Disorders of Amino Acid Absorption or Transport (ICD-11: 5C60)

1. Introduction

Disorders of amino acid absorption or transport represent a heterogeneous group of metabolic conditions characterized by deficiencies in the cellular mechanisms responsible for the uptake, transport, and distribution of amino acids in the body. These conditions may affect intestinal transport as well as renal transport or transport across specific cell membranes, resulting in varied clinical manifestations that depend on the amino acid or group of amino acids involved.

The clinical importance of these disorders lies in the fact that amino acids are essential components for protein synthesis, neurotransmitters, hormones, and various fundamental metabolic pathways. When their absorption or transport is compromised, manifestations may range from mild to severe presentations with neurological, renal, and developmental impairment. Although considered rare conditions individually, together they represent a significant burden for health systems, especially when diagnosis is delayed.

Global prevalence varies according to the specific condition, with some being more frequent in certain populations due to genetic factors. Early diagnosis through newborn screening and appropriate investigation of suspected symptoms is crucial to prevent irreversible complications. Correct coding of these conditions in the ICD-11 system is critical to ensure adequate epidemiological recording, facilitate access to specialized treatments, allow longitudinal follow-up of patients, and enable research studies that contribute to better understanding and management of these rare diseases.

2. Correct ICD-11 Code

Code: 5C60

Description: Disorders of absorption or transport of amino acids

Parent category: null - Disorders of absorption or transport of metabolites

Official definition: Any condition caused by deficiencies in the absorption and/or transport of amino acids.

This code belongs to the chapter on endocrine, nutritional or metabolic diseases of ICD-11 and specifically encompasses conditions where the primary defect is in amino acid transport systems, whether through the intestinal membrane, the renal tubular membrane, or other cell membranes. Code 5C60 serves as a broad category that encompasses various specific disorders that share the common pathophysiological mechanism of deficiency in amino acid transport.

It is important to understand that this code refers to defects in specific amino acid transporters, not to disorders of the metabolism of these compounds after their absorption. The fundamental distinction is that in transport disorders, the enzymatic pathways for degradation or synthesis of amino acids are intact, but there is impairment in the cellular capacity to uptake or transport these compounds across biological membranes. This differentiation is essential for appropriate coding and for correct therapeutic management.

3. When to Use This Code

The code 5C60 should be used in specific clinical situations where there is documented evidence of deficiency in amino acid absorption or transport. Below are detailed practical scenarios:

Scenario 1: Diagnosed cystinuria Patient with recurrent history of kidney stones since childhood or adolescence, with biochemical analysis of stones revealing cystine composition. Urine tests demonstrate increased excretion of cystine, lysine, arginine, and ornithine. Genetic testing confirms mutations in the SLC3A1 or SLC7A9 genes, responsible for the dibasic amino acid transporter in the proximal renal tubule. This is a classic example of renal amino acid transport disorder that requires coding with 5C60.

Scenario 2: Confirmed Hartnup disease Child presenting with intermittent episodes of cerebellar ataxia, photosensitive skin rash similar to pellagra, and delay in neuropsychomotor development. Laboratory investigation reveals increased neutral aminoaciduria, especially of tryptophan, alanine, serine, threonine, and other neutral amino acids. Genetic analysis identifies mutations in the SLC6A19 gene, which encodes the neutral amino acid transporter in the intestine and kidney. This presentation characterizes a specific transport defect and justifies the use of code 5C60.

Scenario 3: Lowe syndrome with aminoaciduria Male infant with congenital cataract, severe hypotonia, and evidence of proximal renal tubulopathy. Metabolic evaluation demonstrates generalized aminoaciduria associated with glycosuria, phosphaturia, and renal tubular acidosis. Although Lowe syndrome is multisystemic, the component of impaired renal amino acid transport justifies additional coding with 5C60 to adequately document this aspect of the clinical presentation.

Scenario 4: Lysinuria with protein intolerance Patient with history of vomiting, food refusal, and episodes of encephalopathy after ingestion of protein-rich meals. Investigation reveals low plasma levels of lysine, arginine, and ornithine, with increased urinary excretion of these amino acids. Liver biopsy shows steatosis and the clinical presentation suggests intermittent hyperammonemia. Molecular testing confirms mutations in the SLC7A7 gene, which encodes the basolateral dibasic amino acid transporter. This is a specific transport disorder codifiable as 5C60.

