High-Sensitivity Troponin: 0/1-Hour Rule-In and Rule-Out Algorithms

Definition and Pathophysiological Basis

High-sensitivity cardiac troponin (hs-cTn) assays detect circulating troponin released from injured cardiomyocytes at substantially lower concentrations than earlier-generation assays. They can identify troponin concentrations in approximately 50–95% of healthy individuals, whereas sensitive, but not high-sensitivity, assays detect troponin in approximately 20–50%. Their greater analytical sensitivity improves diagnostic accuracy and negative predictive value in patients with suspected acute coronary syndrome (ACS).

Cardiac troponin is a marker of myocardial injury, not a disease-specific marker of myocardial infarction (MI). MI requires both:

• Acute myocardial injury, demonstrated by a rising and/or falling troponin pattern with at least one value above the assay-specific 99th-percentile upper reference limit; and

• Evidence that the injury is caused by myocardial ischaemia.

Accordingly, acute troponin elevation without evidence of ischaemia represents acute myocardial injury rather than MI. Persistent elevation without a significant dynamic change is consistent with chronic myocardial injury. Non-ischaemic conditions, including myocarditis, rapid ventricular-rate atrial fibrillation, marked anaemia, hypertensive emergency, heart failure, and end-stage renal disease, may produce elevated troponin concentrations.

The probability of MI increases continuously as the absolute hs-cTn concentration rises. Early absolute changes over 1 hour provide diagnostic information that supplements the initial concentration and can approximate the information obtained from longer serial intervals.

Clinical Presentation and Initial Clinical Context

Chest pain or discomfort is the most common symptom initiating evaluation for ACS. Features that increase the likelihood of ACS include:

• Sudden symptoms at rest or with minimal exertion, lasting at least 10 minutes unless promptly treated

• Severe chest pain, pressure, or discomfort

• An accelerating pattern of angina, with increasing frequency or severity

• Symptoms that awaken the patient from sleep

Clinical assessment must be integrated with vital signs, a 12-lead ECG, and serial hs-cTn measurements. Troponin results should not be interpreted in isolation because myocardial injury is also encountered in non-ACS cardiopulmonary disease.

Patients with persistent or recurrent chest pain require repeat blood sampling even when an accelerated algorithm has already been initiated.

Evaluation and Physical Examination

Patients with suspected ACS should undergo expedited emergency-department assessment. The initial evaluation includes:

• Targeted medical history

• Physical examination

• Vital-sign assessment

• 12-lead ECG

• Initial hs-cTn measurement

• Continuous cardiac monitoring

The source material does not provide a detailed list of physical-examination findings or a specific examination sequence. It does, however, emphasize that symptoms and vital signs contribute to initial triage and that alternative cardiac and thoracic diagnoses must be considered.

The immediate clinical categories include:

• Non-cardiac diagnosis

• Chronic stable angina

• Possible ACS

• Definite ACS

In patients with ST-segment elevation, immediate reperfusion assessment is required. Patients with NSTE-ACS should be admitted for management of acute ischaemia. Those with possible ACS, a non-diagnostic ECG, and normal initial troponin may be observed in a chest-pain unit or another non-intensive-care setting while serial testing is completed.

Diagnostic Framework

The diagnostic pathway combines three principal domains:

• Clinical presentation and vital signs

• The 12-lead ECG

• The hs-cTn concentration at presentation and its serial change

A simplified interpretation is shown below.

The higher the initial hs-cTn concentration or the larger the absolute serial change, the greater the likelihood of MI. Nevertheless, the final diagnosis requires clinical and ECG correlation.

Electrocardiography

A 12-lead ECG should be obtained promptly in all patients with suspected ACS. It is essential for distinguishing suspected STEMI from NSTE-ACS and for integrating the hs-cTn result into the diagnostic pathway.

Relevant ECG patterns include:

• Persistent ST-segment elevation, which directs the patient to an immediate reperfusion pathway

• ST-segment depression, which increases concern for NSTE-ACS and may support direct rule-in when accompanied by compatible clinical and biomarker findings

• A normal or non-diagnostic ECG, which does not by itself exclude MI and requires integration with serial hs-cTn testing

Patients with suspected NSTE-ACS and a non-diagnostic initial ECG remain candidates for accelerated hs-cTn testing and clinical observation when appropriate.

