Guanidinoacetate methyltransferase (
GAMT) deficiency is an autosomal recessive disorder that results in reduced activity of
guanidinoacetate methyltransferase, an accumulation of
guanidinoacetate (GUAC), and a lack of cerebral
creatine (CRE). Lack of CRE in the brain can cause
intellectual disability, autistic-like behavior,
seizures, and
movement disorders. Identification at birth and immediate
therapy can prevent
intellectual disability and
seizures. If started early in life, treatment with
creatine supplements is highly effective. Because there are reliable
biomarkers for
GAMT deficiency, GUAC and CRE, and because the disorder is readily treatable with a significant improvement in outcomes,
GAMT deficiency is an excellent candidate for newborn screening. Several programs have conducted pilot programs or started screening. An isobaric interferant of the GUAC marker has been reported which may cause false positive results. To reduce the number of false positives, a second-tier HPLC test to separate GUAC from unknown, isobaric interferants may be incorporated into the screening algorithm. New York State began screening for
GAMT deficiency in October 2018 using a three-tiered screening approach. Quantification of GUAC and CRE were incorporated into routine screening for
amino acids and acylcarnitines. In the first year of screening a total of 263,739 samples were tested for
GAMT deficiency. Of these, 3382 required second tier testing. After second tier testing, 210 repeat specimens were requested for borderline results and 10 referrals were made to specialty care centers for confirmatory testing. In the first year of screening there were no confirmed cases of
GAMT deficiency detected. To reduce the number of samples needing second tier testing and the number false positives we explored the use of a second MS transition to confirm the identity of the GUAC marker. GUAC and its internal standard are detected as butylated
esters after sample preparation and derivatization. The original method used transition of the GUAC molecular ion of m/z 174.1 to a reactant ion of m/z 101.1. To confirm the identity of the GUAC marker we selected a qualifier ion of 174.1 > 73. The alternative product ion results were found to agree more closely with the second tier HPLC-MS/MS results for GUAC. It was found that the alternative transition may be used for quantification of the GUAC marker with acceptable analytical performance (linearity, accuracy, and precision). On March 5, 2020, the method of analysis for GUAC was modified to use the alternative product ion. For a comparable 6-month period, the modified method reduced the number of samples requiring second tier testing by 98%, reduced the number of borderline results requiring a repeat sample by 87.5%, and reduced the number of referrals to specialty care centers by 85%. Using the modified method, the correlation (r-squared) of the first and second tier screening results for GUAC is greater than 0.95. Since the first-tier results correlate well with the second-tier results, the second-tier screening is no longer necessary with the modified method.