International Journal of Epidemiology, Vol 26, 64-70, Copyright © 1997 by International Epidemiological Association
SA Bashir, SW Duffy and N Qizilbash
BACKGROUND: Haemostatic factors are suspected to be involved in the
aetiology of cerebrovascular events. METHODS: In a case-control study of
105 cases of transient ischaemic attack and minor ischaemic stroke, and 241
controls, data were available on levels of the haemostatic factors-von
Willebrand factor (vWF), plasminogen activator inhibitor-1 (PAI), tissue
plasminogen activator (TPA) and factor VII (FVII). These are subject to
measurement error and within-person fluctuation of true levels, which may
bias relative risk estimates. For all subjects, two determinations were
performed on the same blood sample, which allowed estimation of pure
measurement error. For estimation of within-person fluctuation, levels were
measured from a repeat blood sample on 81 of the controls one year later.
RESULTS: The pure measurement error accounted for a very small proportion
of the total variation in all cases. Uncorrected for within-person
fluctuation, the odds ratio estimates associated with exceeding the median
of vWF, PAI, TPA and FVII respectively were 1.88, 0.87, 1.30 and 0.93.
After correction for within-person fluctuation odds ratios were 3.56, 0.80,
1.41 and 0.91. Because the PAI determination was not robust to storage
conditions, it was estimated that 75% of the variation in this factor was
within- person rather than between-persons. Thus, estimates of relative
risk relation to PAI cannot be regarded as reliable in this study.
CONCLUSIONS: It is likely that elevated levels of vWF are associated with
increased risk of ischaemic stroke, but interpretation must be tentative,
due to relatively large within-person fluctuation of vWF levels.
ARTICLES
Repeat measurement of case-control data: corrections for measurement error in a study of ischaemic stroke and haemostatic factors
MRC Biostatistics Unit, Institute of Public Health, University Forvie Site, Cambridge, UK.
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