Clinical background
Patients presenting acutely with chest pain and suspected acute coronary syndrome form one of the largest patient groups in emergency departments. The proportion who actually have an acute coronary syndrome is often 15 to 20 per cent, but uncertainty about the remainder routinely leads to long observation periods, serial troponin sampling over 6 to 12 hours and sometimes unnecessary admissions. The ADAPT protocol was developed to identify quickly and safely the subgroup whose risk of a major cardiac event within 30 days is so low that accelerated discharge is possible after only 2 hours of observation. The aim is not to diagnose acute myocardial infarction but to exclude it with sufficient certainty to justify early discharge with outpatient follow-up.
Applying the ADAPT protocol
The ADAPT protocol combines three elements into a binary low-risk decision:
All four conditions must be met simultaneously. If any single condition is not met, the patient is not classified as low risk under the protocol and accelerated discharge is not indicated.
The derivation study was a prospective observational study at two acute hospitals in Australia and New Zealand [1]. In total 1,975 patients with chest pain and suspected acute coronary syndrome were included. Troponin I was measured with contemporary (non-high-sensitivity) laboratory assays at 0 and 2 hours. The primary outcome was a major adverse cardiac event (MACE) within 30 days, defined as myocardial infarction, death, resuscitated cardiac arrest, cardiogenic shock, a life-threatening arrhythmia or new atrial fibrillation. Of all the patients, 15.3 per cent had a MACE. The protocol classified 20 per cent as low risk, of whom 1 patient (0.25 per cent) had a MACE. Sensitivity was 99.7 per cent (95 per cent CI 98.1 to 99.9) and the negative predictive value 99.7 per cent (95 per cent CI 98.6 to 100.0). Specificity was only 23.4 per cent, reflecting the fact that the protocol is designed to minimise false negatives, not to maximise the yield.
Interpretation in practice
The ADAPT protocol gives a single decision: low risk or not low risk. There are no score bands to be interpreted gradually.
| Classification | Criteria | Clinical action |
|---|---|---|
| Low risk | TIMI = 0, no new ischaemic ECG changes, normal troponin at 0 and 2 hours | The patient can be considered for discharge from the emergency department within 2 hours with outpatient follow-up shortly afterwards (within a few days). Further investigation can be carried out as an outpatient if symptoms persist. |
| Not low risk | Any of the criteria not met | The protocol gives no support for accelerated discharge. The patient is managed as usual with continued observation, serial troponin sampling and admission where needed. |
It is important to note that low-risk classification does not mean the patient is well. In the derivation cohort, 74 per cent of the patients classified as low risk underwent further investigations during the index attendance or at follow-up, and 18 per cent received therapeutic interventions [1]. The protocol says that the risk of a major cardiac event within 30 days is very low, not that no investigation is needed.
Validation and performance
The first prospective external validation was carried out by Kelly and Klim at an Australian hospital with 840 patients [2]. They used a contemporary sensitive troponin I assay (TnI-Ultra, Siemens). Of 177 patients with a TIMI of 0, 82 met the per-protocol criteria. In this group there were no MACE within 30 days, corresponding to an NPV of 100 per cent (95 per cent CI 95.5 to 100). The study was, however, small and single-centre, with correspondingly wide confidence intervals.
A comparative study of nine risk scores at a tertiary hospital in Brisbane with 401 patients gave a more nuanced picture [3]. ADAPT had the highest NPV of all the scores there (100 per cent, no missed ischaemic heart disease), but at the same time the lowest discrimination, with an AUC of 0.63 (95 per cent CI 0.58 to 0.69). Only 1.7 per cent of patients were classified as low risk, compared with 32 per cent for the HEART score and 10 per cent for the North American Chest Pain Rule. ADAPT was therefore the safest but the least efficient from a capacity perspective: very few patients qualified for accelerated discharge.
A development of the protocol using high-sensitivity troponin I has been validated by Cullen et al. in two cohorts of 1,635 and 909 patients [4]. With a TIMI of 0 and high-sensitivity troponin, 19.6 and 25.3 per cent respectively were classified as low risk, with 0 per cent MACE in both cohorts. A variant using TIMI ≤ 1 classified approximately 40 per cent as low risk with a MACE rate of 0.8 per cent. This variant substantially increases the yield but does not yet have the same weight of validation as the original protocol.
A systematic review and meta-analysis from 2024 collated 37 studies with over 400,000 patients in which various accelerated diagnostic protocols had been implemented [5]. Overall, the protocols reduced the emergency department length of stay by an average of just over 1 hour, reduced the proportion of admissions (pooled RR 0.84) and did not increase the rate of MACE within 30 days (pooled RR 0.95, 95 per cent CI 0.86 to 1.04). The effect on length of stay was most marked at hospitals with a long baseline length of stay.
Limitations
The ADAPT protocol is designed for patients with suspected acute coronary syndrome, not for those with a clear diagnosis on arrival. Patients with ST elevation, haemodynamic instability or an obvious alternative diagnosis should not be evaluated with the protocol.
The greatest limitation is the low yield. With contemporary troponin and a TIMI of 0, only around 20 per cent are classified as low risk in the derivation cohort, and in some populations as few as 1.7 per cent [3]. This means that in practice the protocol has limited operational effect where the proportion of chest pain patients undergoing full evaluation is large. Protocols using high-sensitivity troponin and a higher TIMI threshold (≤ 1) can increase the yield to approximately 40 per cent, but the calculator uses the original and best validated criterion of TIMI = 0.
The protocol requires a troponin assay of adequate sensitivity. With older, less sensitive assays the risk of false negative results at early sampling increases, particularly if the onset of symptoms is close to the time of sampling. The calculator's field design assumes that troponin is measured at 0 and 2 hours from arrival, not from the onset of symptoms. In late-presenting symptoms, the 2-hour interval may be insufficient to exclude an evolving infarction.
A TIMI value of 1 is not sufficient for the accelerated pathway. Any deviation from a TIMI of 0, whatever the reason, means that the protocol cannot be applied for low-risk classification.
References
- Than M, Cullen L, Aldous S, et al. 2-Hour accelerated diagnostic protocol to assess patients with chest pain symptoms using contemporary troponins as the only biomarker: the ADAPT trial. J Am Coll Cardiol. 2012;59(23):2091-2098. PMID: 22578923
- Kelly AM, Klim S. Prospective external validation of an accelerated (2-h) acute coronary syndrome rule-out process using a contemporary troponin assay. Int J Emerg Med. 2014;7:42. PMID: 25635202
- Wamala H, Aggarwal L, Bernard A, et al. Comparison of nine coronary risk scores in evaluating patients presenting to hospital with undifferentiated chest pain. Int J Gen Med. 2018;11:473-481. PMID: 30588062
- Cullen L, Mueller C, Parsonage WA, et al. Validation of high-sensitivity troponin I in a 2-hour diagnostic strategy to assess 30-day outcomes in emergency department patients with possible acute coronary syndrome. J Am Coll Cardiol. 2013;62(14):1242-1249. PMID: 23583250
- Hill J, Essel NOM, Yang EH, et al. Effectiveness of accelerated diagnostic protocols for reducing emergency department length of stay in patients presenting with chest pain: a systematic review and meta-analysis. PLoS One. 2024;19(10):e0309767. PMID: 39436875
