Lysine - Quantitative Plasma Test

The Lysine - Quantitative Plasma test is a specialized biochemical investigation that measures the concentration of lysine in a plasma sample. It may be used to evaluate suspected disorders of amino-acid metabolism or transport, investigate an abnormal newborn-screening or amino-acid result, assess hyperlysinemia, support the evaluation of lysinuric protein intolerance, and monitor selected patients with a confirmed inherited metabolic condition.

Lysine is an essential amino acid, which means the body cannot produce an adequate amount and must obtain it from dietary protein. It is required for growth, tissue maintenance, protein synthesis, production of enzymes and hormones, formation of carnitine, calcium utilisation, immune function, and development of collagen-related structures.

After dietary proteins are digested, lysine is absorbed through the intestine and enters the bloodstream. Cells use it for protein and metabolic processes. Excess lysine is broken down through specific biochemical pathways, while the kidneys filter and normally reabsorb most circulating amino acids.

The plasma lysine concentration depends on age, recent food intake, protein consumption, fasting duration, metabolism, intestinal absorption, kidney handling, liver function, illness, medicines, supplements, and specimen processing. For this reason, the result must be interpreted with the patient's age, diet, symptoms, family history, and other plasma and urinary amino acids.

Lysine is commonly measured as part of a quantitative plasma amino-acid profile because the overall pattern is usually more informative than a single amino acid. A normal or abnormal lysine result does not independently confirm or exclude an inherited metabolic disorder.

Benefits of the Quantitative Plasma Lysine Test

• Provides a quantitative measurement of lysine in plasma.
• Supports the evaluation of suspected hyperlysinemia.
• May contribute to assessing lysinuric protein intolerance.
• Helps investigate abnormal newborn-screening or metabolic results.
• Can be interpreted with other cationic amino acids such as arginine and ornithine.
• May identify patterns suggesting amino-acid transport or degradation disorders.
• Supports monitoring in patients with an established metabolic condition.
• Provides information for specialist nutritional and metabolic management.
• Can be paired with quantitative urine amino-acid testing.
• May guide selection of confirmatory enzyme or genetic investigations.

Clinical Indications and Applications

Hyperlysinemia: Hyperlysinemia describes an increased lysine concentration in blood. Primary hyperlysinemia is a rare inherited metabolic condition commonly associated with reduced activity of the bifunctional enzyme alpha-aminoadipic semialdehyde synthase, encoded by the AASS gene.

Many individuals with primary hyperlysinemia may have few or no symptoms, and the biochemical finding can be relatively benign. However, an elevated lysine result should still be evaluated carefully because it may occur with other metabolic conditions, liver dysfunction, dietary factors, treatment, or specimen-related influences.

Lysinuric Protein Intolerance: Lysinuric protein intolerance is a rare inherited transport disorder associated with pathogenic variants in the SLC7A7 gene. It impairs the transport of cationic amino acids, particularly lysine, arginine, and ornithine, in the intestine and kidneys.

In affected patients, plasma lysine, arginine, and ornithine are often low but may occasionally fall within their age-specific reference intervals. Urinary excretion of these amino acids, particularly lysine, is increased. Therefore, plasma lysine must be interpreted with a quantitative urine amino-acid profile and clinical features.

Symptoms of lysinuric protein intolerance may include poor tolerance of protein-rich food, vomiting after protein intake, poor growth, reduced muscle mass, enlarged liver or spleen, osteoporosis, blood-cell abnormalities, pulmonary involvement, and episodes of high blood ammonia.

Abnormal Newborn Screening: Quantitative plasma amino-acid testing may be requested after an abnormal newborn-screening result. The exact follow-up pathway depends on the marker, screening programme, clinical condition, and suspected disorder.

Developmental or Neurological Symptoms: Plasma amino-acid analysis may be included in the evaluation of unexplained developmental delay, intellectual disability, seizures, movement abnormalities, altered consciousness, feeding problems, or recurrent metabolic illness. These symptoms have many possible causes and are not specific to lysine disorders.