Scenario 5: Benign familial iminoglycinuria Neonatal screening or routine investigation identifies increased urinary excretion of proline, hydroxyproline, and glycine in an asymptomatic individual. Family evaluation reveals an autosomal recessive inheritance pattern. There are no significant clinical manifestations, but the documented defect in the renal iminoglycine transporter justifies registration with code 5C60 for documentation and family follow-up purposes.

Scenario 6: Aminoaciduria diagnosed in the context of tubulopathy Patient with acquired or hereditary Fanconi syndrome presenting with generalized renal loss of amino acids. When the amino acid transport component is clinically relevant and requires specific documentation, code 5C60 may be used in conjunction with codes for the primary condition to adequately characterize the impairment of amino acid transport.

4. When NOT to Use This Code

It is essential to distinguish amino acid transport disorders from other related metabolic conditions. Code 5C60 should not be used in the following situations:

Amino acid metabolism disorders: When the primary defect is in the enzymes that metabolize amino acids after their cellular absorption, and not in the transporters. For example, phenylketonuria, tyrosinemia, homocystinuria, maple syrup urine disease, and other inborn errors of metabolism should be coded in their specific categories of amino acid metabolic disorders, not as 5C60.

Tryptophan metabolism disorders: As specified in the exclusions, conditions that specifically affect tryptophan metabolism should use code 282654317. Although tryptophan is an amino acid, its metabolic disorders have their own classification due to their importance in serotonin synthesis and other specific pathways.

Protein malnutrition: Low amino acid levels resulting from inadequate protein intake or generalized malabsorption due to intestinal disease do not constitute specific amino acid transport disorders. These cases should be coded as malnutrition or intestinal malabsorption as appropriate.

Aminoaciduria secondary to other conditions: When urinary amino acid loss is a consequence of acute kidney injury, chronic kidney disease, use of nephrotoxic medications, or other conditions that damage the renal tubule in a nonspecific manner, the primary code should reflect the underlying condition, not 5C60.

Hyperammonemia from other causes: Although some amino acid transport disorders may present with hyperammonemia, when this is caused by defects in the urea cycle or liver diseases, the specific codes for these conditions should be used.

5. Step-by-Step Coding Process

Step 1: Assess diagnostic criteria

To confirm that the case fits as an amino acid absorption or transport disorder, it is necessary to document:

Initial clinical evaluation: Identify suggestive symptoms such as recurrent renal lithiasis, intermittent neurological manifestations, photosensitive skin lesions, protein intolerance, developmental delay, or symptoms of renal tubulopathy.

Basic laboratory investigation: Perform plasma and urinary amino acid dosage through chromatography or mass spectrometry. Specific patterns of aminoaciduria or abnormal plasma levels of specific amino acids are fundamental for diagnosis.

Functional tests: In some cases, protein overload tests or with specific amino acids can demonstrate the inability for adequate absorption or reabsorption. Evaluation of renal tubular function with dosage of glucose, phosphate, and urinary bicarbonate can identify associated tubulopathies.

Molecular confirmation: Whenever possible, genetic testing identifying mutations in genes encoding amino acid transporters provides definitive diagnostic confirmation. Genes such as SLC3A1, SLC7A9, SLC6A19, SLC7A7, and others are associated with specific transport disorders.

Step 2: Verify specifiers

After confirming the diagnosis of amino acid transport disorder, it is important to characterize:

Specific type of disorder: Identify which amino acid or group of amino acids is involved (cystine, neutral amino acids, dibasic amino acids, iminoglycine, etc.). This determines whether specific subcategories are available in the coding system.

Clinical severity: Document whether the condition is asymptomatic (as in benign iminoglycinuria), mild with occasional manifestations, moderate with recurrent symptoms, or severe with significant systemic complications.

Present complications: Record complications such as renal lithiasis, renal insufficiency, neurological impairment, dermatological changes, or secondary nutritional disorders.

Inheritance pattern: Document family history and genetic inheritance pattern when known, as this has implications for genetic counseling and family screening.

Step 3: Differentiate from other codes

5C61: Disorders of absorption or transport of carbohydrates Key difference: This code applies when the defect is in monosaccharide (glucose, galactose, fructose) or disaccharide transporters. Typical manifestations include osmotic diarrhea, abdominal distension, and intolerance to specific sugars. Laboratory investigation shows carbohydrate malabsorption, not amino acid malabsorption.

