The Biology of HLCS and How It Is Diagnosed

To understand Holocarboxylase Synthetase (HLCS) deficiency, it helps to think of biotin (a B-vitamin) as an essential “key” and your body’s enzymes as “machines.” For these machines to work, the key must be inserted and turned. In your child’s body, the HLCS enzyme is the hand that picks up the biotin key and attaches it to several different machines ^[1][2].

When the HLCS enzyme is not working correctly, it cannot attach the biotin key. This means that multiple machines—specifically four different enzymes called carboxylases—all stop working at the same time ^[1]. This is why doctors call this condition a Multiple Carboxylase Deficiency (MCD) ^[2]. Because these machines are responsible for breaking down proteins, fats, and sugars, their failure causes a buildup of toxic “exhaust” or chemical byproducts in the blood and urine ^[2][3].

The Diagnostic Path: From Screening to Confirmation

Most diagnoses begin with Newborn Screening, the heel-prick test done shortly after birth.

• The Initial Marker (C5-OH): The screening lab looks for an elevated level of C5-OH (3-hydroxyisovaleryl carnitine) ^[4]. While a high C5-OH level is a strong signal, it is not a diagnosis on its own because several different conditions can cause this same result ^[5].
• Urine Organic Acid Testing: This is a more detailed test that looks for the specific “exhaust” chemicals mentioned earlier. If HLCS is the cause, doctors typically see high levels of 3-hydroxyisovaleric acid, 3-methylcrotonylglycine, 3-hydroxypropionate, and methylcitric acid ^[6][7]. High levels of lactate or pyruvate may also be present, especially if the baby is unwell ^[7].
• Genetic Testing: To finalize the diagnosis, doctors look for changes (pathogenic variants) in the HLCS gene. Since this is an autosomal recessive condition, a child must inherit two non-working copies of the gene (one from each parent) to have the deficiency ^[2][8].

Ruling Out “Look-Alike” Conditions

Because the chemical markers for HLCS deficiency can look very similar to other disorders, your doctor must carefully rule out other possibilities.

1. Biotinidase Deficiency (BTD): This is the most common look-alike. While HLCS is the “hand” that attaches biotin to enzymes, Biotinidase is the “recycler” that frees biotin from food and old enzymes so it can be used again ^[9][10]. Because both affect biotin, the symptoms and urine tests can look identical. A specific test for BTD enzyme activity is required to distinguish between the two ^[11].
2. Isolated Carboxylase Deficiencies: Conditions like 3-methylcrotonyl-CoA carboxylase deficiency are important causes of elevated C5-OH and closely resemble the newborn screening footprint of HLCS ^[5].
3. Mitochondrial Disorders: If a child does not respond to biotin treatment as expected, doctors may investigate broader mitochondrial testing as a specialist-directed evaluation to rule out rarer genetic causes ^[12][13].

Diagnostic Components

When reviewing your child’s medical records, the following are tests the metabolic team may use to evaluate and confirm the diagnosis. A diagnosis may sometimes be made even if a genetic test result is incomplete.

• C5-OH Level: Elevated on newborn screening or follow-up blood work ^[4].
• Urine Organic Acids: Elevations in markers like 3-hydroxyisovaleric acid and methylcitrate ^[6].
• Biotinidase (BTD) Activity: Confirming that the biotin-recycling enzyme is working normally (ruling out BTD deficiency) ^[11].
• Genetic Confirmation: Identifying pathogenic or likely pathogenic variants in the HLCS gene ^[2].
• Baseline Labs: Ammonia, lactate, and blood pH, which may show metabolic acidosis ^[6].