  The Callan-Treiman low-energy theorem offers an opportunity to test
electroweak couplings of light quarks to the gauge boson W. To that aim, we
introduce a model-independent and accurate dispersive parametrization of the
two K_{l3} form factors. We then discuss three applications to the analysis of
K_{e3} and K_{mu3} measurements: the prediction of the ratios
Gamma(K_{mu3})/Gamma(K_{e3}), the extraction of |f_+(0)V_{us}| and finally the
possible measurement of m_u - m_d induced isospin breaking asymmetry.
