IGHE and asthma: The available epidemiological evidence suggests that the population-based proportion of asthma cases that are attributable to atopy is usually less than one-half.[19] Higher estimates (up to two-thirds) can be obtained by using very low cut-off levels of total serum IgE, but these should be interpreted with caution since such a definition of atopy has a limited practical use, and these associations may not always be causal.