Cardiac Troponins

Cardiac Troponins

Cardiac troponins (cTnI and cTnT) are structural proteins within the cardiac sarcomere and the preferred biomarkers for myocardial injury. High-sensitivity assays can detect very small amounts of myocardial cell injury, but troponin ≠ myocardial infarction: MI requires a troponin rise/fall plus clinical evidence of acute myocardial ischaemia (symptoms, ECG, imaging, or coronary thrombus).

🔬 What troponin is (structure & physiology)

• Troponin complex sits on the thin filament (actin) with tropomyosin and regulates contraction.
• TnC binds calcium → conformational change → moves tropomyosin off actin’s myosin-binding sites → cross-bridge cycling starts.
• TnI is the inhibitory subunit (prevents actin–myosin interaction at rest).
• TnT anchors the complex to tropomyosin.
• Cardiac-specific isoforms of TnI/TnT allow blood tests to reflect cardiac myocyte injury rather than skeletal muscle damage.

🧪 High-sensitivity troponin (hs-cTn): why it changed practice

• hs-cTn assays detect low troponin concentrations with high analytical precision, enabling earlier rule-out/rule-in pathways in emergency care.
• Interpretation is assay-specific: always use your local lab’s reference ranges and pathway.
• Key concept: the clinically important threshold is the 99th percentile upper reference limit (URL) for that assay.

📌 Definitions you must get right

• Myocardial injury: any troponin value above the assay’s 99th percentile URL.
• Acute myocardial injury: myocardial injury with a rise and/or fall on serial testing (dynamic change).
• Myocardial infarction (MI): acute myocardial injury plus evidence of acute ischaemia (typical symptoms, new ischaemic ECG changes, imaging evidence, or coronary thrombus).

🩺 NICE / UK approach in practice (what you actually do in ED/AMU)

In the UK, NICE supports the use of high-sensitivity troponin testing within structured early rule-out/rule-in strategies for suspected ACS. Your hospital will implement an approved pathway (timings/cut-offs differ by assay), but the principles below stay the same.

• Always do an ECG early and treat STEMI as a time-critical emergency (troponin should not delay reperfusion).
• For possible NSTE-ACS, use hs-cTn with serial sampling when needed (especially if symptom onset is recent).
• If symptoms started very recently (e.g. within ~2 hours), a single troponin can be falsely low → repeat at an appropriate interval per local pathway.
• Discharge decisions should combine: symptoms (including ongoing pain), ECG, troponin pattern, and overall clinical risk (and safety-netting).

⏱️ Typical kinetics (teach the pattern, avoid overpromising)

• With contemporary assays, troponin may rise within a few hours of injury; hs-cTn can detect injury earlier than older assays.
• Early presenters can have an initially normal troponin → serial testing is essential if clinical suspicion remains.
• Troponin can remain elevated for days after significant myocardial necrosis; the exact duration varies by infarct size and assay.

🧭 How to interpret a result (a practical framework)

• Step 1 - Is it above the 99th percentile URL?
  • No: MI is unlikely, but not excluded in early presenters or ongoing ischaemia → repeat if indicated.
  • Yes: myocardial injury is present → move to step 2.
• Step 2 - Is there a significant rise/fall on serial testing?
  • Dynamic change: suggests acute injury (consider MI vs myocarditis vs tachyarrhythmia, etc.).
  • Stable elevation: suggests chronic injury (e.g. structural heart disease, CKD) - still prognostically important.
• Step 3 - Is there evidence of acute ischaemia?
  • If yes → MI (type 1 or type 2 depending on mechanism).
  • If no → acute myocardial injury (non-MI) and treat the cause.

⚠️ Common causes of raised troponin (MI and non-MI)

• Type 1 MI (atherothrombosis): plaque rupture/erosion → coronary thrombosis.
• Type 2 MI (supply–demand mismatch): sepsis, tachyarrhythmia, severe anaemia, hypoxia, hypertensive crisis, coronary spasm, etc.
• Acute myocardial injury (non-ischaemic): myocarditis, acute heart failure, cardiotoxic drugs, post-procedure injury, cardiac contusion.
• Chronic myocardial injury: chronic heart failure, CKD, structural heart disease - often persistently elevated with little change.

🧠 Clinical “gotchas” (very exam- and ward-relevant)

• Renal impairment: baseline troponin may be chronically elevated; the trend and clinical picture are key.
• One troponin is rarely enough if the story suggests early ACS - use serial testing as per pathway.
• Troponin is prognostic even when it’s not MI - an elevated troponin flags higher risk and warrants careful assessment.
• STEMI: treat based on ECG/clinical picture; do not wait for troponin before activating the pathway.

✅ Summary

Troponin tells you “myocardial injury happened”. To diagnose myocardial infarction you need a rise/fall above the 99th percentile URL plus evidence of acute myocardial ischaemia. In UK practice, NICE supports hs-cTn within structured early rule-out strategies - always follow your local lab’s assay-specific cut-offs and timings, and interpret results in the clinical context.