Background:Galactose metabolites, like galactitol and galactose-1-phosphate (G-1-P), can accumulate in galactosemia patients on treatment, however it is yet unknown how these metabolites relate to the clinical outcome. Objectives:The aim of the current work was to measure red blood cells (RBCs) G-1-P and galactitol in healthy and galactosemic infants and children receiving treatment and to demonstrate their effects on clinical outcomes. Patients and methods: The current study included 20 galactosemia patients, 30 patients suspected for galactosemia and 40 healthy controls of matched age and gender. Complete history taking, physical and ophthalmological examination, and abdominal ultrasound were performed. Liver function tests and hemoglobin concentration were tested. RBCs G-1-P and galactitol were quantified using a novel Ultraperformance Liquid Chromatography- Tandem Mass Spectrometry method. Results:Treated galactosemia patients showed significant increase in RBCs G-1-P and galactitol in comparison with healthy controls (p < 0.001). Significant age-related decrease of these metabolites was found. RBCs galactitol was significantly correlated with albumin and ALT. Significant increase of both metabolites was found in patients with hepatomegaly. No significant correlations between these metabolites and Z score of weight and height were found. In suspected patients' group, RBCs galactitol showed significant increase in comparison with age comparable controls. Two cases from the suspected cases (7%) diagnosed classic galactosemia showed highly elevated levels of RBCs G-1-P and galactitol despite starting a galactose-free diet a few days ago. Conclusion:This study's findings imply that galactose metabolic status can be evaluated in galactosemic patients using RBCs galactitol and G-1-P and these can be used to assess liver functions.