A methodological fallacy?
The Brown et al (1) study has generated intense discussion, not only
by clinicians, but very appropriately, by lay people as well. The study
was designed well in most repsects; some of the shortcomings have already
been highlighted in the rapid responses so far.
I would like to highlight another fallacy which has not yet been
commented upon. This study was conducted in a memory clinic environment in
Cambridge on patients suffering from dementia, mostly of Alzheimer’s type
as per NINCDS-ARDRA criteria. The authors mention that patients with
depression were excluded, presumably based on a diagnostic assessment by
the psychiatrist. I would like to comment on the validity of the screening
test, as an important subgroup of patients has been excluded. Most memory
clinics would have a substantial number of patients who present with
cognitive impairment due to depression rather than dementia. It is vitally
important to know how these patients fared on the TYM test as the
specificity of the test to delineate dementia from depression would be
crucial. The authors sadly have not furnished this data/have not attempted
to explore this in the paper.
Professor Black has already commented on the high number of possible false
positives in patients who underwent the TYM. If there is a high false
positive rate in patients with depression as well, it will further reduce
the validity of the TYM test.
The authors also quote that they have been able to demonstrate very
high sensitivity and specificity of the TYM test, 93% and 86%
respectively. However, studies using other cognitive screening tests eg.
the modified Danish version of the ACE have found similar, if not better,
sensitivity and specificity (99% and 94% respectively) (2). The above
paper(2) did though mention that specificity dropped to 64% in patients
with depression. This again highlights the need for a screening test to
differentiate between depression and dementia, which is as yet not readily
1.Brown, J et al. Self administered cognitive screening test (TYM)
for detection of Alzheimer’s Disease: cross sectional study. BMJ
2. Validation of the Danish Addenbrooke's Cognitive Examination as a
screening test in a memory clinic. Stokholm J, Vogel A, Johannsen P,
Waldemar G. Dement Geriatr Cogn Disord. 2009;27(4):361-5. Epub 2009 Mar
Competing interests: No competing interests