A 58-year-old man with hypertension and dyslipidemia presents to the emergency department with chest pain of 2 hours duration, radiating to the left arm. ECG shows ST depression in leads II, III, aVF with T-wave inversion. Troponin I is negative at presentation. What is the investigation of choice to confirm acute coronary syndrome and risk-stratify this patient?

## Investigation of Choice in NSTEMI/Unstable Angina **Key Point:** Serial troponin measurement (at 0 and 3 hours, or 0 and 6 hours) is the gold standard for confirming myocardial necrosis in suspected NSTEMI. A single negative troponin does NOT exclude ACS. ### Rationale for Repeat Troponin **High-Yield:** Troponin I and T are highly specific for myocardial injury but may be negative in the first 2–3 hours after symptom onset. The rise and fall pattern (serial measurement) is diagnostic of myocardial infarction [cite:Harrison 21e Ch 297]. - **Timing:** Troponin typically rises 2–4 hours after symptom onset and peaks at 24–48 hours - **Serial measurement:** 0-hour and 3-hour troponins have >99% sensitivity for MI when both are measured - **Risk stratification:** Elevated troponin identifies high-risk patients who benefit from invasive strategy ### Why This Patient Needs Serial Troponin This patient has: - Ongoing chest pain (within 2 hours) - ECG changes (ST depression + T-wave inversion in inferior leads) - Negative troponin at presentation The combination of clinical symptoms and ECG changes warrants repeat troponin at 3 hours to confirm or exclude myocardial necrosis. **Clinical Pearl:** Even with negative troponin, this patient's ECG abnormalities and symptoms classify him as having **unstable angina** (if troponin remains negative) or **NSTEMI** (if troponin becomes positive). Serial troponin is mandatory before discharge or risk-stratification decisions. ### Investigation Algorithm ```mermaid flowchart TD A[Suspected ACS: Chest pain + ECG changes]:::outcome --> B[Troponin at 0 hours]:::action B --> C{Troponin positive?}:::decision C -->|Yes| D[NSTEMI confirmed]:::outcome C -->|No| E[Repeat troponin at 3 hours]:::action E --> F{Troponin positive?}:::decision F -->|Yes| G[NSTEMI confirmed]:::outcome F -->|No| H[Unstable angina or non-ACS]:::outcome D --> I[Risk stratify + Invasive strategy]:::action G --> I ``` **Mnemonic:** **TRAP** — Troponin Rise And Pattern - **T**roponin at 0 hours (baseline) - **R**epeat at 3 hours (rise confirms MI) - **A**cute coronary syndrome confirmed - **P**attern of rise/fall diagnostic [cite:Harrison 21e Ch 297]