Age >65 years 1
  • Syncope during effort 1
  • Absence of prodromal symptoms 1

• In low-risk patients (younger age, no known cardiac disease, normal ECG), extensive laboratory testing is generally unnecessary 2

Algorithm for Laboratory Testing in Syncope

1. Perform initial evaluation (history, physical exam, ECG) 1
2. Determine if syncope is likely due to volume depletion or metabolic cause 1
3. If yes to #2, order targeted tests:
   • CBC/hematocrit if blood loss suspected 1
   • Electrolytes, glucose if metabolic disorder suspected 2
4. If cardiac cause suspected, consider cardiac biomarkers (BNP, troponin) 1
5. Avoid ordering comprehensive panels without specific indications 1

Common Pitfalls to Avoid

• Ordering comprehensive laboratory panels for all syncope patients without specific indications 1
• Relying solely on laboratory tests without adequate history and physical examination 3
• Failing to recognize that history and physical examination are the most important diagnostic tools in syncope evaluation 3
• Overreliance on laboratory testing when the cause of syncope is likely neurally mediated or orthostatic 2
• Not considering age and presence of heart disease when determining the extent of laboratory testing 4

Remember that laboratory testing should be viewed as supplementary to a thorough history and physical examination, which remain the cornerstone of syncope evaluation 3. The diagnostic yield of laboratory testing in isolation is low, and tests should be ordered based on specific clinical suspicions rather than as a routine panel 1.