Overview: Acylcarnitine C5 TestIntroduction: The Acylcarnitine C5 Test screens for Isovaleric Acidemia, a disorder causing a buildup of toxic substances, leading to vomiting, lethargy, or a distinctive sweaty feet odor, helping diagnose this metabolic condition. With an incidence of 1 in 250,000, its unique odor aids clinical suspicion but requires lab confirmation, especially in newborns during metabolic stress. Following 2023 ACMG guidelines, it uses tandem mass spectrometry (MS/MS) for high specificity, supporting metabolic screening. This test is crucial for diagnosis, treatment planning, and improving outcomes in biochemistry, particularly in preventing acute metabolic crises.
Other Names: C5 Assay, Isovalerylcarnitine Test.
FDA Status: Laboratory-developed test (LDT), meeting biochemistry standards for accuracy.
Historical Milestone: Acylcarnitine testing started in the 1990s with metabolic research by Kevin Strauss, focusing on organic acidemias. Tandem MS advancements in the 2000s by ABI Sciex improved detection of C5 carnitine, surpassing earlier gas chromatography-mass spectrometry (GC-MS) methods that required larger samples and were less sensitive for neonatal screening.
Purpose: Measures C5 acylcarnitine levels to diagnose Isovaleric Acidemia, guides protein restriction and glycine therapy, and evaluates patients with vomiting, lethargy, or metabolic acidosis, aiming to prevent life-threatening episodes like hyperammonemia or coma.
Test Parameters: C5 acylcarnitine levels
Pretest Condition: Fasting preferred for 6-8 hours to assess baseline metabolism and detect abnormalities. Collect dried blood spot, plasma, or serum. Report history of vomiting, lethargy, or metabolic crises.
Specimen: Dried Blood Spot (filter paper, 3-5 spots), Plasma (NaHep, 2-4 mL), Serum (SST, 2-5 mL); 2 mL plasma in Na Heparin tube or dried blood spot on filter paper. Transport in a biohazard container.
Sample Stability at Room Temperature: 24 hours
Sample Stability at Refrigeration: 1 week
Sample Stability at Frozen: 1 month
Medical History: Document vomiting, lethargy, or metabolic crises. Include dietary history, recent infections, or family history of metabolic disorders, noting any protein-rich diet exposure that could trigger symptoms.
Consent: Written consent required, detailing the test's purpose, acidemia risks (e.g., coma, pancreatitis), and potential risks of sample collection, emphasizing neonatal screening and genetic counseling to inform family planning.
Procedural Considerations: Uses tandem mass spectrometry (MS/MS) to measure C5 levels, requiring laboratories with ABI 4000 systems and trained technicians to handle dried blood spots. Results are available in 2-3 days, supporting neonatal care. Performed in labs with sample drying protocols and temperature control to prevent degradation and ensure accurate metabolite detection.
Factors Affecting Result Accuracy: Improper drying, contamination with skin oils, or exposure to heat can affect results. Protein intake, medications, or concurrent illness may alter levels, necessitating controlled conditions and clinical correlation to avoid misdiagnosis.
Clinical Significance: Elevated C5 confirms Isovaleric Acidemia, guiding glycine and carnitine therapy to detoxify isovaleric acid. A child with early treatment might avoid metabolic stroke, while untreated cases can lead to death or severe neurological damage. Normal levels may require genetic testing (e.g., IVD) to rule out carriers or alternative diagnoses.
Specialist Consultation: Consult a metabolic specialist or pediatrician for result interpretation and management, particularly for infants or those with family history, where dietary adjustments and emergency protocols are critical to prevent crises.
Additional Supporting Tests: Urine organic acids, IVD gene analysis, or plasma amino acids to confirm diagnosis and assess glycine status, aiding in personalized care and monitoring long-term outcomes.
Test Limitations: Non-specific for severity; clinical correlation with symptoms and genetics is needed. Sensitivity varies with metabolic state, requiring repeat testing during symptomatic periods.
References: ACMG Guidelines, 2023; Journal of Inherited Metabolic Disease, Vockley J, 2022.
