What additional enzyme finding would be useful for diagnosing a 63-year-old male with a 2-day history of chest pain, tightness, and discomfort upon exertion, with a family history of cardiovascular disease (CVD), hyperlipidemia, and a 25-year smoking history?

Diagnosis of Acute Myocardial Infarction Using LD Isoenzymes

The LD flipped pattern where LD1 > LD2 (option A) would be the most useful additional enzyme finding for diagnosing acute myocardial infarction in this 63-year-old male with chest pain and multiple cardiovascular risk factors.

Clinical Presentation Analysis

This patient presents with classic features suggestive of acute coronary syndrome:

• 2-day history of chest pain and tightness
• Discomfort upon exertion
• Family history of cardiovascular disease
• History of hyperlipidemia
• 25-year smoking history (1 pack/day)

These features strongly suggest coronary artery disease with possible acute myocardial injury.

Diagnostic Value of LD Isoenzymes

LD Flipped Pattern (LD1 > LD2)

• The "flipped" LD pattern occurs when LD1 exceeds LD2 (normally LD2 > LD1)
• This pattern is specifically associated with myocardial injury 1
• The flipped pattern can be detected even when total LD activity remains within normal range 1
• This pattern is particularly valuable when patients present >24 hours after symptom onset, when CK-MB may have already normalized 1

Other LD Patterns

• Elevations of LD3 (option B) are typically associated with pulmonary disorders, not cardiac disease
• Elevations of LD4 and LD5 (option C) are typically associated with liver or skeletal muscle disease
• LD1 > LD5 (option D) is not a recognized diagnostic pattern for myocardial injury

Timing Considerations

The ACC/AHA guidelines recognize that timing is critical in biomarker selection:

• For patients presenting within 6 hours of symptom onset, early markers like myoglobin or CK-MB subforms should be considered 2
• For patients presenting later (as in this case - 2 days), markers with longer detection windows become more valuable

Current Guidelines on Cardiac Biomarkers

While modern guidelines have shifted toward troponin as the preferred biomarker for myocardial injury, the ACC/AHA guidelines specifically note that in certain clinical scenarios, particularly with delayed presentation, additional markers may be valuable 2.

The ACC/AHA guidelines specifically state that "lactate dehydrogenase should not be utilized as primary tests for the diagnosis of myocardial injury" 2, but this doesn't preclude its use as an additional test in specific scenarios.

Clinical Application

For this 63-year-old male with:

1. Chest pain for 2 days (delayed presentation)
2. Multiple cardiovascular risk factors
3. Exertional symptoms consistent with ischemia

The LD1:LD2 ratio provides valuable diagnostic information that may detect myocardial injury even when other markers have normalized. Research shows that the LD1 > LD2 pattern has a sensitivity of 96% and specificity of 97% for diagnosing myocardial infarction 3, making it particularly useful in this clinical scenario.

Conclusion

The flipped LD pattern (LD1 > LD2) would be the most appropriate additional enzyme finding to diagnose myocardial injury in this patient with delayed presentation of chest pain and multiple cardiovascular risk factors.