In this prospective study, we identified three independent predictive factors for delirium: being 85 years old or older, being dependent in five or more ADL (of the six considered), and taking psychotropic drugs (antipsychotics, benzodiazepines, antidepressants, anticonvulsants and/or antidementia drugs). With these factors we developed a clinical prediction rule in which an individual risk score for delirium is obtained by adding one point for each of the factors present. Applying this rule, patients are classified as positive if they have a total score of 1 or more.
In the derivation cohort, 13% of patients developed delirium, while the incidence was somewhat higher, 25%, in the validation cohort. Patients were elderly (mean ages in the derivation and validation cohorts were 75.9±13.3 years and 76.8±
13.3 years, respectively), and there were slightly more women (52%). The mean length of hospital stay was 8±5.8 days and overall mortality was 5%. There is a significant difference in the ADL variables being those from the validation cohort more dependent than the derivation cohort. All the aforementioned variables explain the almost twofold discrepancy in the incidence of delirium between the two cohorts.
There are multiple factors for the development of delirium, the predisposing and triggering factors being well defined. The predisposing factors are mostly related to degenerative brain disease (dementia, arteriosclerosis, Parkinson's disease and depression).10
On the contrary, there is a diverse range of triggering factors, in particular, medication, the presence of infection, surgery, metabolic disorders and water–electrolyte imbalances, among others.13
In the present study, we have only explored variables that are readily available on admission, in order to use the predictive rule at that stage and be able to introduce preventative measures immediately in high-risk patients. These would include trying to avoid triggering factors for the development of delirium (such as changes of room/ward, unnecessary catheterisation, inadequate oral hydration and polypharmacy).
Interestingly, the factors found to be good predictors for the development of delirium in our study (age≥85, high level of dependence and being on psychotropic medication), to some extent, indirectly reflect the severity of the organic brain damage in patients with delirium.
Another predictive rule for delirium in this type of patients has been published21
but showed a significantly lower performance than which we obtained (AUC=0.66 (0.55 to 077) vs AUC=0.85 (0.80 to 0.90) with our rule). Further, in our opinion, it is also more difficult to apply than the rule we propose. The simplicity of the variables included in our rule makes data collection a feasible task for busy healthcare units.
Between 10% and 60% of patients admitted to hospital develop delirium, depending on the type of patient, the incidence in frail elderly patients being at the top of this range. In our study, it was 13% and 25% in the derivation and validations cohorts, respectively. Delirium is well known to be difficult to diagnose and a wide range of instruments have been developed to help detect the condition.26
We used the Confusion Assessment Method22
that has a sensitivity of 96% (95% CI 80% to 100%) and a specificity of 93% (95% CI 84% to 100%). In our study, the doctors in charge of the diagnosis of delirium were specialists in internal medicine, with considerable training and experience in the management of this type of patients, any differences being resolved by consensus with a third specialist. We note, however, that the diagnoses of delirium were not confirmed by a psychiatrist. This may partially explain the low incidence of delirium in our patients, that is, it may be that only the most clinically striking cases, those which required pharmacological treatment, were recognised.
The association between delirium and an increase of morbidity and mortality5
is well known, as are the effectiveness of preventive measures to avoid the development of the disease.15–18
The use of the proposed predictive rule would allow us to classify around half of our inpatients (53%) as high risk. Taking preventative measures in this high-risk group, up to 93.4% of those who developed delirium in our study would have been covered by the measures and might not have then developed the condition.
It would be interesting for the clinical predictive rule we propose to be validated in other cohorts of frail elderly patients with worsening of multiple medical conditions to check its external validity.