Recall rate and positive predictive value of MSUD screening is not influenced by hydroxyproline

Maple syrup urine disease (MSUD) is an autosomal recessive inherited disorder caused by the deficiency of the branched-chain 2-oxo acid dehydrogenase (BCOA-DH) complex. The worldwide incidence is approximately 1 in 185,000. MSUD is integrated in many "expanded" newborn screening (NBS) programs that use electrospray ionization tandem mass spectrometry (ESI-MS/MS). Elevated leucine, isoleucine, and alloisoleucine in the dried blood samples (DBS) of newborns are diagnostic parameters. However, with the applied method, it is not possible to distinguish the amino acids from each other, and also not from the other isobaric amino acid, hydroxyproline. While the branched chain amino acids (BCAA) leucine, isoleucine, and alloisoleucine are no diagnostic problem, because they are all elevated in MSUD patients, and, rather, increase the diagnostic sensitivity and specificity, hydroxyproline may cause false-positive screening results. Hydroxyproline is elevated in the benign familial condition hyperhydroxyprolinemia, which needs no medical intervention. The detection of cases with hyperhydroxyprolinemia have formerly been reported from screening programs that used thin-layer chromatography for phenylketonuria (PKU) screening, and, recently, two more cases have been reported, detected by ESI-MS/MS-based NBS. However, the detection of non-diseases is a heavy burden for screening programs and should be avoided if possible. With optimal settings for the quantitation of BCAAs and interpretation rules, elevated hydroxyproline should not cause false-positive screening results.