  Fluctuations of conserved charges such as baryon number, electric charge and
strangeness may provide a test for completeness of states in lattice QCD for
three light flavors. We elaborate on the idea that the corresponding
susceptibilities can be saturated with excited baryonic states with an
underlying quark-diquark structure with a linearly confining interaction. Using
Polyakov-loop correlators we show that in the static limit, the quark-diquark
potential coincides with the quark-antiquark potential in marked agreement with
recent lattice studies. We thus study in a quark-diquark model the baryonic
fluctuations of electric charge, baryon number and strangeness: $\chi_{BQ}$,
$\chi_{BB}$ and $\chi_{BS}$; by considering a realization of the hadron
resonance gas model in the light flavor sector of QCD. These results have been
obtained by using the baryon spectrum computed within a relativistic
quark-diquark model, leading to an overall good agreement with the spectrum
obtained with other quark models and with lattice data for the fluctuations.
