The purpose of this study was to investigate whether  differential phosphorylation states of blood markers can identify patients with LRRK2 Parkinson's disease (PD). We assessed phospho(P)‐Ser‐935‐LRRK2 and P‐Ser‐473‐AKT levels in peripheral blood cells from patients with G2019S LRRK2‐associated PD (L2PD, n = 31), G2019S LRRK2 non‐manifesting carriers (L2NMC, n = 26), idiopathic PD (iPD, n = 25), and controls (n = 40, total n = 122). We found no differences at P‐Ser‐935‐LRRK2 between groups but detected a specific increase of P‐Ser‐473‐AKT levels in all G2019S carriers, either L2PD or L2NMC, absent in iPD. Although insensitive to LRRK2 inhibition, our study identifies P‐Ser‐473‐AKT as an endogenous candidate biomarker for peripheral inflammation in G2019S carriers using accessible blood cells. ANN NEUROL 2022;92:888–894