Middle cerebral artery flow velocity decreases with age. When studying subjects with transcranial Doppler, these age-related changes should be taken into account. A recent paper by dr. A. Schaafsma promotes Z-scores as a way to correct for age. Z-scores express the distance in standard deviations to the middle cerebral artery flow velocity expected for age by linear regression.

A Z-score expresses how many standard deviations a given measurement is above (positive) or below (negative) the value expected for a given age. Z-scores for different parameters can be combined into a radar plot. An example is give to the right: data are provided for average Z-scores during hyperventilation (red), normoventilation (green) and CO2-retention (yellow).

A. Schaafsma, A new method for correcting middle cerebral artery flow velocity for age by calculating Z-scores. *Journal of Neuroscience Methods*. **307**, 1–7 (2018).