5C62: Disorders of absorption or transport of lipids Key difference: Applies to defects in intestinal fat absorption or lipoprotein transport. Manifestations include steatorrhea, fat-soluble vitamin deficiency, and in some cases, plasma lipid alterations. Amino acids are not primarily involved.

5C63: Disorders of absorption or transport of vitamins or non-protein cofactors Key difference: This code is for specific defects in vitamin (B12, folate, thiamine, biotin, etc.) or mineral transporters. Although they may have neurological or hematological manifestations similar to some amino acid disorders, biochemical investigation identifies specific vitamin deficiency, not aminoaciduria.

Step 4: Required documentation

For appropriate coding with 5C60, the medical record must contain:

Checklist of mandatory information:

Detailed clinical history with symptoms and temporal evolution
Results of plasma and urinary amino acid chromatography
Reports of imaging tests when relevant (renal ultrasound in cases of lithiasis)
Results of genetic testing when available
Evaluation of renal function and other affected systems
Documentation of instituted treatment and therapeutic response
Family history and pedigree when applicable

Appropriate record: The documentation must clearly specify that it is a transport disorder, not amino acid metabolism disorder, describing the specific pattern of aminoaciduria or abnormal plasma levels identified.

6. Complete Practical Example

Clinical Case

A 16-year-old patient seeks medical care for an episode of intense right-sided renal colic. He reports that this is the third similar episode in the past two years, having spontaneously passed small stones on previous occasions. Family history reveals that his father also has recurrent renal lithiasis since youth. On physical examination, he presents with pain on percussion in the right lumbar region, without other significant findings.

Initial investigation:

Renal ultrasound: 4mm stone in right proximal ureter, multiple smaller stones in both kidneys
Analysis of previously passed stone: pure cystine composition
Urinalysis type I: pH 6.0, presence of characteristic hexagonal crystals

Metabolic evaluation:

Urinary amino acid chromatography: increased cystine excretion (>250 mg/g creatinine), lysine, arginine and ornithine
Plasma amino acids: normal levels of all amino acids
Renal function: normal serum creatinine, preserved creatinine clearance
Urinary nitroprusside test: positive for cystine

Genetic investigation:

Sequencing of SLC3A1 and SLC7A9 genes
Mutations identified in compound heterozygosity in the SLC3A1 gene
Diagnostic confirmation of cystinuria type A

Family evaluation:

Father screening: also presents cystinuria, previously undiagnosed
12-year-old sister: asymptomatic carrier of one mutation

Step-by-Step Coding

Criteria analysis:

Confirmation of transport disorder: Cystinuria is caused by a defect in dibasic amino acid transporters in the proximal renal tubule and small intestine. Normal plasma cystine levels with increased urinary excretion confirm that the defect is in renal transport, not metabolism.
Characteristic pattern: Increased excretion of cystine, lysine, arginine and ornithine is typical of a defect in the transport system shared by these dibasic amino acids.
Molecular confirmation: Mutations identified in the SLC3A1 gene confirm the genetic diagnosis of cystinuria.
Clinical manifestation: Recurrent renal lithiasis from cystine crystallization is the primary clinical manifestation of this transport disorder.

Selected code: 5C60

Complete justification:

Code 5C60 (Disorders of amino acid absorption or transport) is the correct code because:

The primary defect is in the amino acid transporter, not in metabolic enzymes
There is clear laboratory documentation of the transport defect (specific aminoaciduria)
Genetic confirmation of the defect in the transporter gene
Does not fit into other categories of metabolic disorders
The clinical manifestation (lithiasis) is a direct consequence of the transport defect

Applicable complementary codes:

Code for recurrent renal lithiasis: to document the primary complication
Z code for family history of renal disease: relevant to the family context
Procedure codes: to document surgeries or urological procedures performed

Documented treatment plan:

Abundant hydration (target urine output >3L/day)
Urinary alkalinization with potassium citrate
Moderate restriction of sodium and proteins
Cystine chelating agent (penicillamine or tiopronin) if conservative measures fail
Regular nephrology follow-up with semi-annual renal ultrasound
Family genetic counseling

7. Related Codes and Differentiation

Within the Same Category

5C61: Disorders of carbohydrate absorption or transport

When to use 5C61 vs. 5C60: Use 5C61 when the primary defect involves sugar transporters such as SGLT1 (sodium-glucose transporter), GLUT5 (fructose transporter), or disaccharidase deficiencies. Typical clinical manifestations include chronic osmotic diarrhea, abdominal distension, and flatulence following ingestion of specific carbohydrates. Diagnostic tests include hydrogen breath test and intestinal biopsy with disaccharidase assay. The main difference is that symptoms are predominantly gastrointestinal and related to sugar ingestion, not proteins.

