AbstractCongenital disorders of glycosylation are a rapidly growing group of inherited (neuro)metabolic disorders characterized by defects in glycosylation of proteins and lipids. This study discusses an analytical problem in the differentiation between hypoglycosylation and transferrin (Tf) protein variants. Analysis of serum Tf by isoelectric focusing is used as a common method suitable for screening 19 out of a total of 22 types of glycosylation defects identified so far. In three members of a family, several indicators showed evidence of a Tf protein variant, however, routine neuraminidase‐based demonstration failed to confirm this result. On the assumption that we should be able to exclude Tf protein variants at the screening‐level of the diagnostic algorithm, our concern is a possible cause of our failure to confirm some of the Tf D variants (in contrast to the other C, B and D allelic combinations that are commonly well identified). Several explanations are discussed. J. Clin. Lab. Anal. 23:77–81, 2009. © 2009 Wiley‐Liss,Inc.