MicroRNA-122 (miR-122) is a sensitive biomarker for paracetamol-induced liver injury. This study assessed the diagnostic performance of dynamic chemical labelling (DCL) combined with an immunoassay platform to quantify miR-122 in human serum, as a proof-of-concept for hospital use. This study used serum samples from 19 healthy individuals and 19 patients with paracetamol drug-induced liver injury (DILI) collected in previous studies. miR-122 was labelled with a biotinylated SMART-Base and quantified using a multiplex, bead-based, fluorescence detection system. The median miR-122 concentration was significantly elevated at 233.4 pg/mL (IQR: 74.5–363.4) compared to 15.2 pg/mL (IQR: 9.4–24) in the control group. The Mann–Whitney U test showed significant group differences (p < 0.0001). ROC analysis yielded an area under the curve (AUC) of 0.96, with a 29.7 pg/mL cut-off providing 92.6% sensitivity (95% CI: 76.6%–98.7%) and 89.4% specificity (95% CI: 68.6%–98.1%). miR-122 moderately correlated with ALT (r = 0.56, p = 0.03). DCL with fluorescence detection is a promising clinical tool for miR-122 quantification.