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

Laboratory Tests for Bradycardia Evaluation

For patients with bradycardia, targeted laboratory tests including thyroid function tests, electrolytes panel (especially potassium), Lyme disease titer, and acid-base status (pH) should be ordered based on clinical suspicion for potential underlying causes. 1

Essential Laboratory Tests for Bradycardia

The 2018 ACC/AHA/HRS guidelines recommend the following laboratory tests for bradycardia evaluation:

• Thyroid function tests (TSH, T3, T4) - Hypothyroidism is a common reversible cause of bradycardia 2, 1
• Electrolyte panel - Particularly potassium (both hyperkalemia and hypokalemia can cause bradycardia) 2, 1
• Renal function tests - BUN/creatinine to assess kidney function 1
• Lyme disease titer - Especially in young patients with new high-grade AV block in endemic areas 2, 1
• Acid-base status (pH) - Acidosis can contribute to bradycardia 2, 1

Additional Tests Based on Clinical Context

• Drug levels - For patients on medications that can cause bradycardia:

  • Digoxin level
  • Beta-blocker levels
  • Calcium channel blocker levels 1, 3

• Blood glucose - Hypoglycemia can be associated with bradycardia 2

• Inflammatory markers - Consider if infection or inflammatory condition suspected:

  • ESR (erythrocyte sedimentation rate)
  • CRP (C-reactive protein)

Special Considerations

• Genetic testing may be considered in patients with suspected inherited conduction disease to facilitate cascade screening of relatives 2, 1

• Sleep study evaluation should be considered in patients with bradycardia during sleep to rule out sleep apnea 1

Clinical Approach to Laboratory Testing

1. Target testing based on clinical presentation - Not all tests are necessary for every patient with bradycardia 2, 1

2. Consider common reversible causes first - Hypothyroidism, electrolyte abnormalities, and medication effects are frequent causes 2, 1

3. Regional considerations - In Lyme endemic areas, consider Lyme titer for patients with new conduction disorders 2, 1

Common Pitfalls to Avoid

• Overlooking medication effects - Many medications can cause bradycardia, including beta-blockers, calcium channel blockers, digoxin, and antiarrhythmic drugs 2, 3

• Missing thyroid dysfunction - Hypothyroidism is a common reversible cause of bradycardia that can be easily treated 4, 5

• Neglecting rare but treatable causes - Conditions like Lyme carditis may be missed if not specifically tested for in appropriate clinical scenarios 2

• Ordering unnecessary tests - Laboratory testing should be directed toward specific diagnoses based on clinical suspicion rather than ordering a comprehensive panel for all patients 2

By following this targeted approach to laboratory testing, clinicians can efficiently identify reversible causes of bradycardia and avoid unnecessary testing in patients with physiologic or chronic bradycardia.