High-Sensitivity Troponin Assays

Preferred assay type

High-sensitivity assays are preferred over lower-sensitivity assays because they offer greater diagnostic accuracy at similar low cost and provide a higher negative predictive value for MI. Automated central-laboratory assays have undergone more extensive evaluation than point-of-care tests and are currently preferred.

Most point-of-care tests are not high-sensitivity assays. Their principal advantage is shorter turnaround time, but this is offset by lower sensitivity, lower diagnostic accuracy, and lower negative predictive value.

Analytical considerations

Troponin assays are not standardized. Results from different assays, assay generations, instruments, or platforms must not be directly compared. Thresholds are assay-specific and should be expressed in nanograms per litre. Values should not be interpreted by mixing units from conventional and high-sensitivity assays.

Analytical and biological variation together may be approximately 50–60%. When troponin values are elevated, a change of approximately 20% may be used to characterize values as stable in the appropriate clinical setting. Dynamic changes may be difficult to detect in:

• Very early presenters, before substantial troponin release

• Late presenters on the declining part of the concentration-time curve

• Patients near the peak concentration, where the trajectory may be changing from rising to falling

False-positive and false-negative analytical results are uncommon and occur less often with hs-cTn assays.

The ESC 0/1-Hour Algorithm

Principle

The 0/1-hour algorithm uses:

• A baseline hs-cTn concentration obtained immediately at emergency-department admission

• A second hs-cTn concentration obtained 1 hour later

• The absolute change between the two values

Patients are assigned to one of three pathways:

• Rule out

• Observe

• Rule in

The algorithm is assay-specific. Thresholds must therefore be taken from the validated protocol for the local hs-cTn assay.

Operational requirements

For safe and feasible implementation:

• The initial sample should be obtained immediately after admission.

• The 1-hour sample should be obtained irrespective of other clinical details or pending results.

• The timing of the second sample is critical.

• The algorithm must always be interpreted alongside clinical assessment and the 12-lead ECG.

Routine collection of both samples may result in some unnecessary second measurements, particularly in patients with very low initial concentrations and symptom onset more than 3 hours earlier. This approach nevertheless simplifies implementation and supports patient safety.

Rule-out pathway

Patients assigned to rule out have a very low probability of MI. In large validation cohorts, the negative predictive value has exceeded 99%.

Rule-out assignment does not automatically mean that outpatient management is appropriate. Early discharge may be considered only after integration with:

• Clinical findings

• ECG results

• Overall risk assessment

• The possibility of an alternative diagnosis

Even when MI has been excluded, elective non-invasive or invasive imaging may be appropriate according to the clinical risk profile.

Rule-in pathway

The rule-in pathway is based on a sufficiently high baseline hs-cTn concentration, a sufficiently large absolute rise at 1 hour, or both. Across studies, the positive predictive value has generally been approximately 70–75%.

A rule-in result does not establish MI in every patient. Some patients have other cardiac diseases or non-ischaemic causes of myocardial injury. However, most patients meeting rule-in criteria require hospital admission, specialist cardiology assessment, and either invasive coronary angiography or non-invasive imaging to establish the final diagnosis.

Observe pathway

Patients who meet neither rule-out nor rule-in criteria enter the observe pathway. This is a heterogeneous group with a mortality rate comparable to that of patients assigned to rule in. Clinical risk assessment is therefore essential.

The next steps generally include:

• Individualized risk assessment

• A third hs-cTn measurement at 3 hours

• Echocardiography when appropriate

• Further invasive or non-invasive imaging according to the likelihood of ACS

Patients with a high clinical suspicion of ACS, particularly those with a relevant troponin increase by 3 hours, are often candidates for invasive coronary angiography. Patients with low or intermediate clinical likelihood may undergo non-invasive imaging after transfer from the emergency department.

Assay-Specific Thresholds

The following values are examples from a summarized table of validated rapid algorithms. They must not be transferred between assays or platforms.

These criteria are assay-specific examples rather than universal thresholds. The source material reports the following ranges for the 0/1-hour strategy:

• Negative predictive value for MI: 98.9–100%

• Sensitivity for MI: 96.7–100%

• Proportion ruled out: 47.9–64.2%

• Positive predictive value for MI: 63.4–84.0%

• Specificity for MI: 93.8–97%

• Proportion ruled in: 13.1–23.0%

The rule-out threshold is selected to achieve a sensitivity and negative predictive value of at least 99% in the derivation and validation framework, whereas rule-in thresholds were selected to provide a positive predictive value of at least 70%. Performance varies with assay, population, timing of presentation, and implementation.

