The a-index samples the flux of the 5200 Å region by comparing the flux at the center with the adjacent regions. The final intrinsic peculiarity index ∆a was defined as the difference between the individual a-values and the a-values of normal stars of the same colour (spectral type). Here we present, for the first time, a case study to detect and analyse Asymptotic Giant Branch (AGB) stars in the Magellanic Clouds. For this, we use our photometric survey of the Magellanic Clouds within the a-index. We find that AGB stars can be easily detected on the basis of their ∆a index in an efficient way.