The visualization of Golgi glycosylation defects in patients' cells with Congenital Disorders of Glycosylation (CDG) is challenging and necessitates the use of cumbersome glycan analysis methods that are barely adapted to clinical research. We show here that metabolic labelling of patient cells with alkyne-tagged sialic-acid (SiaNAl) enables an easy and reliable readout assay for the detection of CDG occurrence. It also provides valuable clues regarding the pathological processes by assessing the distribution of sialic acid analogues within the cells.