How to Order Serum Carnitine Levels for Suspected Primary Carnitine Deficiency
Order plasma or serum for free carnitine, total carnitine, and acylcarnitine profile analysis using tandem mass spectrometry (MS/MS), which is the standard analytical platform used by virtually all clinical laboratories. 1
Specimen Type and Collection
Preferred sample: Plasma or serum is the preferred specimen type for symptomatic patients and asymptomatic at-risk individuals. 1, 2
Alternative option: Some laboratories also accept dried blood spots, which facilitate specimen transport and may increase sensitivity for detecting long-chain fatty acid oxidation disorders. 1, 2
Sample volume: The typical testing volume required is approximately 20 microliters (μL) of plasma or serum. 2
Handling and Shipping Requirements
- Plasma/serum specimens must be frozen as soon as possible after collection and shipped to the testing laboratory on dry ice. 1
- Dried blood spots on filter paper can be shipped at room temperature. 1
- Long-term storage at -80°C maintains indefinite stability of acylcarnitines, while room temperature storage causes progressive degradation, particularly of short-chain species. 1
Comprehensive Metabolic Evaluation
The American College of Medical Genetics recommends that carnitine testing be part of a comprehensive metabolic evaluation that includes: 1
- Plasma free and total carnitine (to calculate acyl-to-free carnitine ratio)
- Plasma acylcarnitine profile (the key diagnostic test showing characteristic patterns)
- Plasma amino acids
- Urine organic acids
- Blood triglycerides, liver function tests (AST, ALT), glucose, lactate, and ammonia
- Urine ketones 1, 3, 4
Interpretation of Results
The acyl-to-free carnitine ratio is critical for diagnosis: 1, 4
- Normal status: Ratio ≤ 0.25
- Carnitine deficiency: Ratio > 0.4
Primary carnitine deficiency (carnitine uptake defect) is specifically detectable by acylcarnitine profile analysis and will show markedly reduced free carnitine levels with abnormal ratios. 1
Clinical Context
Primary carnitine deficiency is a genetic disorder of the cellular carnitine-transporter system that typically manifests by age five with cardiomyopathy, skeletal muscle weakness, and hypoglycemia. 1 The diagnosis is almost exclusively a laboratory process, making proper specimen collection and handling essential to avoid false results. 1
Common Pitfalls to Avoid
- Do not allow plasma/serum specimens to remain at room temperature, as this causes degradation of acylcarnitines and may lead to false-negative results. 1
- Ensure the laboratory uses tandem mass spectrometry (MS/MS), as this is the standard methodology with appropriate analytical proficiency. 1
- Coordinate with your laboratory regarding specific collection volumes and shipping requirements, as these may vary slightly between facilities. 1