Comparison With Other High-Sensitivity Troponin Strategies

0/2-hour algorithm

The 0/2-hour algorithm uses the same general structure as the 0/1-hour pathway but assesses the absolute troponin change after 2 hours.

Advantages include:

• Greater assay precision than a single measurement

• Better suitability for some early presenters

• Practicality in centres unable to obtain 1-hour samples routinely

Limitations include:

• Longer time to rule out than with the 0/1-hour pathway

• Similar algorithmic complexity

• Potential to miss late-presenting MI on the relatively flat portion of a declining troponin curve

• Lack of validation in randomized controlled trials in the material provided

0/3-hour strategy

The 0/3-hour approach uses the 99th-percentile upper reference limit. A single hs-cTn below that limit may rule out MI when symptoms began more than 6 hours earlier and the patient is pain free. If symptoms began less than 6 hours earlier, both the baseline and 3-hour values must remain below the 99th percentile.

This strategy is familiar and conceptually straightforward, but it has lower sensitivity and negative predictive value than more rapid strategies and rules out fewer patients.

Single-draw strategy

A single baseline measurement may rule out MI in selected low-risk patients when the hs-cTn concentration is below the assay limit of quantification or an optimized low cutoff, such as hs-cTnI <5 ng/L. It is not suitable for early presenters and applies to fewer than half of patients.

High-STEACS strategy

The High-STEACS strategy may rule out MI when:

• Initial hs-cTnI is <5 ng/L or hs-cTnT is <6 ng/L more than 3 hours after symptom onset; or

• The change between the initial and 3-hour hs-cTn is <3 ng/L and the value remains below the sex-specific 99th-percentile upper reference limit

It has a reported negative predictive value of 99.5%, but it generally requires longer observation for patients whose initial concentration exceeds the low rule-out threshold and discharges fewer patients than the 0/1-hour or 0/2-hour pathways.

Imaging in Patients Not Immediately Classified

Non-invasive imaging can improve diagnostic accuracy and risk assessment.

Echocardiography

Echocardiography may be used in the observe pathway, particularly alongside a third troponin measurement at 3 hours. The source material does not specify a detailed echocardiographic protocol or diagnostic threshold.

Coronary CT angiography

Coronary CT angiography may assist diagnosis in patients with low to intermediate likelihood of ACS. It can:

• Identify patients with non-obstructed coronary arteries who may be discharged after relevant alternative diseases have been excluded

• Detect obstructive coronary disease in patients who may require consideration of revascularization

Invasive coronary angiography

Invasive coronary angiography is generally considered for:

• Patients assigned to rule in

• Patients in the observe zone with high clinical suspicion of ACS

• Patients with a relevant increase in troponin over serial testing

• Patients with high-risk clinical or ECG features

In patients with an alternative diagnosis that explains the troponin elevation, such as rapid atrial fibrillation, marked anaemia, or hypertensive emergency, invasive testing may not be necessary.

Biomarkers and Laboratory Findings

Cardiac troponin is the preferred biomarker for diagnosing MI and should be measured in patients with suspected ACS. It should not be measured routinely in patients without suspicion of ACS unless it is being used for a specific risk-stratification purpose.

Recommended timing is:

• hs-cTn at presentation and repeated at 1–3 hours

• Conventional troponin at presentation and repeated at 3–6 hours

• Additional testing beyond these intervals when clinical suspicion persists or diagnostic uncertainty remains

In patients presenting more than 2–3 hours after symptom onset, a very low hs-cTn concentration at presentation may exclude MI with a negative predictive value greater than 99%.

Other laboratory tests may be selected according to the differential diagnosis. D-dimer may assist evaluation for pulmonary embolism, and B-type natriuretic peptide may be useful with the clinical history and examination when heart failure is suspected. Biomarkers other than troponin are not recommended for ACS diagnosis unless troponin is unavailable. Creatine kinase myocardial band, myosin-binding protein C, and copeptin may have limited incremental value when combined with conventional troponin, but their additional contribution is usually small.

Immediate Management While Diagnosis Is Established

Patients with suspected ACS should receive continuous cardiac monitoring. The source material identifies the following immediate measures:

• Supplemental oxygen when the patient is hypoxaemic or in heart failure

• Sublingual nitroglycerin for persistent angina

• An aspirin loading dose

• Parenteral anticoagulation

• A beta blocker when there are no contraindications

• Morphine for pain refractory to other measures

ST-segment elevation requires immediate assessment for reperfusion therapy. NSTE-ACS requires hospital admission for management of acute ischaemia.

