What laboratory tests should I order for a patient with a borderline electrocardiogram?

Laboratory Testing for Borderline ECG Findings

For a patient with a borderline ECG, order a basic metabolic panel (electrolytes, creatinine, glucose) and thyroid function tests (TSH, free T4) as your initial laboratory workup, reserving additional testing for specific clinical scenarios. 1

Core Laboratory Tests

Electrolyte Panel

• Order serum potassium, magnesium, and calcium to identify electrolyte disturbances that can cause ECG abnormalities, particularly in patients taking diuretics or with suspected volume depletion 1
• Hypokalemia and hypomagnesemia predispose to QT prolongation and arrhythmias 2
• These tests are Class I indicated when syncope may be due to loss of circulating volume or when a metabolic cause is suspected 1

Renal Function

• Measure serum creatinine and calculate GFR as baseline for medication decisions and to assess for chronic kidney disease that may contribute to electrolyte abnormalities 3
• Renal impairment affects drug dosing for antiarrhythmic agents and increases risk of electrolyte disorders 3

Thyroid Function Tests

• Order TSH and free T4 to exclude hyperthyroidism or hypothyroidism, both of which can cause ECG abnormalities and arrhythmias 3
• Hyperthyroidism causes atrial fibrillation in 5-15% of patients, more commonly in those over 60 years 3
• Thyroid dysfunction is a common, treatable cause of palpitations and rhythm disturbances 3

Glucose

• Check fasting glucose or HbA1c to identify diabetes, which increases cardiovascular risk and may contribute to autonomic dysfunction 1

Conditional Laboratory Tests

Complete Blood Count

• Order CBC only if anemia is suspected based on symptoms of fatigue, dyspnea, or history suggesting blood loss 1, 3
• Hemoglobin/hematocrit determination may confirm suspected acute blood loss, though examination of stool for blood may be more accurate early in acute blood loss 1

Cardiac Biomarkers

• Order troponin only if chest pain suggestive of ischemia is present before or after loss of consciousness 1
• Do not routinely order cardiac enzymes for borderline ECG findings without clinical suspicion of acute coronary syndrome 1

Additional Testing for Athletes

• If the patient is an athlete with two or more borderline ECG findings, order echocardiography to exclude myocardial disease 1
• Consider additional testing based on clinical suspicion and specific ECG abnormalities 1

What NOT to Order

Avoid Routine Comprehensive Panels

• Do not order comprehensive metabolic panels routinely as blood tests rarely yield diagnostically useful information in syncope or palpitation evaluation 1, 3
• Basic laboratory tests are only indicated if syncope may be due to loss of circulating volume or if a metabolic cause is suspected 1

Pregnancy Testing

• Order pregnancy test only in women of childbearing potential with syncope, though pregnancy should not dissuade evaluation for other risk factors 1

Clinical Context Matters

For Patients with Suspected Cardiac Disease

• Laboratory findings suggesting interval development of cardiac disease warrant repeat ECG (Class I indication) 1
• Changes in relevant laboratory findings are Class I indications for follow-up ECG in patients with known cardiovascular disease 1

For Patients on Medications

• Monitor electrolytes in patients taking diuretics, lithium, or agents known to alter serum electrolyte concentrations 1
• Assess response to administration of any agent known to result in cardiac or ECG abnormalities (antineoplastic drugs, lithium, tranquilizers, antidepressants) 1

Common Pitfalls to Avoid

• Do not order comprehensive laboratory panels routinely without specific clinical indication—this increases cost without improving diagnostic yield 1, 3
• Do not rely solely on laboratory tests when history, physical examination, and ECG provide sufficient diagnostic information 4
• Do not delay urgent evaluation for high-risk features (syncope, chest pain, dyspnea) while waiting for laboratory results 3