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Twin study using mortality data: a new sampling method.

BACKGROUND
To evaluate the success of a novel approach to twin studies using death discordant twin pairs in a disease of low prevalence.


METHOD
A population study based on all registered deaths at the Office of Population Censuses and Surveys of England and Wales, classified under the ICD code 335.2 (motor neuron disease [MND]) for the period 1979-1989 inclusive. From the above database of 10,872 people, individuals born after 31 December 1899 were traced in the Birth Indices for England & Wales to enable identification of possible twins. In all 131 twin pairs were found and the co-twin details were sent to the National Health Service Central Register (NHS-CR) to enable location of the relevant Family Health Services Authority and thence the co-twin's general practitioner (GP). A letter requesting access was sent to the family practitioners. If given, the co-twin can be approached and interviewed.


RESULTS
The search produced: 54 living co-twins; 31 who died as adults; 29 infant deaths; 5 emigrated; 3 incorrectly diagnosed index twins; and 9 untraceable co-twins. Among the adult deaths two concordant pairs were identified. This has created the largest twin population sample worldwide for MND.


CONCLUSION
This new twin study method is clearly viable, and has produced a large unbiased sample compared to that possible using traditional methods. It relies heavily on the accuracy of death certificates and zygosity reporting by living co-twins, but is possibly the only way of collecting twins in rare conditions.