Measures of Twin Concordance

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Abstract
The pairwise concordance rate indicates the proportion of pairs in which both members are affected, whereas the proband concordance rate states the proportion of affected individuals among co-twins of previously defined index cases. Concordant pairs in the population have a double chance of ascertainment compared with discordant pairs. Thus the pairwise concordance rate of the population, from which a sample is derived, is lower than the rate calculated from the directly observed proportions in the sample. In order to give an estimate of the morbid risk or lifetime expectancy of disease for MZ [monozyose] and DZ [dizyose] co-twins of affected persons, the crude proband concordance rate must be adjusted with regard to the age structure of the material. This risk can be compared with the risk figures for other groups of relatives or for the general population. Thus there is no doubt about the theoretical advantages of the proband concordance rate, especially with regard to its value as an estimate of a population parameters. Different ascertainment problems affecting one or both of these rates are discussed. It is stressed that in all descriptions of twin materials it should be specified, whether concordance as stated in pairwise or or probandwise, whether some pairs are doubly ascertained, and whether figures are raw or age-corrected.

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