This is a method for stressing the pterin system, but may be unreliable. It was developed as a biochemical marker for pterin synthesis defects associated with dopamine-responsive dystonia, especially GTP-CH deficiency. A dose of 100mg/kg of phenylalanine is used. A phenylalanine/tyrosine ratio >7.5 is suggestive, but not diagnostic, of a pterin synthesis defect. The phenylalanine must rise above 600umol/l. for the test to be valid. This test has low sensitivity and specificity.
Add comment