Hello Brent,
Regarding a case-control approach, the problem always comes when selecting
the controls. The controls are patients without the outcome (in this case
Colonic dysplasia ) yet who have had the same chance as the cases of
receiving the exposure (Ursodiol- ursodeoxycholic acid - English
translation!). The problem in your study, I think, would be getting an
adequate number of controls. Normally, the ratio of controls:cases is 4:1 in
order to achieve maximum power. From the numbers quoted, you're more likely
to get a 1:1 ratio (with a much lower power).
Another concern is the ascertainment of colonic dysplasia. How accurately is
this done in practice and is the test reliable i.e. would a group of
pathologists come up with the same results when repeatedly given the same
sample?
My major concern, however, is (1) the length of time cases and controls have
been exposed to Ursodiol and (2) the length of time patients had suffered
from Ulcerative Colitis. Could these factors be a major confounder? I wonder
if a method employing survival analysis might be more appropriate?
John
Mr John P. Hampson
Teacher Practitioner Pharmacist
Gwenfro Academic Unit
Croesnewydd Road
Wrexham LL13 7YP
United Kingdom
Tel. 01978 727407 or
01978 291100 bleep 5881
Fax 01978 727167
E Mail [log in to unmask]
> ----------
> From: Beasley , Brent[SMTP:[log in to unmask]]
> Reply To: Beasley , Brent
> Sent: 19 February 2001 22:52
> To: [log in to unmask]
> Subject: Cross sectional studies?
>
> RE: Annals of Intern Med Jan 16th, 2001, vol 134(2):88-95.
>
> Hello Colleagues,
> The study is cross-sectional. 59 pts with Ulc Colitis who get
> surveillance
> colonoscopies. Analysis was done based upon dividing this group into
> those
> with vs. without use of Ursodiol (41/59). Use of Ursodiol was assessed 1)
> by chart records and 2) by direct patient contact if not found in records.
> Association of interest was Colonic dysplasia assessed by biopsy (26/59).
> Ursodiol was strongly associated with decreased prevalence of colonic
> dyplasia (32% vs. 72%, OR=0.18).
>
> Questions:
> It seems this study could have been done in a case-control format,
> dividing
> groups on the basis of dysplasia.
> 1. What would have been the BEST methodology?
> 2. Which methodology would be most conservative/least biased?
> 3.Does it matter?
>
> Brent W. Beasley, M.D.
> St. Luke's Hospital, Dept. of Medical Education
> 4401 Wornall Rd
> Kansas City, MO 64111
> (816) 932-3409
> Fax: (816) 932-5179
> Pager: (816) 440-3968
> [log in to unmask]
>
> "When men have come to the edge of a precipice,
> it is the lover of life who has the spirit to leap backwards,
> and only the pessimist who continues to believe in progress."
> --G. K. Chesterton
>
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