Landau-de Gennes (LdeG) theory and a microscopic model of interacting quadrupolar tensors, also known as the dispersion model, are used to analyze the effect of an external field on a nematogenic liquid crystal material having the zero field phase sequence biaxial nematic-uniaxial nematic-isotropic liquid as temperature increases. Although both approaches give qualitatively the same results the dispersion model does not reproduce all features of the phenomenological phase diagrams. For example, the biaxial-uniaxial transition for positive uniaxial anisotropic susceptibility is found to shift with field to either higher or lower temperature, depending on the values of the coefficients in the LdeG expansion, while the molecular model shows only the first type of behavior.