  This paper discusses some features of the spectral line profile theory used
in the treatment of measured atomic transitions. It is shown that going beyond
the established linear approximation for the spectral line contour in the case
of its nonresonant extension, the potential for a more accurate extraction of
atomic characteristics from experimental data arises. Using the example of the
Lyman-$\alpha$ (Ly$_\alpha$) transition in hydrogen, a simple analysis of the
observed spectral line distorted by a possible interfering transitions is
given. In particular, the results obtained in the present work clearly
demonstrate that the processing of the same experimental data at different
settings can provide an accurate determination of the transition frequency, the
centre of gravity as well as the hyperfine splitting of the ground state in
hydrogen-like atomic systems. The latter is especially important for setting up
precision spectroscopic experiments on the antihydrogen atom.
