False-Positive Newborn Screen Using the Beutler Spot Assay for Galactosemia in Glucose-6-Phosphate Dehydrogenase Deficiency.

Classical galactosemia is detected through newborn screening by measuring galactose-1-phosphate uridylyltransferase (GALT) in the USA primarily via the Beutler spot assay. We report on an 18-month-old patient with glucose-6-phosphate dehydrogenase (G6PD) deficiency that was originally diagnosed with classical galactosemia. The patient presented with elevated liver function enzymes and bilirubinemia and was immediately treated with soy-based formula. Confirmatory tests revealed deficiency of the GALT enzyme, however, full-sequencing of GALT was normal, suggestive of a different ideology. The Beutler spot assay uses three other enzymatic steps in addition to GALT. A deficiency in either of these enzymes can result in suspected decreased GALT activity when using the Beutler assay. Congenital Disorders of Glycosylation screening for phosphoglucomutase-1 deficiency was negative. Quantitative analysis of G6PD enzyme in red blood cells showed a severe deficiency and a deletion in G6PD. Soy-formula, the standard treatment for galactosemia, has been reported to trigger hemolysis in G6PD deficient patients. G6PD and phosphoglucomutase-1 deficiencies should be considered when confirmatory tests are negative for pathogenic variants in GALT and galactose-1-phosphate level is normal.