Repeat measurement of case-control data: corrections for measurement error in a study of ischaemic stroke and haemostatic factors.

Haemostatic factors are suspected to be involved in the aetiology of cerebrovascular events. 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. 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. 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.