An object of the present invention is to provide a method and apparatus for predicting an effective dose of or the sensitivity to 5-hydroxy-1H-imidazole-4-carboxamide, which are capable of performing a determination in a simple operation and a short time, a method for determining amounts of xanthosine monophosphate, and a treatment agent and treatment method for treating myelodysplastic syndrome. According to the present invention, provided are a method for predicting an effective dose of or the sensitivity to 5-hydroxy-1H-imidazole-4-carboxamide, including determining amounts of xanthosine monophosphate in blood and a prediction apparatus, a method for determining amounts of xanthosine monophosphate, including determining xanthosine monophosphate in blood in two different determining conditions by mass spectrometry, and a treatment agent and treatment method for treating myelodysplastic syndrome.
