CSF Sialic Acid is useful for diagnosing free sialic acid storage diseases. This testing may also be used for assessment of Variants of Uncertain Significance (VUS) identified during genetic testing (e.g. Next Generation Sequencing or Capillary Sequencing Testing). Mutations in the SLC17A5 gene encoding the lysosomal transporter sialin are associated with the free sialic acid storage diseases (SASD): Salla disease (or the Finnish type of sialuria), the more severe infantile free sialic acid storage disease (ISSD), and intermediate phenotypes with clinical findings of both Salla disease and ISSD.1 SASD are characterized by the abnormal retention of free sialic acid in the lysosome (Online Mendelian Inheritance in Man 604369 and 269920). Patients with SASD usually present with nystagmus, progressive cerebellar ataxia, spasticity, and severe psychomotor delay. Cerebellar ataxia may be the primary symptom. These symptoms are associated with diffuse supratentorial hypomyelination, thin corpus callosum, and cortical and cerebellar atrophy. In some patients, sialic acid increases are identified only in CSF.