The 4 'A's test for detecting delirium in acute medical patients: a diagnostic accuracy study

Abstract

Background: Delirium is a common and serious neuropsychiatric syndrome, usually triggered by illness or drugs. It remains underdetected. One reason for this is a lack of brief, pragmatic assessment tools. The 4 'A's test (Arousal, Attention, Abbreviated Mental Test - 4, Acute change) (4AT) is a screening tool designed for routine use. This project evaluated its usability, diagnostic accuracy and cost.

Methods: Phase 1 - the usability of the 4AT in routine practice was measured with two surveys and two qualitative studies of health-care professionals, and a review of current clinical use of the 4AT as well as its presence in guidelines and reports. Phase 2 - the 4AT's diagnostic accuracy was assessed in newly admitted acute medical patients aged ≥ 70 years. Its performance was compared with that of the Confusion Assessment Method (CAM; a longer screening tool). The performance of individual 4AT test items was related to cognitive status, length of stay, new institutionalisation, mortality at 12 weeks and outcomes. The method used was a prospective, double-blind diagnostic test accuracy study in emergency departments or in acute general medical wards in three UK sites. Each patient underwent a reference standard delirium assessment and was also randomised to receive an assessment with either the 4AT (n = 421) or the CAM (n = 420). A health economics analysis was also conducted.

Results: Phase 1 found evidence that delirium awareness is increasing, but also that there is a need for education on delirium in general and on the 4AT in particular. Most users reported that the 4AT was useful, and it was in widespread use both in the UK and beyond. No changes to the 4AT were considered necessary. Phase 2 involved 785 individuals who had data for analysis; their mean age was 81.4 (standard deviation 6.4) years, 45% were male, 99% were white and 9% had a known dementia diagnosis. The 4AT (n = 392) had an area under the receiver operating characteristic curve of 0.90. A positive 4AT score (> 3) had a specificity of 95% [95% confidence interval (CI) 92% to 97%] and a sensitivity of 76% (95% CI 61% to 87%) for reference standard delirium. The CAM (n = 382) had a specificity of 100% (95% CI 98% to 100%) and a sensitivity of 40% (95% CI 26% to 57%) in the subset of participants whom it was possible to assess using this. Patients with positive 4AT scores had longer lengths of stay (median 5 days, interquartile range 2.0-14.0 days) than did those with negative 4AT scores (median 2 days, interquartile range 1.0-6.0 days), and they had a higher 12-week mortality rate (16.1% and 9.2%, respectively). The estimated 12-week costs of an initial inpatient stay for patients with delirium were more than double the costs of an inpatient stay for patients without delirium (e.g. in Scotland, £7559, 95% CI £7362 to £7755, vs. £4215, 95% CI £4175 to £4254). The estimated cost of false-positive cases was £4653, of false-negative cases was £8956, and of a missed diagnosis was £2067.

Limitations: Patients were aged ≥ 70 years and were assessed soon after they were admitted, limiting generalisability. The treatment of patients in accordance with reference standard diagnosis limited the ability to assess comparative cost-effectiveness.

Conclusions: These findings support the use of the 4AT as a rapid delirium assessment instrument. The 4AT has acceptable diagnostic accuracy for acute older patients aged > 70 years.

Future work: Further research should address the real-world implementation of delirium assessment. The 4AT should be tested in other populations.

Trial registration: Current Controlled Trials ISRCTN53388093.

Funding: This project was funded by the National Institute for Health Research (NIHR) Health Technology Assessment programme and will be published in full in Health Technology Assessment; Vol. 23, No. 40. See the NIHR Journals Library website for further project information. The funder specified that any new delirium assessment tool should be compared against the CAM, but had no other role in the study design or conduct of the study.

Keywords: 4AT; ACUTE HOSPITALS; CAM; DELIRIUM; DEMENTIA; DIAGNOSTIC ACCURACY.