Transferrin isoform analysis is an established laboratory test for congenital disorders of glycosylation (CDG). Despite its long history of clinical use, little has been published about its empirical sensitivity for specific conditions. We conducted a retrospective analysis of ten years of testing data and report our experience with transferrin testing for type I profiles and its sensitivity for the most common congenital disorder of glycosylation, PMM2-CDG. The data demonstrate 94% overall test sensitivity for PMM2-CDG and importantly demonstrate two known, recurrent variants enriched in false positive cases highlighting an important limitation of the test. The data confirm the clinical validity of transferrin isotype analysis as a screening test for disorders of protein N-linked glycosylation and as functional test for PMM2 genotypes of uncertain significance.