In glycine encephalopathy there is decreased metabolism of glycine, resulting in its accumulation in the brain and other organs. The concentration of glycine in plasma and cerebrospinal fluid (CSF) can help to identify a patient who should have molecular testing for glycine encephalopathy.
Patient selection: possible glycine encephalopathy
Patient preparation:
• The CSF specimen should be free of blood or serum.
• The samples of blood and CSF should be collected at the same time.
Parameters:
(1) plasma glycine in µmol/L
(2) upper limit of the normal (ULN) reference range for plasma glycine
(3) CSF glycine in µmol/L
(4) upper limit of the normal (ULN) reference range for plasma glycine
plasma glycine as multiple of ULN =
= (plasma glycine) / (ULN)
CSF glycine as multople of ULN =
= (CSF glycine) / (ULN)
ratio of CSF to plasma glycine =
= (CSF glycine) / (plasma glycine)
Interpretation:
• In healthy newborns and infants the CSF to plasma ratio is <= 0.04.
• The more severe the glycine encephalopathy the higher the ratio, the CSF glycine and plasma glycine. The classic form has a ratio > 0.08 while attenuated disease may have a ratio >= 0.04.
• Additional testing is needed to establish the final diagnosis. The ratio has rarely been reported to be elevated in pyridoxine responsive seizures and other conditions.
• Valproate therapy may raise glycine concentrations.