Protein Intolerance or Hyperammonemia: Patients who develop vomiting, lethargy, confusion, behavioural changes, or high ammonia after protein intake may require urgent metabolic evaluation. Plasma lysine, arginine, ornithine, citrulline, glutamine, and other amino acids may contribute to interpretation.

Monitoring a Confirmed Disorder: A metabolic specialist may request serial plasma lysine measurements to evaluate biochemical control, dietary management, amino-acid supplementation, or response to treatment. Monitoring plans are individualised according to the diagnosis.

Why Doctors Recommend This Test

Doctors recommend this test when a patient's symptoms, family history, newborn-screening result, urinary amino-acid pattern, or previous laboratory findings suggest abnormal lysine metabolism or transport.

The plasma concentration helps determine whether lysine is increased, reduced, or within its age-specific expected interval. This distinction can help guide further investigation, but the overall amino-acid pattern remains essential.

For example, significantly elevated plasma lysine may raise consideration of hyperlysinemia or another metabolic cause. Low plasma lysine with increased urinary loss of lysine, arginine, and ornithine may support lysinuric protein intolerance. In cystinuria, urinary cystine and dibasic amino acids are increased, while plasma lysine may remain normal.

A doctor may also request urine amino acids, plasma ammonia, urine organic acids, acylcarnitine profile, lactate, liver function, kidney function, glucose, blood gases, enzyme analysis, or molecular genetic testing. No single lysine measurement provides a complete metabolic diagnosis.

Preparation Before the Test

Follow the instructions provided by the referring metabolic specialist and Focus Diagnostics. Fasting is commonly preferred for quantitative plasma amino-acid testing because recent protein intake can temporarily change amino-acid concentrations. The required fasting period depends on age and clinical condition.

Infants, young children, pregnant patients, people with diabetes, and patients at risk of metabolic decompensation should not undergo prolonged fasting unless specifically supervised by a doctor. Safety takes priority over standard fasting preparation.

Inform the healthcare provider about all prescription medicines, non-prescription medicines, amino-acid supplements, protein powders, nutritional formulas, vitamins, herbal products, and specialised metabolic diets.

Do not stop prescribed treatment, change protein intake, or discontinue a medical formula without instructions from the metabolic team. The doctor may require testing while the patient follows the usual treatment plan.

Provide the patient's age, weight, fasting status, collection time, clinical condition, family history, diet, medicines, supplements, and suspected diagnosis. Age is essential because normal amino-acid concentrations differ considerably between newborns, infants, children, adolescents, and adults.

Tell the laboratory if the sample is collected during an acute metabolic episode, after protein intake, during intravenous nutrition, after transfusion, or following dialysis. These circumstances may influence interpretation.

What Happens During the Test?

A trained phlebotomist cleans the skin over a suitable vein and collects a small blood sample into the laboratory-approved anticoagulant tube. In infants, the collection technique and sample volume are adapted to age and clinical circumstances.

The blood sample is processed promptly to separate plasma from blood cells. Delayed separation or unsuitable storage may alter amino-acid concentrations. The plasma may need to be frozen and transported under controlled conditions to a specialised metabolic laboratory.

The laboratory measures lysine using a validated quantitative method, commonly liquid chromatography with tandem mass spectrometry or another amino-acid analysis technique. A full plasma amino-acid profile may be generated even when lysine is the principal analyte of interest.

The blood draw may cause brief discomfort, mild bruising, tenderness, or minor bleeding. Serious complications are uncommon. Inform the phlebotomist if the patient has a bleeding disorder, takes anticoagulants, or has previously fainted during blood collection.

Normal Reporting Time

The report for the Lysine - Quantitative Plasma test is generally available within 7 to 10 working days. It is a specialised metabolic investigation that may require frozen transport, batch processing, and referral-laboratory analysis.

