What abbreviations are used for initial diagnosis of Inborn Errors of Metabolism (IEM), such as MS/MS (Tandem Mass Spectrometry), OA (Organic Acids), and AA (Amino Acids)?

Common Abbreviations for Initial Diagnosis of Inborn Errors of Metabolism

MS/MS (Tandem Mass Spectrometry) is the primary abbreviation and gold-standard screening method for initial diagnosis of IEMs, detecting amino acidemias (AA), organic acidemias (OA), and fatty acid oxidation disorders (FAO) simultaneously from a single dried blood spot. 1

Primary Diagnostic Abbreviations

MS/MS - Tandem Mass Spectrometry

• MS/MS is the comprehensive screening platform that identifies and quantifies metabolites based on mass-to-charge ratios, completing analysis in seconds rather than hours required by traditional methods. 1
• The American College of Medical Genetics established MS/MS as the standard for newborn screening, capable of detecting over 30 different IEMs from a single sample. 1
• MS/MS analyzes acylcarnitine profiles for fatty acid oxidation disorders and amino acid profiles for aminoacidemias simultaneously. 1

Metabolite Category Abbreviations

AA (Amino Acids/Amino Acidemias)

• Refers to disorders of amino acid metabolism including phenylketonuria (PKU), maple syrup urine disease (MSUD), homocystinuria, and citrullinemia. 1
• MS/MS detects elevated phenylalanine, leucine/isoleucine, methionine, and citrulline as diagnostic markers. 1

OA (Organic Acids/Organic Acidemias)

• Encompasses disorders like propionic acidemia, methylmalonic acidemia, isovaleric acidemia, and glutaric aciduria type I. 1
• Traditional organic acid analysis uses GC/MS (Gas Chromatography-Mass Spectrometry) on urine samples, but MS/MS detects these via acylcarnitine profiles in blood. 2, 3

FAO (Fatty Acid Oxidation Disorders)

• Includes MCAD (Medium-Chain Acyl-CoA Dehydrogenase), VLCAD (Very Long-Chain Acyl-CoA Dehydrogenase), LCHAD (Long-Chain 3-Hydroxy Acyl-CoA Dehydrogenase), and CPT deficiencies. 1
• Acylcarnitine analysis via MS/MS is superior to organic acid analysis for FAO disorders because it detects abnormalities even when patients are not acutely ill. 1

Secondary Testing Abbreviations

GC/MS (Gas Chromatography-Mass Spectrometry)

• Used for confirmatory urine organic acid profiling when MS/MS blood spot screening suggests an organic acidemia. 2, 3
• GC/MS provides comprehensive organic acid profiles but requires longer analysis time (not suitable for rapid screening). 2

DBS (Dried Blood Spots)

• The sample type used for MS/MS screening, typically collected 24-48 hours after birth. 4, 3

Clinical Algorithm for IEM Screening

Initial screening pathway:

• Collect DBS at 24-48 hours of life for MS/MS analysis of amino acids and acylcarnitines. 4
• If collected before 24 hours, repeat screening at 1-2 weeks per standard protocol. 4
• MS/MS cannot detect biotinidase deficiency, hypothyroidism, hemoglobinopathies, congenital adrenal hyperplasia, or galactosemia—these require separate screening methods. 1

Confirmatory testing when MS/MS is abnormal:

• Obtain plasma acylcarnitine profile for quantification. 5
• Collect urine for organic acid analysis by GC/MS. 2, 5
• Measure urine acylglycine profiles for specific organic acidemias. 5
• Proceed to enzyme activity assays or molecular genetic testing for definitive diagnosis. 5

Important Clinical Pitfalls

• Do not delay or repeat newborn screening solely because an infant is receiving dopamine or other vasoactive medications—these do not interfere with MS/MS metabolite detection. 4
• Critically ill neonates may have confounding factors affecting results including TPN administration, blood transfusions, hemodilution from fluid resuscitation, and timing of sample collection—investigate these before attributing abnormal results to true IEM. 4
• MS/MS detects 30+ disorders but prevalence patterns vary by population; consanguinity increases detection rates of autosomal recessive IEMs. 6
• Combined MS/MS and GC/MS screening in sick infants yields approximately 2% detection rate for IEMs, with amino acid disorders being most common (66%), followed by organic acidemias (30%) and FAO disorders (4%). 3