Disorders of Aminoacids, Organic acids & Fatty acid oxidation can be tested by Tandem Mass Spectrometer (TMS).
Why test with Tandem Mass Spectrometer?
More cases of inborn errors of metabolism are diagnosed by screening with tandem mass spectrometry than are diagnosed clinically. (New England Journal of Medicine 2003; 348: 2304-12).
SIDS or Metabolic Disorder? – TMS can differentiate
3-6% of SIDS are due to inherited disorders of Fatty Acid Oxidation, (Clinical Chemistry; June 2001:47 (7)1166-1182)
What Analytes are measured by Tandem Mass Spectrometry?
- Aminoacids: building blocks of proteins
- Carnitine: transportation system for fats in and out of mitochondria
- Acylcarnitine: Acylcarnitines are identified by the size of fat molecule attached to it and categorised simply as short, medium amd long chain. The important medium sized fat attached to carnitine that is measured in MCAD is eight carbon fatty acid known as Octanoylcarnitine and is abbreviated as C8
Disorders detected by TMS
I. Aminoacid Disorders:
- Urea cycle disorders
- Non-ketotic hyperglycinemia
- Hyperammonemia, Hyperornithinemia, Homocitrullinuria
II. Organic Acid Disorders:
- Glutaric acidemia type 1
- Isovaleric acidemia
- 3-ketothiolase deficiency
- 3-MethylcrotonylC0A carboxylase deficiency
- 3-Methylglutaconyl CoA hydratase deficiency
- Multiple carboxylase deficiency
- Beta-Ketothiolase deficiency
- Methylmalonic acidemia
- Methylmalonic acidemia with homocystinuria
- Propionic acidemia
III. Fatty acid Oxidation Disorders:
- Shortchain acyl CoA dehydrogenase deficiency (SCAD)
- Medium chain acyl CoA dehydrogenase deficiency (MCAD)
- Mitochondrial trifunctional protein deficiency
- Longchain 3-hydroxy acyl CoA dehydrogenase deficiency (LCHAD)
- Very long chain acyl CoA dehydrogenase deficiency ( VLCAD)
- Carnitine transport defect
- Carnitine palmitoyl transferase deficiency type 1 (CPT 1)
- Carnitine palmitoyl transferase deficiency type 2 ( CPT 2)
- Glutaric acidemia type 2
- Carnitine acylcarnitine translocase deficiency
SENSITIVITY, SPECIFICITY, POSITIVE PREDICTIVE VALUE of TMS
Sensitivity close to 100% with high positive predictive value (PPV) (Ref:J Inherit
Metab Dis (2007)30:129-133)
– Classical MSUD -PKU
-UCD -Earlyonset organic acidemias
-Tyrosinemia -Fatty acid oxidation disorders
FALSE POSITIVE RATE (TMS); (Ref:J Inherit Metab Dis (2007)30:129-133)
- Low • 0.2-0.33 % of babies screened needed further testing
What is the Screening Sample ?
5 droops of blood are taken form the babies heel. Baby’s heel is pricked with a lancet and drops of blood are collected on special filter paper.
When should the newborn screening sample be collected ?
Infant must be 48 hours of age or older to obtain a satisfactory sample for screening.Ideal time is 4 days of age. Sample should be taken prior to adminstering antibiotics or transfusing blood or blood products.
What is the cutoff age for an infant when accepting sample for newborn screening?
Till one month of age except for disorders by TMS. Test methodologies for other tests are set to obtain valid values till 30 days of age. Result values of tests carried out beyond 30 days may overlap considerably making it difficult to distinguish presence or absence of disorders.
Factors which affect Result & Interpretation:
- Size of blood spot
- Blood for test aspirated from iv catheter
- IV fluid
- Intravenous fluid hydration with dextrose prior to collection of sample can flush out short lived active acylcarnitine metabolite from the intravascular blood stream
- Blood transfusion
- Low Protein diet
- Supplemented with aminoacids
- Drugs such as valporic acid, MCT Oil, antibiotics containing a derivative of pivalic acid
How long to wait to screen an infant who has been transfused? Given the presumption that the mean age of RBC’s of the transfused blood is 60-80 days, it is suggested that sample should be collected 60 days after discontinuation of transfusion.
What to do for an infant who needs antibiotics prior to 24 hours of age?
Sample taken from infants receiving antibiotics at the time of collection may yield false results. Sample should be collected prior to starting antibiotic therapy or 24 to 36 hours after discontinuation of antibiotic therapy.
GC-MS- Confirmation of screen Positive cases for Organic Acid Disorders
GC-MS is gold standard technology for confirming organic acid disorders in urine. The test is performed in a random urine from which organic compounds are extracted and analysed by gas chromatography with mass spectrometric detector.Urine organic acid profile also provides a diagnostic clue to other inborn errors of metabolism including aminoacid disorders, urea cycle disorders and fatty acid oxidation disorders. Aneonate affected with an organic academia is usually well at birth and for the first few days or months of life. The usual clinical presentation is that of toxic encephalopathy and includes vomiting, poor feeding, seizures , abnormal tone and lethargy leading to coma. Clinical laboratory finding that suggests an organic academia include acidosis, ketosis, hyperammonemia, abnormal liver function tests, hypoglycemia and neutropenia.
Several rare types of organic acid disorders present with neurologic signs without concomitant biochemical findings such as hyperammonemia and acidosis; however, these disorders have a distinctive pattern of organic acids. Such disorders include 4- hydroxyglutaric aciduria, 3-methylglutaconic acid dehydrase deficiency, and malonic
aciduria Propionic academia, Methylmalonic academia, Cobalmin defect variants, may present with developmental delay without acidosis.
Late-onset 3-methylcrotonyl carboxylase deficiency may present as developmental
delay without Reye-like syndrome, in contrast to the early-onset form.
In the older child or adolescent, variant forms of the organic acid disorders can present as loss of intellectual function, ataxia or other focal neurologic signs, Reye syndrome, recurrent ketoacidosis, or psychiatric symptoms.
A variety of MRI abnormalities have been described in the organic acid disorders, including distinctive basal ganglia lesions in glutaric acidemia type I (GA l), white matter changes in maple syrup urine disease (MSUD), and abnormalities of the globus pallidus in methylmalonic acidemia. Macrocephaly is common in GA I