The Diagnostic Path: Testing and Look-Alikes

The journey to a diagnosis for CBAS1 (HSD3B7 deficiency) often feels like a race against time, especially when doctors are trying to rule out other serious conditions that require urgent surgery. Because the symptoms of many neonatal liver diseases look similar on the surface, doctors rely on a specific set of laboratory “fingerprints” to identify this rare condition ^[1][2].

The Laboratory Fingerprint

In most cases of infant liver disease, blood markers like GGT (gamma-glutamyl transferase) and serum bile acids are very high. However, CBAS1 behaves differently, which can be confusing for families.

The two key diagnostic clues your doctor will look for in blood work are:

1. Normal or Low GGT: Even when the liver is struggling and the skin is yellow (cholestasis), the GGT level in CBAS1 typically stays within or below the normal range ^[1][3].
2. Normal or Low Serum Bile Acids: While most liver blocks cause bile acids to flood the bloodstream, the liver in CBAS1 cannot make normal bile acids in the first place. Therefore, the total bile acid level in the blood is often unexpectedly low or normal ^[3][4].

If your child has high bilirubin (jaundice) but normal GGT and low bile acids, it is a strong signal to test for a bile acid synthesis defect ^[1]. However, these are useful clues, not rules that confirm CBAS1 or exclude another disease.

Definitive Testing: Urine and Genetics

While blood tests provide clues, highly specialized tests are needed to confirm the diagnosis.

• Urine Mass Spectrometry (FAB-MS or LC-MS/MS): This is often considered the most important biochemical test. It looks for “atypical” bile acids that the body makes when the HSD3B7 enzyme is missing ^[5]. On a lab report, you may see mentions of 3β-hydroxy-Δ5 bile-acid sulfates or 3β-sulfated-Δ5-cholenoic acids ^[4]. In children with CBAS1, these abnormal metabolites are vastly higher than in healthy children ^[5][4].
• Genetic Testing: A definitive diagnosis requires compatible clinical findings plus finding pathogenic or likely pathogenic variants (changes) in both copies of the HSD3B7 gene ^[6]. This test proves the underlying cause is a genetic blueprint error. Finding only one variant or a variant of uncertain significance does not establish CBAS1.

Ruling Out “Look-Alike” Conditions

Because treatment for CBAS1 is very different from other liver diseases, doctors must carefully distinguish it from its “look-alikes”:

• Biliary Atresia: This is a life-threatening blockage of the bile ducts that requires a time-sensitive surgery (the Kasai procedure). Unlike CBAS1, biliary atresia usually causes a very high GGT ^[7]. However, a low GGT or a normal routine ultrasound does not rule it out. The hepatology or surgical team must determine if urgent evaluation—which may include specialized imaging or an operative cholangiogram—is necessary to definitively ensure the bile ducts are physically open ^[8][9].
• PFIC (Progressive Familial Intrahepatic Cholestasis): These are other genetic liver diseases. While some types (like PFIC1) also have a low GGT, they may have different serum bile acid profiles and require different management ^[10][11].

What a Liver Biopsy Can (and Cannot) Tell Us

Your child may undergo a liver biopsy, where a tiny piece of liver tissue is examined under a microscope. In CBAS1, the biopsy often shows:

• Giant-cell transformation: Large, abnormal liver cells that are common in many infant liver diseases ^[12].
• Bile plugs: Small deposits of bile stuck in the liver’s microscopic channels ^[1].

While these findings show the liver is under stress, they are not unique to CBAS1. The biopsy helps the medical team see how much damage or scarring (fibrosis) is present, but the urine and genetic tests are what provide the final name for the condition ^[1][13].