By-
Dr Arihant Jain, MD | lifeonthefrontline.com
Instagram: @humans.of.em
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Before you go any further…

This is Part 3.

If you haven’t read Part 1 and Part 2, stop here and read them first.

Part 1 built the foundation:
Troponin → Myocardial Injury ≠ Myocardial Infarction

Part 2 took it further:
Troponin Matrix → Trop-Zones → Delta → Five questions

This post assumes you already have that framework.

Because the Fifth Universal Definition of Myocardial Infarction (2026) doesn’t make us throw that framework away.

It makes us update it.

Start here:
→ Part 1: Troponin Is Not an MI Test
→ Part 2: Troponin Is Not an MI Test

Then come back here.

Let’s see what changed in 2026.
If you have read Part 1 and Part 2 of this series, you already know the central message:

Troponin detects myocardial injury. It does not diagnose myocardial infarction.

We built the Troponin Triangle.

We moved from:

“Is the troponin positive?”

to:

“Is there myocardial injury → Is it acute → Is there evidence of ischemia → If yes, what is the mechanism?”

We discussed the Troponin Matrix, Type 1 versus Type 2 MI, the Trop-Zones, and why the delta is often more informative than the first troponin.

All of that came from the framework of the Fourth Universal Definition of Myocardial Infarction (2018). Now, the Fifth Universal Definition of Myocardial Infarction (2026) has arrived.

And importantly, it does not throw away the mental model. It makes it more sophisticated.

The fundamental question remains:

Is this myocardial injury or myocardial infarction?

But the Fifth UDMI gives us better tools for answering the questions that come after the troponin comes back.

What actually changed?

There are several important updates. But for the acute-care physician, I think three deserve particular attention:

1. The delta is no longer just “rise or fall”

2. Imaging becomes much more important in determining what the injury actually represents

3. The classification of MI has been reorganized around clinical setting and pathophysiology

Let’s unpack these.

1. The Delta Is Not a Number

In Part 2, we called the delta:

“The most important number you never memorized.”

The Fifth UDMI adds an important qualification:

There is no single absolute or relative troponin change that works for every patient, every assay, and every clinical situation.

Why?

Because the meaning of a delta depends on three things.

Factor 1: Time from symptom onset

Troponin does not behave like a light switch. It follows a biological curve. Early after an ischemic event, the absolute change may be small. Several hours later, the rise can become much more pronounced. Around the peak, the change may become relatively small again. And during the later phase, the troponin begins to fall.

The Fifth UDMI therefore emphasizes that the same delta can mean very different things depending on when the patient presents.

Think about two patients:

Patient A

Chest pain started 45 minutes ago.

Troponin:
10 → 14 ng/L

Patient B

Chest pain started 10 hours ago.

Troponin:
10 → 14 ng/L

Same delta.

Very different biological stories.

This is why:

Delta without a timeline is incomplete information.

Factor 2: Time between samples

The second troponin is not simply “another troponin.” It is a measurement taken at a particular point in time.

A 1-hour delta and a 6-hour delta are not interchangeable.

The longer the interval, the greater the opportunity for a biological rise or fall to occur. This is one reason accelerated diagnostic pathways use fixed sampling intervals, such as 0/1-hour or 0/2-hour strategies. But even here, the Fifth UDMI emphasizes that the thresholds and deltas are assay-specific.

So:

A delta without knowing the interval between samples is also incomplete information.

Factor 3: The baseline troponin

This one is particularly important.

Suppose the troponin is:

8 → 18 ng/L

A 10 ng/L rise. That absolute change may be quite meaningful when the baseline is low.

Now consider:

800 → 810 ng/L

Same absolute change. But the biological interpretation is very different.

The Fifth UDMI highlights that the relationship between absolute and relative change depends on the baseline concentration.

When troponin is around or below the 99th percentile, absolute changes can be particularly useful. When baseline troponin is already substantially elevated, relative change may be easier to interpret.

And importantly:

Absolute delta thresholds cannot simply be transferred from one assay to another.

They are assay-specific. So the new mental model becomes:

Don’t ask:

“What is the delta?”

Ask:

“What is the delta, over what interval, from what baseline, and at what point in the patient’s illness?”

The New Delta Triangle

I would therefore modify our Part 2 framework.

Previously:

TROPONIN → DELTA → ACUTE INJURY

Now:

DELTA =

Time from symptom onset

+

Time between samples

+

Baseline troponin

Only then does the number start to make biological sense.

2. Imaging Moves Further Into the Story

This may be the most important conceptual change for me. And there is an important caveat:

Imaging is not new to the Universal Definition.

