AbstractPurposeCongenital disorders of glycosylation (CDG) are one of the fastest growing groups of inborn errors of metabolism. Despite the availability of next‐generation sequencing techniques and advanced methods for evaluation of glycosylation, CDG screening mainly relies on the analysis of serum transferrin (Tf) by isoelectric focusing, HPLC or capillary electrophoresis. The main pitfall of this screening method is the presence of Tf protein variants within the general population. Although reports describe the role of Tf variants leading to falsely abnormal results, their significance in confounding diagnosis in patients with CDG has not been documented so far. Here, we describe two PMM2‐CDG cases, in which Tf variants complicated the diagnostic.Experimental DesignGlycosylation investigations included classical screening techniques (capillary electrophoresis, isoelectric focusing and HPLC of Tf) and various confirmation techniques (two‐dimensional electrophoresis, western blot, N‐glycome, UPLC‐FLR/QTOF MS with Rapifluor). Tf variants were highlighted following neuraminidase treatment. Sequencing of PMM2 was performed.ResultsIn both patients, Tf screening pointed to CDG‐II, while second‐line analyses pointed to CDG‐I. Tf variants were found in both patients, explaining these discrepancies. PMM2 causative variants were identified in both patients.Conclusion and Clinical RelevanceWe suggest that a neuraminidase treatment should be performed when a typical CDG Tf pattern is found upon initial screening analysis.