Abstract: Methods are disclosed for diagnosing a hyposialylation disorder. Methods are also disclosed for determining the effectiveness of a therapeutic agent for treatment of a hyposialylation disorder in a subject. These methods include measuring an amount of monosialylated Thomsen-Friedenreich (ST) antigen and measuring an amount of non-sialylated Thomsen-Friedenreich antigen (T) in a biological sample, such as a serum or plasma sample from the subject and determining the ratio of T to ST. A ratio of T to monosialylated ST of about 0.06 or higher diagnoses the hyposialylation disorder or indicates that the therapeutic agent is not effective for the treatment of the hyposialylation disorder. In other embodiments, a ratio of T to ST less than about 0.06 indicates that the therapeutic agent is effective for the treatment of the hyposialylation disorder, or the subject does not have the hyposialylation disorder.