The Fourth UDMI already included imaging as evidence of ischemia and myocardial infarction. So the Fifth UDMI does not suddenly “introduce imaging.”

Instead, it expands and organizes the role of multimodality coronary and cardiac imaging in determining the mechanism and consequences of myocardial injury.

And this is where things become particularly interesting for emergency and acute-care medicine. Because sometimes:

The troponin tells you that myocardium is injured.

But it cannot tell you:

What happened to the myocardium?

And it certainly cannot always tell you:

What happened to the coronary artery?

That is where imaging comes in.

Think of the modalities as answering different questions

Troponin

Is the myocardium injured?

ECG

Is there evidence of electrical ischemia?

Echocardiography

Is there a new functional consequence?

But then we have:

CMR

What happened to the myocardium?

CCTA

What does the coronary anatomy and plaque look like?

IVUS/OCT

What actually happened inside the coronary artery?

This is a very different way of thinking about imaging.

CMR: What happened to the myocardium?

Cardiac MRI is particularly powerful because it can characterize myocardial tissue.

It can demonstrate:

myocardial infarction
edema
inflammation
microvascular obstruction
intramyocardial haemorrhage
patterns of myocardial injury

using multimodality tissue-characterization sequences. This becomes particularly important when the diagnosis is not obvious from the initial presentation.

A patient has:

Chest pain + troponin elevation + non-obstructive coronaries.

What now? The answer is not:

“Angiogram is normal, so nothing happened.”

The myocardium may have infarcted. Or the patient may have myocarditis. Or Takotsubo syndrome. Or another cardiomyopathy.

This is where CMR can help establish the final diagnosis. The Fifth UDMI specifically emphasizes its role in situations such as MINOCA and unrecognized myocardial infarction.

CCTA: What do the coronaries look like?

CT coronary angiography provides another piece of the puzzle.

It can evaluate:

coronary anatomy
obstructive CAD
plaque characteristics
graft patency

and newer CT techniques are expanding its ability to characterize myocardial viability and alternative diagnoses. But this does not mean:

“Every patient with a raised troponin needs a CCTA.”

Absolutely not. The Fifth UDMI still recognizes that its role depends on clinical context and pre-test probability.

The important change is conceptual:

Coronary anatomy can become part of the diagnostic explanation for myocardial injury.

IVUS/OCT: What actually happened to the plaque?

This is perhaps the most fascinating part.

Sometimes angiography tells us:

“There is no major obstruction.”

But that does not necessarily mean:

“There is no coronary pathology.”

Intravascular imaging with IVUS or OCT can reveal pathology that angiography may miss, including:

plaque rupture
plaque erosion
mural thrombus
spontaneous coronary artery dissection
stent-related pathology
other subtle coronary abnormalities.

And this is especially relevant to MINOCA.

Because:

MINOCA is not necessarily the final diagnosis.

It can be a working diagnosis that tells us:

“The patient appears to have had an MI, but we have not yet established why.”

Further investigation may involve intravascular imaging, functional coronary testing and CMR to identify the underlying mechanism.

The New Imaging Mental Model

This is how I would now think about it:

TROPONIN

↓

MYOCARDIAL INJURY

↓

IS IT ISCHEMIC?

↓

If yes:

WHAT HAPPENED TO THE CORONARY?
→ Angiography
→ IVUS/OCT
→ Functional testing

WHAT HAPPENED TO THE MYOCARDIUM?
→ CMR
→ Echo
→ Other appropriate imaging

WHAT IS THE UNDERLYING MECHANISM?

That is a much richer diagnostic pathway than:

Troponin positive → NSTEMI

3. MINOCA: “Nothing Significant on Angiography” Is Not the End

This is where the imaging emphasis becomes clinically relevant.

Imagine:

Chest pain

Dynamic troponin rise

Ischemic ECG changes

Angiography: no obstructive coronary lesion

The old instinct might be:

“The angiogram is normal.”

The better question is:

“Why did this patient have an infarction?”

Possibilities include plaque disruption, coronary spasm, microvascular dysfunction, SCAD and other mechanisms.

The Fifth UDMI therefore reinforces MINOCA as a working diagnosis requiring further investigation, rather than simply a final label.

And that is exactly where multimodality imaging becomes clinically useful.

4. The Type 1–5 World Has Been Reorganized

There is another major change.

For years, we have taught:

Type 1 MI
Type 2 MI
Type 3 MI
Type 4 MI
Type 5 MI

The Fifth UDMI replaces this numerical classification with three broader clinical settings:

Primary MI

Secondary MI

Procedure-related MI

This is not simply a change in vocabulary. It is an attempt to align the classification more closely with the clinical setting and underlying pathophysiology. The underlying concepts have not disappeared. The point is to make the classification more clinically intuitive.

