[en] When measuring a radioactive source composed of a radionuclide ^NX together with its metastable state ^NX^m decaying to the ground state, special attention should be given to the decay correction. Indeed the activity A of the radionuclide ^NX is the summation of the usual decaying activity A_D of the initial ^NX with the growing activity A_G of the ^NX coming from the disintegration of the metastable state to the ground state. If t₀ is the time of production of the source (which is often not known by the user), the usual decay correction A_D(t) = A_D,0 exp[-(t-t₀)/τ] allows an easy calculation of the correction D for any time difference Δt = t₂-t₁. Depending on the respective half-lives τ and τ_m of both states, the equilibrium (constant activity ratio) between ^NX^m and the daughter ^NX may or may not be reached. In this short report, it is demonstrated that, perhaps surprisingly, the decay correction for the total activity A = A_D + A_G is independent of t₀. No conditions on the respective half-lives of ^NX and ^NX^m are imposed. The decay correction for the total activity of ^NX is given by C(t₁, t₂, R(t₁)) for any time difference, i.e. whether equilibrium is reached or not. This expression is independent of the time of source production and is valid for any values of τ and τ_m