  Kaonic hydrogen is studied with various realistic potentials in an accurate
numerical approach based on Sturmian functions. The kaon-proton scattering
length extracted from the 1s energy shift of the kaonic hydrogen by applying
the Deser-Trueman formula is severely inconsistent with the one derived by
directly solving the scattering Schoedinger equation. We pay special attention
to the recent measurement of the energy shift and decay width of the 1s kaonic
hydrogen state by the DEAR Collaboration. After taking into account the large
discrepancy between the extracted and directly-evaluated scattering lengths, we
found theoretical predictions of most chiral SU(3) based models for the kaonic
hydrogen decay width are consistent with the DEAR data. We warn the SIDDHARTA
collaboration that it may not be reasonable to extract kaon-nucleon scattering
lengths, by using the Coulomb-interaction corrected Deser-Truemab formula, from
the planned measurement of kaonic hydrogen.
