The Informant Questionnaire is one of the methods used for the clinical assessment of cognitive impairments. Among various questionnaires, the IQCODE is one of the most widely used methods [
12]. As the IQCODE is a reliable and validated informant-based instrument for assessing changes in everyday cognitive dysfunction over a 10-year period, it has been widely adopted by clinical researchers across different cultures and languages [
8,
24]. In general, informant-based assessments do not rely on direct patient testing, thus making capturing changes over time possible and operable and making these assessments less prone to social-cultural biases [
25,
26]. However, having an informant who has known the older individual in question for at least 10 years respond to the questions is a key requirement for the IQCODE [
12]. The IQCODE has performed at least as well as cognitive screening tests, such as the MMSE. Furthermore, an important advantage of the IQCODE over the MMSE lies in that it is not affected by the patient’s premorbid intelligence or education [
27]. These properties are important for obtaining acceptable diagnostic accuracy, which is required for screening tools [
28]. Moreover, from a clinician’s perspective, the strengths of the IQCODE are that it is copyright free, available in different languages, and relatively easy to complete. From the original version with 26 questions that was developed in 1989, the short 16-item IQCODE version was developed in 1994, and its performance was essentially identical to that of the original version [
16,
29]. There have been many studies supporting the validity of the IQCODE for dementia screening, with a sensitivity ranging from 79 to 100% and a specificity ranging from 68 to 100% [
16,
30,
31]. However, no studies evaluating the efficacy of IQCODE for assessing the severity of cognitive impairments in patients with AD have been published.
This study was based on the 16-item version, and the cognitive changes were scored on a 5-point scale. Higher scores corresponded to greater cognitive decline. The ratings were averaged, composing a mean score between 1 and 5. The severity of cognitive impairments of the total of 394 AD patients was assessed by the CDR and the short IQCODE and was tested by the MMSE, DRS and ADAS-Cog. These patients were classified into mild, moderate and severe dementia groups according to the severity of dementia. Our results demonstrated that in the moderate group, there was a statistically significant association between the IQCODE scores and the MMSE, DRS and ADAS-Cog scores. However, no statistical significance was observed in the mild and severe groups. These findings suggest that IQCODE is more accurate and efficient for assessing moderate dementia than for assessing mild or severe dementia.
Significant difference in each item in the 16-item IQCODE were found in the mild and moderate groups but not in the severe group. ROC curve analyses for the IQCODE discriminating between mild-moderate dementia showed an AUC of 0.666. The sensitivity was 66.1%, and the specificity was 59.8% with 65 used as the cut-off point. The AUC under the ROC curve for the IQCODE discriminating between moderate-severe dementia was 0.768; the sensitivity was 73.9%, and the specificity was 67.7% when using 75 as the cut-off score. Our results revealed that the sensitivity and specificity of the IQCODE were not high enough. Hence, we proposed that there might be a relatively large deviation when using the IQCODE alone to assess the severity of cognitive impairments. Furthermore, the IQCODE is unsatisfactory for evaluating conditions or treatment effects of patients with AD. Collectively, we do not advocate the use of the IQCODE alone for assessing treatment efficacy. A combination of the IQCODE with objective cognitive function tests and evaluation of patients’ behavioural and psychiatric symptoms is required to provide the basis for further treatment.