Several recent studies have investigated whether plasma glutamatergic amino acid levels or NMDA-related genes are diagnostic, therapeutic, or symptomatic biomarkers. First, a comprehensive literature review [7▪▪] suggests that serum D-serine, glycine, glutathione, and alanine could be useful biomarkers. These findings were further supported by a case–control study by Hons et al. [8] that included 50 nonacute-schizophrenia patients and 50 age-matched and sex-matched controls. Glycine serum levels were measured, and the Positive and Negative Symptom Scale (PANSS) and the Scale for the Assessment of Negative Symptoms (SANS) were used to assess relationship of glycine levels with negative symptoms. As predicted, mean glycine serum levels were significantly lower in patients than in controls. Low levels of plasma glycine were associated with higher levels of negative symptoms assessed by the PANSS negative subscale and the SANS total scores in the patients.
