Differentiating Incident from Recurrent Stroke Using Administrative Data: The Impact of Varying Lengths of Look-Back Periods on the Risk of Misclassification.

BACKGROUND/AIMS: Administrative data are widely used to monitor epidemiological trends in stroke and outcomes; yet there is scant empirical guidance on how to best differentiate incident from recurrent stroke.
METHODS: We identified all hospital admissions in New South Wales, Australia, with a principal stroke diagnosis from July 1, 2013 to June 30, 2014, linked to 12 years of previous admissions. We calculated the proportion of cases identified with a prior stroke to determine the number of years of look-back required to minimise misclassification of incident and recurrent strokes.
RESULTS: Using the maximum available look-back period of 12 years, 1,171 out of 8,364 eligible stroke cases (14.0%) had a stroke history. A 1-year look-back period identified only 25.1% of these patients and 1 in 10 stroke cases were misclassified as incident. With a 10-year clearance period, less than 1 in 100 stroke cases were misclassified as incident. The risk of misclassification was lower in patients younger than 65 years and in those with haemorrhagic stroke.
CONCLUSION: Hospital administrative data sets linked to prior admissions can be used to distinguish recurrent from incident stroke. The risk of misclassifying recurrent stroke cases as incident events is negligible with a look-back period of 10 years.