Management After NSTE-ACS Is Diagnosed

The initial strategy is determined by risk assessment:

• Early invasive approach: coronary angiography within 2 hours for very-high-risk patients, or otherwise within 24 hours

• Delayed invasive approach: coronary angiography more than 24 hours after presentation

• Ischaemia-guided approach: initial medical treatment with selective angiography for haemodynamic instability, recurrent symptoms at rest, or ischaemia during stress testing

A P2Y12 inhibitor, preferentially prasugrel or ticlopidine in the source material, should be administered after angiography when angiography is planned within 24 hours, or on day 1 otherwise, provided coronary artery bypass surgery is not planned before discharge.

Antithrombotic and Lipid-Lowering Therapy

For patients not requiring oral anticoagulation, dual-antiplatelet therapy is recommended for 12 months as the default discharge strategy. Shorter courses of 3–6 months or less intensive treatment may be used when bleeding risk is high. Extension beyond 12 months may be considered in patients with high ischaemic risk, low bleeding risk, and good tolerance of dual therapy.

When long-term oral anticoagulation is required, such as in atrial fibrillation, a short course of triple therapy may be considered:

• A non-vitamin K antagonist oral anticoagulant

• Clopidogrel

• Aspirin for up to 1 week

After this initial period, treatment generally consists of a non-vitamin K antagonist oral anticoagulant plus clopidogrel through 1 year, followed by long-term anticoagulation without an antiplatelet agent. A 1-day triple-therapy course and 3–6 months of combination therapy may be considered in patients at high bleeding risk. In patients at high ischaemic risk, triple therapy may be extended to 30 days.

High-intensity statin therapy should be started at presentation:

• Atorvastatin 40–80 mg

• Rosuvastatin 20–40 mg

The stated goals are:

• At least a 50% reduction in low-density lipoprotein cholesterol

• Low-density lipoprotein cholesterol <55 mg/dL

Ezetimibe should also be given when the patient was already receiving high-intensity statin therapy or is unlikely to reach these targets. A PCSK9 inhibitor may be added as a third lipid-lowering agent if the targets remain unmet.

Guideline Recommendations

The principal recommendations for suspected ACS are:

• Obtain a targeted history, physical examination, 12-lead ECG, and hs-cTn promptly after arrival.

• Use hs-cTn assays in preference to lower-sensitivity assays.

• Prefer the ESC 0/1-hour algorithm; use the 0/2-hour algorithm as the next option.

• Obtain baseline and 1-hour blood samples irrespective of other clinical details when implementing the 0/1-hour pathway.

• Interpret the algorithm with the clinical assessment and ECG.

• Use assay-specific thresholds and do not transfer cutoffs between platforms.

• Assign patients who meet neither rule-out nor rule-in criteria to an observe pathway with individualized risk assessment, a third troponin measurement at 3 hours, and echocardiography when appropriate.

• Do not equate rule-out assignment automatically with discharge; clinical assessment and alternative diagnoses remain necessary.

• Consider non-invasive or invasive imaging after MI has been excluded when clinical risk warrants further evaluation.

• Treat patients assigned to rule in as requiring specialist assessment, usually with hospital admission and additional imaging or angiography.

• Do not use biomarkers other than troponin routinely for ACS diagnosis unless troponin is unavailable.

• Repeat troponin testing when pain is ongoing or recurrent or when diagnostic uncertainty persists.

Prognosis and Follow-Up

Patients assigned to the rule-out pathway by the ESC 0/1-hour or 0/2-hour algorithms have a very low rate of clinical events through 30 days in reported validation cohorts. Nevertheless, rule-out of MI does not eliminate the need to identify alternative diagnoses or evaluate residual coronary risk.

Patients assigned to the observe zone require particular caution. They constitute a heterogeneous population, and their mortality has been reported as comparable to that of rule-in patients. Management should therefore be individualized using clinical risk assessment, serial troponin testing, ECG findings, echocardiography, and appropriate non-invasive or invasive imaging.

The source material does not specify a separate long-term surveillance schedule after discharge following a negative 0/1-hour algorithm. Follow-up should therefore be determined by the final diagnosis, residual clinical risk, findings on subsequent imaging, and whether ACS or another cardiac disorder is ultimately established.