5C62: Disorders of lipid absorption or transport

When to use 5C62 vs. 5C60: Code 5C62 is appropriate for conditions such as abetalipoproteinemia, hypobetalipoproteinemia, or fatty acid transporter deficiency. Manifestations include significant steatorrhea, deficiency of fat-soluble vitamins (A, D, E, K), retinitis pigmentosa, and peripheral neuropathy. Laboratory findings show very low levels of total cholesterol, LDL, and triglycerides. The fundamental difference is that the defect affects lipid transport, resulting in fat malabsorption, whereas 5C60 specifically involves amino acids.

5C63: Disorders of vitamin or non-protein cofactor absorption or transport

When to use 5C63 vs. 5C60: This code applies to defects in specific vitamin transporters such as Imerslund-Gräsbeck disease (vitamin B12 malabsorption), defects in thiamine, biotin, or folate transport. Manifestations depend on the vitamin involved: megaloblastic anemia in B12 or folate deficiency, beriberi in thiamine deficiency, dermatitis and alopecia in biotin deficiency. The differentiation is clear: specific serum vitamin levels are low, there is no characteristic aminoaciduria, and vitamin supplementation corrects the condition.

Differential Diagnoses

Fanconi syndrome: Although presenting with generalized aminoaciduria, Fanconi syndrome is a global proximal tubulopathy affecting the transport of multiple substances (amino acids, glucose, phosphate, bicarbonate, uric acid). When the diagnosis is Fanconi syndrome, the primary code should reflect this condition, with 5C60 potentially used as an additional code if the amino acid transport component is clinically relevant and requires specific documentation.

Inborn errors of amino acid metabolism: Conditions such as phenylketonuria, tyrosinemia, or maple syrup urine disease cause elevated plasma levels of specific amino acids due to enzymatic defects, not transport defects. Aminoaciduria, when present, is secondary to renal overflow from elevated plasma levels. These cases should not be coded as 5C60.

Protein-calorie malnutrition: Low plasma amino acid levels from inadequate intake do not constitute a transport disorder and should be coded as malnutrition. Dietary history and absence of a characteristic aminoaciduria pattern aid in differentiation.

8. Differences with ICD-10

In ICD-10, amino acid transport disorders were coded primarily in category E72 (Other disorders of amino acid metabolism). Specifically:

E72.0: Disorders of amino acid transport
Subcategories included cystinuria (E72.01), Hartnup disease (E72.02), and others

Main changes in ICD-11:

ICD-11 introduces greater specificity and hierarchical organization by clearly separating transport disorders (code 5C60) from metabolic disorders of amino acids. This separation better reflects distinct pathophysiology: defects in transporters versus defects in metabolic enzymes.

The ICD-11 structure allows better grouping of transport disorders of different metabolites (amino acids, carbohydrates, lipids, vitamins) under a common category, facilitating understanding that they share similar pathophysiological mechanisms, even when affecting different substances.

Practical impact of these changes:

For health information systems, the transition requires updating databases and training coders to understand the new structure. Historical epidemiological studies that used ICD-10 codes will need conversion tables for comparison with data coded in ICD-11.

Clinically, the clearer separation between transport and metabolic disorders facilitates diagnostic and therapeutic targeting, as approaches can differ significantly. Researchers also benefit from more precise classification when studying disease mechanisms and developing targeted therapies.

For administrative and reimbursement purposes, health systems need to recognize the equivalence between old and new codes to ensure continuity in access to specialized treatments and high-cost medications frequently necessary for these rare conditions.

9. Frequently Asked Questions

How is the diagnosis of amino acid transport disorders made?

Diagnosis usually begins with clinical suspicion based on characteristic symptoms such as recurrent renal lithiasis, intermittent neurological manifestations, or specific cutaneous lesions. Confirmation requires quantitative analysis of amino acids in plasma and urine through liquid chromatography or mass spectrometry. Specific patterns of aminoaciduria (increased urinary excretion of specific amino acids with normal or low plasma levels) suggest a transport defect. Definitive confirmation is obtained through genetic testing that identifies mutations in genes encoding amino acid transporters. In some cases, intestinal or renal biopsy with functional studies may be necessary.

