What blood work is indicated for a patient presenting with chest pain in an outpatient setting?

Outpatient Blood Work for Chest Pain Evaluation

High-sensitivity cardiac troponin testing is the cornerstone laboratory test for evaluating chest pain in the outpatient setting, with serial measurements recommended for optimal diagnostic accuracy. 1

Initial Laboratory Evaluation

Primary Tests

• High-sensitivity cardiac troponin (hs-cTn):

  • Gold standard biomarker for detecting myocardial injury
  • Serial measurements (0h and 1-3h) provide greater diagnostic accuracy than single measurement 1
  • Sex-specific 99th percentile upper reference limits should be used 1

• Complete Blood Count (CBC):

  • Helps identify anemia (can cause demand ischemia)
  • Evaluates for infection or inflammation

• Basic Metabolic Panel (BMP):

  • Assesses electrolyte abnormalities that may cause chest pain or ECG changes
  • Evaluates renal function which affects troponin clearance

• D-dimer:

  • Consider when pulmonary embolism is in the differential diagnosis
  • Higher D-dimer levels (>3.18 ng/mL) may help distinguish pulmonary embolism from NSTEMI 2

Additional Tests Based on Clinical Suspicion

• Lipid panel: For cardiovascular risk assessment
• HbA1c: To screen for diabetes in patients with cardiovascular risk factors
• BNP/NT-proBNP: When heart failure is suspected
• CRP: To evaluate for inflammatory conditions

Troponin Testing Strategy

High-Sensitivity Troponin Algorithm

1. Initial hs-cTn measurement:

   • hs-cTnI <5 ng/L or hs-cTnT <6 ng/L: Very low risk, consider discharge with appropriate follow-up 1
   • hs-cTnI or hs-cTnT between detection limit and 99th percentile: Observation zone, requires repeat testing
   • hs-cTnI or hs-cTnT >99th percentile: Higher risk, requires additional evaluation

2. Repeat hs-cTn at 3 hours if initial value is in observation zone or above 99th percentile 1

   • No significant change: Lower risk
   • Significant change (defined as >3 ng/L absolute change for low values or >20% relative change for higher values): Suggests acute myocardial injury 1

Risk Stratification Tools

Combine laboratory results with validated risk scores:

• Modified HEART score: History, ECG, Age, Risk factors, Troponin
• EDACS: Emergency Department Assessment of Chest Pain Score

Important Caveats

• Single troponin measurement limitations:

  • A single negative troponin does not rule out ACS in patients presenting <3 hours from symptom onset 1
  • Serial testing is strongly recommended for patients with recent onset chest pain

• Non-ACS causes of elevated troponin:

  • Pulmonary embolism 3
  • Myocarditis 4
  • Aortic dissection 5
  • Heart failure
  • Renal dysfunction (but should not be assumed to be the sole cause of elevation) 6

• Follow-up recommendations:

  • Low-risk patients with negative serial troponins should have follow-up within 1-2 weeks 1
  • If follow-up cannot be arranged, consider additional testing before discharge

Common Pitfalls to Avoid

1. Relying on a single troponin measurement when symptoms started <3 hours before presentation
2. Failing to use sex-specific 99th percentile cutoffs for troponin interpretation
3. Attributing elevated troponin solely to renal dysfunction without excluding ACS
4. Using point-of-care troponin tests which are less sensitive than laboratory methods
5. Not considering non-coronary causes of troponin elevation in patients with normal coronary arteries

Remember that patients with mildly elevated hs-cTn who are discharged have increased risk of mortality and subsequent MI at 90 and 360 days, highlighting the importance of appropriate follow-up 7.