A 3-year-old child from rural Maharashtra presents with recurrent episodes of hypoketotic hypoglycemia, hepatomegaly, and developmental delay. Serum carnitine is low. The parents report that symptoms worsen during fasting and improve with frequent feeding. Which investigation is most appropriate to confirm the suspected disorder of fatty acid oxidation?

Explanation

## Diagnosis of Carnitine Deficiency and Fatty Acid Oxidation Disorders ### Clinical Context The presentation of hypoketotic hypoglycemia, hepatomegaly, low serum carnitine, and worsening during fasting is classic for **primary carnitine deficiency** or other fatty acid oxidation defects. The hypoketotic pattern (low ketones despite hypoglycemia) is pathognomonic — it indicates impaired fatty acid oxidation, preventing ketone body production. ### Why Acylcarnitine Profile is Diagnostic **Key Point:** Plasma acylcarnitine profile by tandem mass spectrometry (MS/MS) is the gold-standard first-tier investigation for all fatty acid oxidation disorders. **High-Yield:** Acylcarnitine profiling reveals: - **Primary carnitine deficiency:** Low free carnitine (C0), normal acylcarnitines - **Medium-chain acyl-CoA dehydrogenase (MCAD) deficiency:** Elevated C6–C10 acylcarnitines - **Long-chain 3-hydroxyacyl-CoA dehydrogenase (LCHAD) deficiency:** Elevated C16 and C18 hydroxylated acylcarnitines - **Carnitine palmitoyltransferase (CPT) deficiencies:** Variable patterns depending on type This single test identifies the specific defect, guides therapy, and enables newborn screening. ### Comparison with Other Investigations | Investigation | Utility | Limitation | |---|---|---| | **Acylcarnitine profile (MS/MS)** | Identifies specific FAO defect; rapid; non-invasive | Requires specialized lab | | Serum free fatty acid levels | Non-specific; elevated in many metabolic states | Does not identify the enzymatic defect | | Urine organic acid analysis | May show secondary metabolites in some disorders | Less sensitive for primary carnitine deficiency | | Liver biopsy with EM | Invasive; shows lipid accumulation | Does not identify biochemical defect; high risk in a hypoglycemic child | **Clinical Pearl:** In India, tandem MS is increasingly available in tertiary centres and is part of expanded newborn screening programs. It is non-invasive, requires only a dried blood spot, and provides rapid diagnosis. **Mnemonic — FAO Defect Diagnosis:** **MASS** = **M**ass spectrometry for **A**cylcarnitine **S**creening **S**ystem.