A method for predicting an effective dose of 5-hydroxy-1H-imidazole-4-carboxamide or a salt thereof or a hydrate thereof for a myelodysplastic syndrome patient, comprising: (a) a step of determining (i) amounts of xanthosine monophosphate in whole blood collected from the patient before the administration of 5-hydroxy-1H-imidazole-4-carboxamide or a salt thereof or a hydrate thereof, and (ii) amounts of xanthosine monophosphate in the whole blood collected from the patient after the administration of 5-hydroxy-1H-imidazole-4-carboxamide or a salt thereof or a hydrate thereof, or amounts of xanthosine monophosphate in the whole blood obtained by bringing the blood collected from the patient before the administration of 5-hydroxy-1H-imidazole-4-carboxamide or a salt thereof or a hydrate thereof into contact with 5-hydroxy-1H-imidazole-4-carboxamide or a salt thereof or a hydrate thereof; (b) a step of determining a variation of xanthosine monophosphate from the amounts of xanthosine monophosphate obtained in step (a); and (c) a step of predicting an effective dose of 5-hydroxy-1H-imidazole-4-carboxamide or a salt thereof or a hydrate thereof from the variation of xanthosine monophosphate obtained in step (b), wherein step (a) includes the following steps: a step of determining xanthosine monophosphate in whole blood in two different determining conditions by mass spectrometry; a step of determining an ionic strength ratio of xanthosine monophosphate in whole blood in two different determining conditions obtained in step (al); and a step of checking whether the ionic strength ratio of xanthosine monophosphate in whole blood obtained in step is less than 1, wherein the two different determining conditions are detection ions and/or ionization conditions in mass spectrometry, wherein the ionic strength ratio is calculated as follows: ionic strength ratio = strength of ions under the other determining condition / strength of product ion of 97.0 m / z, when 365.0 m / z is taken as a precursor ion. (a) a step of determining (i) amounts of xanthosine monophosphate in whole blood collected from the patient before the administration of 5-hydroxy-1H-imidazole-4-carboxamide or a salt thereof or a hydrate thereof, and (ii) amounts of xanthosine monophosphate in the whole blood collected from the patient after the administration of 5-hydroxy-1H-imidazole-4-carboxamide or a salt thereof or a hydrate thereof, or amounts of xanthosine monophosphate in the whole blood obtained by bringing the blood collected from the patient before the administration of 5-hydroxy-1H-imidazole-4-carboxamide or a salt thereof or a hydrate thereof into contact with 5-hydroxy-1H-imidazole-4-carboxamide or a salt thereof or a hydrate thereof; (b) a step of determining a variation of xanthosine monophosphate from the amounts of xanthosine monophosphate obtained in step (a); and (c) a step of predicting an effective dose of 5-hydroxy-1H-imidazole-4-carboxamide or a salt thereof or a hydrate thereof from the variation of xanthosine monophosphate obtained in step (b), a step of determining xanthosine monophosphate in whole blood in two different determining conditions by mass spectrometry; a step of determining an ionic strength ratio of xanthosine monophosphate in whole blood in two different determining conditions obtained in step (al); and a step of checking whether the ionic strength ratio of xanthosine monophosphate in whole blood obtained in step is less than 1,