Table 3. Biochemical genetics tests that should be ordered depending on the suspected IEM.
| Assay | Special sample considerations | Diagnosable disorders |
|---|---|---|
| Plasma acylcarnitine analysis | Avoid hemolysis | Fatty acid oxidation disorders; organic acidurias |
| Urine organic acids analysis | Avoid bacterial contamination of the urine. | Organic acidurias; fatty acid oxidation disorders |
| Plasma/CSF/urine amino acids analysis | For plasma amino acid analysis, collect sample after an overnight fast if possible, or a minimum 4 hours fast; for pediatric samples, collect sample before next scheduled feeding; for plasma amino acids, separate plasma from cells as soon as possible after collection; avoid leaving blood at room temperature prior to centrifugation; hemolysis can affect results | Aminoacidopathies/aminoaciduria |
| Urinary acylglycines | – | Organic acidurias; fatty acid oxidation disorders |
| Serum very long chain fatty acids analysis; plasma/urine pipecolic acid; plasma/urine bile acids; red blood cells plasmalogen | Collect sample after an overnight fast. For pediatric patients, collect sample before next scheduled feeding; patients should avoid alcohol for 24 hours prior to sample collection; Icterus and lipemia can affect results | Peroxisomal disorders |
| Urine/serum; guanidinoacetate/creatine/creatinine analysis | Creatine biosynthesis disorders | |
| Serum carnitine quantification | Carnitine deficiencies | |
| Urine/serum purines & pyrimidines | Purine & pyrimidine disorders | |
| Serum/urine homocysteine quantification | For serum homocysteine, place tube in an ice bath or refrigerator for 30 minutes after collection | Hyperhomocysteinemia |
| Serum/urine methylmalonic acid quantification | Methylmalonic acidemias | |
| Urine/plasma/fecal/RBC porphyrins, urine D-aminolaevulinic acid & porphobilinogen analysis | For D-ALA, PBG, and porphyrins analysis, samples must be protected from UV light by using an amber tube/container, or by wrapping the container completely in foil | Porphyrias; hyperbilirubinemias |
| Plasma/CSF lactate & blood/pyruvic acid quantifications | For serum and blood, tourniquet technique may influence results | Mitochondrial disorders (in general) |
| Transferrin isoelectric focusing analysis & LCMSMS | Congenital disorders of glycosylation | |
| Serum biotinidase enzyme activity quantification | Biotinidase deficiency | |
| Urine oligosaccharides analysis & LCMSMS | Oligosaccharides | |
| Urine, blood & CSF glycosaminoglycans (heparan sulfate, dermatan sulfate, keratan sulfate, and chondriotin sulfate) analysis & LCMSMS | Mucopolysaccharidosis | |
| Galactose-1-Phosphate | Galactosemia | |
| Frctose-1-Phosphate | Hereditary Fructose Intolerance | |
| Plasma/urine globotriaosylsphingosine (LYSO-Gb3); plasma ceramide; plasma chitotriosidase (CHITO); plasma angiotensin-converting enzyme (ACE); plasma tartrate-resistant acid phosphatase (TRAP); plasma pulmonary and activation-regulated chemokine (PARC/CCL18) | homozygous 24-bp duplication in the chitotriosidase gene is found in 6% of the population causing chitotriosidase deficiency; therefore, monitoring CHITO in these individuals is not useful [44] | *Sphingolipidoses; Fabry disease; Fabre disease; Gaucher disease |
| Plasma chitotriosidase (CHITO); plasma oxysterols; urine sulfatides; plasma creatine kinase (CK), aspartate aminotransferase (AST) and alanine aminotransferase (ALT), lactate dehydrogenase (LDH), & urine hexose tetrasaccharide (HEX4 or glucose tetrasaccharide, Glc4) | Niemann Pick A/B/C; Niemann Pick C; metachromatic leukodystrophy (MLD); Pompe disease |
In all groups of disorders, there are individual conditions for which specific mass spectrometry-based analysis, specific enzyme analysis and/or molecular genetics analysis might be required to accurately make a diagnosis. CSF, cerebrospinal fluid; RBC, red blood cell; LCMSMS, liquid chromatography tandem mass-spectrometry.