Rj. Davenport et al., EFFECT OF CORRECTING OUTCOME DATA FOR CASE-MIX - AN EXAMPLE FROM STROKE MEDICINE, BMJ. British medical journal, 312(7045), 1996, pp. 1503-1505
Objective-To show the influence of variations in case mix on clinical
outcome indicators for patients admitted to hospital with acute stroke
. Design-''Before and after'' cohort study, with prospective, consecut
ive identification of patients and prospective follow up; multiple log
istic regression analyses to correct for case mix variations. Setting-
University teaching hospital. Subjects-216 patients with stroke identi
fied before the introduction of an organised stroke service, and 252 p
atients with stroke identified after its introduction. Main outcome me
asures-Case fatality at 30 days and 12 months; for survivors at 12 mon
ths, proportions of patients who were independent (according to the Ox
ford handicap scale) and of those living at home. Results-Crude outcom
e data suggested that patients in the cohort identified after the intr
oduction of the stroke service were significantly more likely to be al
ive, independent, and living at home than patients managed before the
stroke service. After adjustment for age and sex these ''improvements'
' were less impressive but still significant. After adjustment for man
y other possible prognostic indicators, however, the differences betwe
en the two groups for all four outcomes were non-significant, suggesti
ng that the ''improvements'' may have been entirely due to differences
in case mix between the two cohorts, rather than the new stroke servi
ce. Conclusions-Variations in case mix have a crucial influence on the
interpretation of outcome data, and this is particularly important in
non-randomised comparative studies. Such studies, comparing performan
ce within and between different provider units, are likely to become i
ncreasingly common in the new reformed NHS. To allow meaningful interp
retation, these studies must try to correct for case mix.