The C5-OH cluster

A handful of metabolic conditions share a single newborn screening flag: an elevated 3-hydroxyisovalerylcarnitine, C5-OH, on tandem mass spectrometry of the dried blood spot. The shared marker reflects shared biochemistry. Five autosomal recessive disorders, each rare and each with a distinct clinical phenotype, all funnel through the C5-OH signal and are then sorted in second-tier testing. The cluster is a useful object for a different reason. It demonstrates how an expanded newborn screening panel actually works in practice. The model is one signal per pathway, with confirmatory testing doing the disease-specific work.

This piece is a reference. Each condition has its own page.

What C5-OH is and what produces it

3-hydroxyisovalerylcarnitine, C5-OH, is the carnitine ester of 3-hydroxyisovaleric acid. The five-carbon ketoacid arises in the leucine catabolic pathway and at branch points of biotin-dependent carboxylase chemistry. When any of the enzymes downstream of 3-methylcrotonyl-CoA fails, or when the biotin recycling or biotin-attachment chemistry fails, 3-hydroxyisovaleric acid accumulates and is exported as the carnitine ester to the bloodstream. The dried blood spot tandem mass spectrometry assay detects the elevation reliably.

A sixth condition, biotinidase deficiency, can also flag C5-OH but is more reliably identified through its dedicated enzyme activity assay on the dried blood spot. That condition is covered separately.

The conditions that funnel through C5-OH:

3-Methylcrotonyl-CoA carboxylase deficiency, 3-MCC

Variants in MCCC1 or MCCC2. The fourth step of leucine catabolism. The clinical phenotype is broad. Many people identified by screening are asymptomatic. A subset present in infancy or during catabolic illness with hypoglycemia, metabolic acidosis, hyperammonemia, vomiting, lethargy, and hypotonia. The wide range, including a substantial fraction of biochemically affected people who never develop symptoms, is the central clinical feature. Maternal 3-MCC is a recognized cause of an elevated newborn C5-OH where the affected person is the mother rather than the baby.

Distinguishing on second-tier testing: urine organic acids show elevated 3-methylcrotonylglycine and 3-hydroxyisovaleric acid without methylcitrate, and gene panels return MCCC1 or MCCC2 variants. Standard of care is supportive, with L-carnitine in selected cases, sick-day protocols, and dietary protein restriction reserved for symptomatic cases.

Holocarboxylase synthase deficiency, HLCS

Biallelic variants in HLCS, encoding the enzyme that attaches biotin to four downstream carboxylases. The neonatal form of multiple carboxylase deficiency. Lactic acidosis, organic aciduria, ketoacidosis, hypotonia, characteristic skin rash, and alopecia in the first weeks to months of life.

Distinguishing on second-tier testing: urine organic acids show elevated 3-methylcrotonylglycine, 3-hydroxyisovaleric acid, methylcitrate, and lactate. Plasma propionylcarnitine (C3) is often elevated alongside C5-OH. Biotinidase activity is normal, distinguishing HLCS from biotinidase deficiency. HLCS sequencing confirms.

Standard of care is lifelong oral biotin at pharmacological doses. Most cases respond completely or near-completely.

Beta-ketothiolase deficiency, BKT

Biallelic variants in ACAT1, encoding mitochondrial acetoacetyl-CoA thiolase, an enzyme in isoleucine catabolism and ketone body utilization. Episodic ketoacidosis between months 6 and 24 of life, triggered by infection or fasting. Children appear well between episodes.

Distinguishing on second-tier testing: urine organic acids show 2-methyl-3-hydroxybutyric acid, 2-methylacetoacetic acid, and tiglylglycine. Plasma acylcarnitines show elevated tiglylcarnitine (C5:1) alongside C5-OH. ACAT1 sequencing confirms.

Standard of care is avoidance of prolonged fasting and aggressive sick-day management. Most affected children identified by screening reach adulthood without significant cognitive sequelae.

3-Hydroxy-3-methylglutaric aciduria, HMG

Biallelic variants in HMGCL, encoding the final enzyme of leucine catabolism and ketogenesis. The crisis biology is hypoketotic hypoglycemia rather than ketoacidosis, because affected children cannot make ketones during fasting. The picture can mimic Reye syndrome.

Distinguishing on second-tier testing: urine organic acids show 3-hydroxy-3-methylglutaric acid, 3-methylglutaconic acid, 3-methylglutaric acid, and 3-hydroxyisovaleric acid. Plasma acylcarnitines show elevated 3-methylglutarylcarnitine (C6DC) alongside C5-OH. HMGCL sequencing confirms.

Standard of care is avoidance of prolonged fasting and aggressive sick-day management with intravenous dextrose during illness. Modest fat or protein restriction is used in some centers.

Isovaleric acidemia, IVA

Variants in IVD, the third enzyme of leucine catabolism. Two phenotypes: severe neonatal-onset metabolic crisis with the characteristic sweaty-feet odor of accumulated isovaleric acid, and chronic intermittent disease with later episodes. A common mild variant identified through expanded screening, c.932C>T, often does not produce clinical disease.

Distinguishing on second-tier testing: urine organic acids show elevated isovalerylglycine and 3-hydroxyisovaleric acid. Plasma acylcarnitines show elevated isovalerylcarnitine (C5) and the C5/C5-OH pattern is distinctive. IVD sequencing confirms.

Standard of care is a leucine-restricted diet, oral glycine, oral L-carnitine, and sick-day protocols. Outcomes in screen-detected cases are generally excellent.

What the cluster shows

The C5-OH cluster is a small example of a structural feature of expanded newborn screening. The original screening assays were one-test-per-disease. The shift to tandem mass spectrometry in the late 1990s and early 2000s allowed simultaneous measurement of dozens of acylcarnitine and amino acid markers. Each marker reflects metabolic flux at a particular pathway node, and disease-specific identification depends on the pattern across markers plus second-tier testing rather than on a single result.

The trade-off is broader detection of a wider range of conditions, balanced against an increase in incidental findings and biochemical abnormalities of uncertain clinical significance. The 3-MCC question of whether asymptomatic biochemical disease should be called disease at all is the cluster's clearest illustration.