Is treatment available in public health systems?

Treatment availability varies according to the health system and the specific condition. Many amino acid transport disorders require relatively simple treatments such as adequate hydration, dietary modifications, and specific supplementation, which are generally available in public systems. More specialized medications, such as cystine chelators for severe cystinuria or specific amino acid supplements, may have variable availability. Rare disease programs in various countries include coverage for these conditions. It is important that the diagnosis be properly documented and coded to facilitate access to treatments through specialized programs.

How long does treatment last?

Most amino acid transport disorders are chronic genetic conditions that require treatment and follow-up throughout life. There is no definitive cure, but appropriate management can prevent or minimize complications. For example, patients with cystinuria require continuous abundant hydration and urinary alkalinization to prevent stone formation. Individuals with Hartnup disease may require prolonged vitamin supplementation. Regular medical follow-up is essential to adjust treatment as needed, monitor complications, and optimize quality of life.

Can this code be used in medical certificates?

Yes, code 5C60 can and should be used in medical certificates when appropriate. For work or school absences related to acute complications (such as renal colic from lithiasis in a patient with cystinuria or encephalopathy episode in a patient with lysinuria), the code appropriately documents the underlying condition. For requests for workplace or school environment adaptations, appropriate coding facilitates the process. In medical reports for disability benefits or rare disease assistance programs, accurate coding is essential to prove the diagnosis.

Are these disorders hereditary?

Yes, the vast majority of amino acid transport disorders are hereditary genetic conditions, usually with an autosomal recessive inheritance pattern. This means that affected individuals inherited mutations from both parents, who are asymptomatic carriers. Some disorders may have autosomal dominant or X-linked inheritance. Genetic counseling is important for affected families, as it allows estimation of recurrence risks, guidance on prenatal testing when available and appropriate, and facilitation of screening of relatives who may be carriers or asymptomatic affected individuals.

Can children with these disorders have normal development?

With early diagnosis and appropriate treatment, many children with amino acid transport disorders can have normal or near-normal neuropsychomotor development. The prognosis depends on the specific condition, the timeliness of diagnosis, and treatment adherence. Some conditions, such as iminoglycinuria, are completely benign and do not affect development. Others, such as Hartnup disease, may cause delay if untreated, but respond well to vitamin supplementation. Cystinuria generally does not affect neurological development, but requires management to prevent renal complications. Neonatal screening and early investigation of suspected symptoms are fundamental to optimize outcomes.

Is it possible to prevent complications of these disorders?

Yes, many complications can be prevented or minimized with appropriate treatment. In cystinuria, abundant hydration, urinary alkalinization, and when necessary, use of cystine chelators can significantly reduce renal stone formation and preserve renal function. In Hartnup disease, niacin supplementation prevents dermatological and neurological manifestations. In lysinuria with protein intolerance, moderate protein restriction and citrulline supplementation can prevent hyperammonemia crises. Regular medical follow-up with laboratory and imaging monitoring allows early detection of complications and timely therapeutic adjustment.

Is there a difference between transport aminoaciduria and overflow aminoaciduria?

Yes, this is a fundamental distinction. Transport aminoaciduria (coded as 5C60) occurs when there is a defect in renal or intestinal amino acid transporters, resulting in increased urinary excretion despite normal or low plasma levels. Overflow aminoaciduria occurs when very elevated plasma levels of amino acids (due to metabolic defects) exceed the capacity of renal tubular reabsorption, resulting in secondary urinary loss. The latter should not be coded as 5C60, but rather with the code of the primary metabolic disorder. Simultaneous measurement of plasma and urinary amino acids allows this crucial differentiation.

Conclusion

Disorders of amino acid absorption or transport represent an important group of rare metabolic conditions that require appropriate clinical recognition, specific laboratory investigation, and accurate coding. The ICD-11 code 5C60 provides the appropriate classification for these conditions, clearly distinguishing them from metabolic disorders of amino acids and other metabolite transport defects. Proper understanding of diagnostic criteria, clinical manifestations, and differentiation of similar conditions is essential for health professionals involved in the diagnosis, treatment, and coding of these diseases. With appropriate management, many patients can have preserved quality of life and prevention of serious complications, highlighting the importance of early diagnosis and continued specialized follow-up.

External References

This article was prepared based on reliable scientific sources:

References verified on 2026-02-03

Disorders of Amino Acid Absorption or Transport

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