Informant accuracy of IQCODE, AD8 and GPCOGi for diagnosis of dementia: does your friend know best?

Increasing numbers of people require evaluation for possible dementia. However, research on the accuracy of informant questionnaires in primary care remains limited.

This study assessed the diagnostic accuracy of IQCODE, AD8, and GPCOGi based on the informant's relationship to the patient. We recruited 240 participants from 21 general practices in South West England. The reference standard for a diagnosis of dementia was made by a specialist clinician using ICD-10 criteria. A threshold of greater than 3.3 on IQCODE, greater or equal to 2 on AD8 and less than 5 on the informant component of GPCOG was used to indicate an abnormal test.

Of 238 participants with informant data, 131 had dementia, 60 had CIND, and 47 had normal cognition. Median informant age was 70 years (IQR 60 years to 78 years). 71% of informants were female and 56% were spouses. On all three questionnaires, compared to spouses, adult descendants tended to score participants more cognitively impaired, whereas friends scored participants less cognitively impaired. However, there was little evidence of difference by informant type once fully adjusted. Sensitivity by informant type ranged from 91 to 100% for IQCODE, 94-100% for AD8 and 99% to100% for GPCOGi. There was no significant difference in sensitivity by informant type. Specificity by informant type ranged from 25 to 79% for IQCODE, 13-75% for AD8 and 17-38% for GPCOGi. Adult descendants tended to have the lowest specificity at 25% (95% CI 10-47%) for IQCODE, 13% (95% CI 3-32%) for AD8 and 17% (95% CI 5-37%) for GPCOGi. Friends tended to have the highest specificity at 79% (95% CI 49-95%) for IQCODE, 75% (95% CI 48-93%) for AD8 and 38% (95% CI 15-64%) for GPCOGi.

An informant of any relationship type, using IQCODE, AD8 or GPCOGi may be useful for ruling out dementia but not for ruling it in. We found no evidence of difference between spouse or adult descendants but friends performed significantly better overall on IQCODE and AD8.