This invention addresses the problem of providing a 5-hydroxy-1H-imidazole-4-carboxamide effective-dose/sensitivity prediction method and prediction device whereby measurement can be performed in a short amount of time with a simple operation, a xanthosine-monophosphate-amount measurement method, and a myelodysplastic-syndrome treatment agent and treatment method. This invention provides a 5-hydroxy-1H-imidazole-4-carboxamide effective-dose/sensitivity prediction method and prediction device that involve the measurement of the amount of xanthosine monophosphate in blood, a xanthosine-monophosphate-amount measurement method that includes a step in which mass spectrometry is used to measure the amount of xanthosine monophosphate in blood under two different sets of measurement conditions, and a myelodysplastic-syndrome treatment agent and treatment method.
