A Predictive Model for Delirium in Hospitalized Elderly Medical Patients Based on Admission Characteristics

Delirium, defined as an acute disorder of attention and cognition, occurs in 14% to 56% of elderly, hospitalized patients [1-16] and may be the most frequent complication of hospitalization in this population [6]. Delirium is associated with increased rates of morbidity, mortality, and nursing home placement and with longer, costlier hospitalizations [17-29]. Effective prevention requires the identification of risk factors for delirium. Seven prospective studies have systematically examined risk factors for delirium in elderly, hospitalized patients [9, 12-14, 16, 30, 31]. The risk factors generally identified included age, dementia, severe illness, metabolic and electrolyte imbalance, the use of psychoactive medications, and infections. However, most studies failed to distinguish prevalent cases of delirium at admission from incident cases occurring during hospitalization, a necessary step to define risk factors that truly precede the onset of delirium. In addition, only one of the previous studies [16] used a standardized, validated instrument to identify delirium. Finally, no study proposed a predictive model for delirium that was validated in an independent sample. Previous studies focused on identifying precipitating factors for delirium (hospitalization-related factors such as medications, procedures, and intercurrent infections) or have simultaneously examined precipitating and predisposing conditions [9, 12-14, 16, 30, 31]. Although identifying these precipitating factors is undoubtedly important, an essential, prerequisite step is elucidation of baseline (or host) characteristics that indicate a particular vulnerability to delirium. The focus of our study was to better define host or baseline vulnerability factors that would assist clinicians in identifying, at admission, those patients with a high risk for developing delirium during hospitalization. Our objectives were to estimate the incidence of delirium in an elderly hospitalized cohort using a standardized, validated instrument; to identify admission characteristics associated with the new occurrence of delirium; and to develop and validate a predictive model for the occurrence of delirium in the hospital based on the characteristics present at admission. Methods Patients The potential study patients were consecutive weekday admissions from the emergency service to six general medicine floors at Yale-New Haven Hospital from June 1988 through March 1989. Eligibility criteria, which were met by 135 patients, included age of 70 years or more; no evidence of delirium at admission by clinical evaluation; and no history of severe dementia reported by the admitting attending physician or nurse or documented in the medical record. The latter criterion was selected to exclude patients with severe underlying dementia; patients with mild-to-moderate cognitive impairment were still eligible for the study. Exclusion criteria included an inability to undergo interviewing for such reasons as terminal illness, violent behavior, or intubation (n = 14); brief hospitalization (< 48 hours) (n = 5); language barrier (n = 2); refusal by the patient, family, or physician (n = 3); and other reasons (n = 4). Of the 135 eligible patients, 107 (79%) were enrolled in the study. Clinical Evaluation Trained clinician-researchers carried out daily structured interviews with the patients and their primary nurses from entry until hospital discharge. The baseline patient interview, completed within 48 hours of admission, was designed to gather demographic information; to discover whether the patient had a history of confusion; and to determine the patient's pre-illness social activity level, activities of daily living [32], instrumental activities of daily living [33], and social network and supports [34]. The interview also included the Mini-Mental State Examination [35], the Geriatric Depression Scale [36], standard vision (Jaeger- and Snellen-type tests) and hearing tests (Welch-Allyn audioscope), and questions designed to screen for hearing loss [37, 38]. Baseline information obtained from the nurses included an overall rating of illness severity [39]. Early in the hospital admission, family members or caregivers underwent a structured interview that included a modified Blessed Dementia Rating Scale [40, 41] and an estimation of the duration of any cognitive impairment. Thereafter, the clinician-researchers interviewed the patients and their nurses daily and reviewed medical records to detect any new cases of delirium. The discharge assessment included determination of discharge location. Medical records were reviewed for admission medical diagnoses (inclusive list based on review of all admission notes), medications, and laboratory data. Informed consent was obtained from the patient or, for those with significant cognitive impairment, from the closest relative. The study was approved by the institutional review board