5. What About the 99th Percentile?

Here is another important nuance.

The Fifth UDMI continues to define acute myocardial injury using a rise and/or fall in cardiac troponin with at least one value above the assay’s sex-specific 99th percentile URL.

But don’t read this as:

“Sex-specific 99th percentile is a completely new 2026 concept.”

It isn’t. The Fourth UDMI had already recommended sex-specific 99th-percentile values for hs-cTn assays.

What the Fifth UDMI does is further emphasize and operationalize this issue, recognizing the potential for under-recognition of myocardial injury when a single threshold is applied across sexes. And another important distinction:

The 99th percentile defines myocardial injury.

It is not automatically the same thing as a rule-in threshold for MI. Accelerated diagnostic pathways may use assay-specific thresholds and deltas that are different from the 99th percentile.

So What Should We Change at the Bedside?

Let’s go back to the five questions from Part 2.

They still work. But the Fifth UDMI makes them better.

Question 1

Is there myocardial injury?

Look at the assay-specific 99th percentile.

Question 2

Is it acute or chronic?

Look at the pattern over time.

Question 3

Is there evidence of ischemia?

Symptoms.
ECG.
Imaging.
Coronary findings.

Question 4

If ischemic, what is the mechanism?

Primary?
Secondary?
Procedure-related?

Question 5

If the mechanism is unclear, what additional information do I need?

And this is where the Fifth UDMI pushes us further.

Maybe the answer is:

Repeat troponin.

Maybe:

Repeat ECG.

Maybe:

Echo.

But sometimes:

CMR.

CCTA.

IVUS/OCT.

Functional coronary assessment.

The Updated Troponin Mental Model

So perhaps our original framework needs one more layer.

The old shortcut:

TROPONIN → MI

was wrong.

Our Part 1 model:

TROPONIN

↓

INJURY

↓

ACUTE OR CHRONIC

↓

ISCHEMIC OR NON-ISCHEMIC

↓

MI

was much better.

The 2026 model:

TROPONIN

↓

MYOCARDIAL INJURY

↓

ACUTE OR CHRONIC

↓

INTERPRET THE DELTA

Time from onset
Sampling interval
Baseline concentration

↓

IS THERE ISCHAEMIA?

↓

USE CLINICAL + ECG + IMAGING EVIDENCE

↓

IS IT INFARCTION?

↓

WHAT IS THE MECHANISM?

↓

PRIMARY / SECONDARY / PROCEDURE-RELATED

And when the answer remains unclear:

↓

IMAGE THE CORONARY. IMAGE THE MYOCARDIUM.

The Take-Home Message

The Fifth Universal Definition does not overturn the central lesson of our previous two posts. It reinforces it. Troponin still detects myocardial injury. It does not tell you whether that injury is an infarction. But the 2026 update makes us more precise about two things.

First:

The delta is not a magic number.

Its interpretation depends on:

when the injury occurred + when you sampled + where the troponin started.

Second:

The diagnosis does not necessarily end with the troponin—or even the angiogram.

Modern MI diagnosis increasingly asks:

What happened to the myocardium?

and

What happened to the coronary artery?

Sometimes the answer comes from the ECG.

Sometimes from echocardiography.

Sometimes from angiography.

But increasingly, the answer may require:

CMR.

CCTA.

IVUS/OCT.

Functional coronary testing.

And perhaps that is the most important evolution from our original mental model.

One Last Time

The next time someone tells you:

“The troponin is positive.”

Don’t ask:

“Is this an NSTEMI?”

Ask:

Is there myocardial injury?

Is it acute?

Does the delta make biological sense given the timing, sampling interval and baseline?

Is there evidence of ischemia?

If there is ischemia, what is the mechanism?

And if the answer is still unclear—what does the coronary artery and the myocardium actually look like?

Because:

Troponin tells you that the myocardium is speaking.
The Fifth UDMI reminds us that we still have to listen to the whole story.

References

Fifth Universal Definition of Myocardial Infarction. ESC/ACC/AHA/WHF, 2026.
Jain A. Troponin Is Not an MI Test: Part 1 — A New Mental Model for Acute Care Physicians. Life on the Frontline. July 1, 2026. Read Part 1
Jain A. Troponin Is Not an MI Test: Part 2 — A New Mental Model for Acute Care Physicians. Life on the Frontline. July 4, 2026. Read Part 2

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