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PubMed This is a summary of 16 peer-reviewed journal articles Updated
Pediatric Hepatology

The Biology and Getting the Right Diagnosis

At a Glance

CBAS5 is confirmed by finding abnormal bile acids such as THCA and DHCA on specialized testing together with disease-causing changes in both copies of ABCD3. Normal or low GGT can be a clue, but it never rules out urgent evaluation for biliary atresia.

To understand congenital bile acid synthesis defect 5 (CBAS5), it helps to think of the liver as a factory that refines “raw” bile acids into a finished product. In children with CBAS5, the refining process is interrupted because of a missing biological “gatekeeper” [1][2]. While this analogy is a simplification, it helps explain why standard liver labs can be misleading.

Glossary of Key Terms

  • GGT: An enzyme often elevated in liver blockages, but usually normal or low in CBAS5.
  • THCA/DHCA: Toxic, unfinished bile acid intermediates that build up in CBAS5.
  • Pathogenic Variant: A genetic change known to cause disease.
  • LC-MS/FAB-MS: Highly sensitive tests used to detect specific bile acids.

The Peroxisome and the PMP70 Gatekeeper

Inside your liver cells are tiny compartments called peroxisomes. These compartments are responsible for the final steps of making bile acids [3][1]. To get inside these compartments, raw bile acids must pass through a transporter protein called PMP70, which is created by the ABCD3 gene [1][2].

In CBAS5, the ABCD3 gene has disease-causing variants, meaning the PMP70 gatekeeper has reduced or absent function [1]. Because the raw bile acids cannot get into the peroxisome to be refined, they remain as “unfinished” intermediates called THCA and DHCA [4][5]. These intermediates are not just useless; they can build up and become toxic to the liver [4].

The Danger of Misdiagnosis: CBAS5 vs. Biliary Atresia

One of the most critical parts of early infant liver disease is distinguishing it from biliary atresia (BA). BA is a time-sensitive condition where the bile ducts outside the liver are blocked or missing [6]. Because both conditions cause jaundice and pale stools in infants, they can look identical on the surface [7][8].

When you look at your child’s lab reports, there are two key markers that often differ between these two conditions:

  • Gamma-Glutamyl Transferase (GGT): In biliary atresia, GGT is almost always very high [6][9]. In CBAS5 and related bile acid defects, the GGT level is often surprisingly normal or low, even when the child is very jaundiced [10][11].
  • Total Serum Bile Acids: The total bile acid count might appear normal or low because routine assays detect some abnormal bile acids variably or only look for finished bile acids, rather than the toxic intermediates (THCA/DHCA) that are actually present [10][12].

Crucially, a normal or low GGT does NOT rule out biliary atresia. Biliary atresia can have atypical lab results and requires an urgent surgery called the Kasai procedure to create a new drainage path [13]. If your child’s GGT is low but they are still jaundiced, the metabolic workup and the time-sensitive surgical evaluation for biliary atresia must happen urgently and in parallel [12]. Never delay a surgical evaluation while waiting for metabolic tests.

Definitive Diagnostic Tests

To confirm a diagnosis of CBAS5, doctors use specialized tests in conjunction with clinical findings:

  1. Specialized Bile-Acid Profile: A sensitive test (such as LC-MS or FAB-MS) that specifically looks for the elevated “unfinished” bile acids, THCA and DHCA in blood or urine [4][14].
  2. Genetic Testing: This supports the diagnosis when finding appropriately classified pathogenic or likely pathogenic variants on both copies of the ABCD3 gene, confirming their phase (one from each parent), and ensuring the genetic result fits the biochemical profile [15][16].

Liver biopsies may also be performed, but they often show nonspecific findings like “giant cell hepatitis,” which occurs in many infant liver diseases and does not automatically confirm CBAS5 [12]. Getting the correct expert interpretation of both the genetic and biochemical tests ensures your child receives the right care from the start.

Common questions in this guide

What does a normal or low GGT mean when a baby is jaundiced?
A normal or low GGT can occur in CBAS5 because the problem is with bile-acid processing rather than the typical blockage pattern seen in biliary atresia. It is an important clue, but it does not rule out biliary atresia, so urgent surgical evaluation is still needed.
How do doctors confirm congenital bile acid synthesis defect 5?
Doctors combine a specialized blood or urine bile-acid profile with genetic testing. LC-MS or FAB-MS can detect the unfinished bile acids THCA and DHCA, while genetic testing looks for disease-causing or likely disease-causing variants in both copies of ABCD3, with one copy inherited from each parent.
Why can total bile acids look normal in CBAS5?
Routine total bile-acid tests may detect finished bile acids more reliably than the abnormal intermediates that build up in CBAS5. As a result, the total value can be normal or low even when THCA and DHCA are present, so specialized testing may be needed.
What do ABCD3 and PMP70 do in this condition?
The ABCD3 gene makes PMP70, a transporter that helps move raw bile acids into peroxisomes inside liver cells. When ABCD3 changes reduce or eliminate PMP70 function, THCA and DHCA cannot be processed normally and may accumulate.
Can a liver biopsy by itself diagnose CBAS5?
No. A biopsy may show giant cell hepatitis or ductular proliferation, but these findings can occur in several infant liver diseases and are not specific for CBAS5. The diagnosis depends on the biochemical profile, genetic results, and clinical context.
Why does a baby with suspected CBAS5 need an urgent biliary atresia evaluation?
Biliary atresia can cause jaundice and pale stools that look like CBAS5, and it may require a time-sensitive Kasai procedure to establish bile drainage. A low or normal GGT does not safely exclude it, so surgical assessment and metabolic testing should proceed in parallel.

Get notified when new evidence is published on congenital bile acid synthesis defect 5.

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References

References (16)
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This page is for informational purposes only and does not constitute medical advice about your child's jaundice or test results. If biliary atresia is possible, seek urgent guidance from your child's pediatric liver and surgical teams.

Guide contents congenital bile acid synthesis defect 5
Research & Literature
02 Symptoms and Urgent Warning Signs 04 Standard of Care Treatment