  It is well known that, Tan's contact could be calculated by using any of
following three methods: by the asymptotic behavior of momentum distribution;
by Tan's adiabatic sweep theorem; or by the operator product expansion as an
expectation value of the interaction term. We argue that, if a theory
describing Bose (or Fermi) system with the only contact interaction is self
consistent, then it should lead to the same result in all three cases. As an
example we considered MFT based approaches and established that among existing
approximations of MFT, the Hartree - Fock - Bogoliubov (HFB) approach is the
most self consistent. Actually, HFB is able to describe existing experimental
data on Tan's contact for dilute Bose gas, but fails to predict its expected
behavior at large gas parameter $(\gamma > 0.015)$. So, for appropriate
description of properties of a Bose gas even at zero temperature, this
approximation needs to be expanded by taking into account fluctuations in
higher order then the second one.