Reporting may take longer if the laboratory requires sample dilution, repeat analysis, confirmation of an unusual amino-acid pattern, or specialist biochemical interpretation. Patients should confirm the expected turnaround time with Focus Diagnostics.

Who Should Consider This Test?

• Patients with suspected hyperlysinemia or another lysine-metabolism disorder.
• Individuals undergoing assessment for lysinuric protein intolerance.
• Babies requiring follow-up after an abnormal newborn-screening result.
• Children with poor growth, feeding difficulties, or protein intolerance.
• Patients with unexplained developmental or neurological symptoms.
• Individuals with recurrent vomiting, lethargy, or high ammonia after protein intake.
• Patients with an abnormal quantitative urine amino-acid result.
• Individuals with a confirmed metabolic disorder requiring monitoring.
• Patients referred by a metabolic physician, geneticist, paediatrician, neurologist, or dietitian.
• Individuals specifically advised to undergo quantitative plasma amino-acid testing.

Understanding the Test Results

The result is reported as a lysine concentration with a method-specific unit and an age-specific reference interval. Newborns, infants, children, and adults may have different reference ranges. The laboratory's reference information should always be used.

Elevated Plasma Lysine: Increased lysine may occur in primary hyperlysinemia, related lysine-degradation abnormalities, liver dysfunction, certain nutritional circumstances, high recent protein intake, supplementation, or another metabolic pattern. An elevated value does not by itself establish a diagnosis.

When hyperlysinemia is suspected, the doctor may review other plasma amino acids, urine amino acids, clinical symptoms, family history, and genetic testing involving the AASS gene or other relevant genes. Many individuals with isolated primary hyperlysinemia may be asymptomatic.

Low Plasma Lysine: Reduced lysine may be associated with lysinuric protein intolerance, inadequate protein intake, malnutrition, malabsorption, increased renal loss, or another medical condition. The urinary amino-acid pattern is particularly important.

Low or normal plasma lysine with markedly increased urinary lysine, arginine, and ornithine may support a cationic amino-acid transport defect. Molecular testing can confirm lysinuric protein intolerance when clinical and biochemical findings are compatible.

Result Within the Reference Interval: A normal result means that plasma lysine was within the expected range at the time of collection. It does not completely exclude lysinuric protein intolerance because plasma cationic amino acids may occasionally be within their normal ranges.

The doctor may request a repeat fasting sample if the result is inconsistent with the clinical findings or if collection occurred after recent food intake, during acute illness, or under unsuitable pre-analytical conditions.

Clinical Limitations

The plasma lysine concentration is influenced by fasting status, recent protein intake, age, diet, supplements, medicines, liver function, kidney function, illness, intravenous nutrition, and specimen processing.

A single amino-acid result cannot diagnose a genetic disorder. The entire plasma and urine amino-acid pattern, symptoms, family history, enzyme studies, and molecular genetic results must be considered.

Reference intervals differ by age, laboratory method, unit, and patient population. Results from different laboratories may not be directly comparable.

Delayed plasma separation, improper temperature control, haemolysis, or prolonged transport may affect amino-acid stability and lead to an unreliable result.

Important Safety Information

Seek emergency medical attention for confusion, severe vomiting, unusual sleepiness, breathing difficulty, seizures, loss of consciousness, or suspected hyperammonemia. Acute metabolic decompensation may require urgent treatment and should not wait for a routine report.

Do not increase or restrict dietary protein, lysine, arginine, ornithine, or another amino acid based solely on this result. Incorrect dietary restriction can impair growth and nutrition, particularly in children.

Do not begin supplements or change a prescribed metabolic formula without guidance from a metabolic physician and specialist dietitian.

This information is intended for general education and does not replace diagnosis or advice from a metabolic specialist, geneticist, paediatrician, or dietitian. Always discuss the result with the referring healthcare provider.