of Yale University School of Medicine. Identification of Delirium Suspected cases of delirium were identified during the daily patient and nurse interviews. The Confusion Assessment Method [42] questionnaire, a previously validated instrument with a sensitivity rate ranging from 94% to 100% and a specificity rate ranging from 90% to 95%, was completed daily by two observers (a clinician-researcher and a nurse). The principal investigator (SKI) saw each patient with suspected delirium within 24 hours and established all final diagnoses of delirium based on fulfillment of the Confusion Assessment Method diagnostic criteria, which included the following: acute onset and fluctuating course; inattention; and either disorganized thinking or altered level of consciousness [42]. A case of delirium developing at any time during hospitalization was included in the analysis; however, a given patient could only develop delirium once (recurrent episodes of delirium were not counted when determining the relative-risk estimates). Definition of Variables We used clinically meaningful cutpoints to categorize variables. Patients were considered to have vision impairment if their corrected vision was worse than 20/70 on both near and distant binocular tests. Patients were considered to have hearing loss if they heard fewer than three of eight tones on the audioscope test (tones at a hearing level of 40 dB and at frequencies of 500, 1000, 2000, and 4000 Hz), had a score of 4 of 8 or less on the questions screening for hearing loss [37, 38], or wore a hearing aid. Patients were considered to have chronic cognitive impairment if they scored less than 24 of 30 on the Mini-Mental State Examination at admission [35]. The chronicity of the cognitive impairment in each patient with a score of less than 24 was confirmed by a modified Blessed Dementia Rating Scale score of 4 or more or by a duration of cognitive symptoms of at least 6 months. Illness severity was rated at admission using an APACHE II score [43] and a subjective overall rating by the nurse (adapted from Charlson and colleagues' observer-rated ordinal scale [39]) trichotomized as mild, moderate, or severe. Severe illness was considered present if the nurse gave a rating of severe or the APACHE score exceeded 16. An abnormal blood urea nitrogen/creatinine ratio, used as an index of dehydration, was defined as 18 or more [4]. Prominent depressive symptoms were considered present if the Geriatric Depression Scale score was equal to or exceeded the median score of 8 of 30. The size of a patient's social support network was estimated by the sum of the number of children, close relatives, and friends he or she saw at least once a month. A low number of social supports was indicated by six or fewer such contacts per month. Three social support typesinstrumental support, emotional support, and presence of a confidantewere rated as present or absent. Having a few support types was indicated by the presence of one or none of these support types. Validation Study To validate the predictive model in an independent sample, a second cohort of patients was assembled. Patients Potential participants in the validation study included 202 patients who were enrolled from November 1989 through June 1990 in another ongoing study examining the frequency of functional decline in hospitalized elderly persons at Yale-New Haven Hospital. The inclusion and exclusion criteria for this study were identical to those for the study in the development cohort, except that patients with preexisting dementia were not excluded. For comparability with the initial development cohort, therefore, 25 patients (12%) with a documented history of dementia, Alzheimer disease, or chronic organic brain syndrome in their medical record were not considered eligible for the validation sample. Of the 177 eligible patients, 174 (98%) were included in the validation sample; 3 patients were excluded because missing data did not allow assessment of the outcome of delirium. Procedure The clinical evaluation and identification of patients with delirium were similar in the two cohorts. In all but three instances, the same assessment instruments were used. Only near binocular vision (Jaeger-type test), not distant vision, was tested in the validation cohort. For assessment of hearing, the Whisper test [44] was used. Hearing impairment was defined as present if the patient heard correctly fewer than 7 of 12 numbers on the Whisper test or wore a hearing aid. A shortened 15-item version of the Geriatric Depression Scale [45] was used. Significant depressive symptoms were considered present if the patient had a score that was equal to or exceeded the median of 4 of 15. Statistical Analyses In bivariate analyses, rates of delirium were calculated when each risk factor was present or absent. Crude relative risks were determined as the number of events (delirium) per person-days of observation in the group with the risk factor present relative to the number of such events per person-days of