Holotranscobalamin – what the test shows and how it differs from regular B12

What is S-holotranscobalamin?

Holotranscobalamin (HTC) is the active form of vitamin B12 found in the blood, i.e. the form that can actually be absorbed and used by the body's cells. Vitamin B12, also known as cobalamin, is transported in the blood bound to two main proteins:

• Transcobalamin (TC) – binds approximately 20-30% of B12 and forms holotranscobalamin, i.e. the active and biologically available form of the vitamin. HTC is the fraction of B12 that can be delivered to the tissues and participate in enzymatic reactions, for example in the synthesis of DNA and the metabolism of homocysteine ​​and methylmalonyl-CoA.
• Haptocorrin (HC) – binds the majority of B12 but this form is inactive and cannot be directly utilized by the cells. The haptocorrin-bound B12 is mainly transported to the liver and excreted in the bile. HC acts as a reservoir protein but does not reflect the functional B12 status in the body.

Holotranscobalamin has a shorter half-life in the blood than total B12 (about 6 hours compared to several days for total B12), making it an early indicator of changes in B12 status.

What is the difference between S-holotranscobalamin and total B12?

Total B12 shows the sum of both active and inactive B12, while S-holotranscobalamin only measures the amount of active, bioavailable B12 in the blood. This means that S-holotranscobalamin provides a more direct picture of how much functional B12 the body has available. Because the active fraction decreases faster than total B12, S-holotranscobalamin can reveal an early B12 deficiency even when total B12 is still normal.

Clinically, S-holotranscobalamin is often used together with other biomarkers, for example:

• Homocysteine – elevated levels may indicate functional B12 deficiency.
• Methylmalonic acid (MMA) – increases in B12 deficiency and provides information about intracellular B12 deficiency.

Why test S-holotranscobalamin?

The test is mainly used to confirm suspected B12 deficiency, especially when total B12 is normal but symptoms suggest deficiency. S-holotranscobalamin can also be used to:

• Identify early B12 deficiency before the total amount of B12 has decreased.
• Investigate symptoms such as fatigue, dizziness, numbness, memory problems or depression.
• Measure B12 status in people with impaired absorption, for example in celiac disease, inflammatory bowel disease, gastritis, after gastric bypass or in vegetarians/vegans.
• Follow up treatment with B12 supplements, both oral and intravenous, as well as drugs that can affect absorption (e.g. metformin or proton pump inhibitors).
• Provide early guidance in neurological symptoms where B12 deficiency is suspected, since low levels of holotranscobalamin can occur before clinical signs become apparent.

Normal levels and clinical interpretation

Reference ranges may vary between laboratories, but typical levels of S-holotranscobalamin are approximately 35–90 pmol/L.
- <35 pmol/L: often indicates B12 deficiency and should be investigated further.
- 35–50 pmol/L: may indicate early B12 deficiency, especially if symptoms or other biomarkers such as MMA or homocysteine ​​are elevated.
- >50 pmol/L: normally active B12.

When should I get tested?

A S-holotranscobalamin test is relevant if you experience:

• Lack of energy or prolonged fatigue without a clear cause
• Tingling, numbness or impaired balance
• Depression, mood swings or difficulty concentrating
• Palour, palpitations or shortness of breath

Can S-holotranscobalamin indicate autism?

No, the test is not used to detect or diagnose autism. It only measures the amount of active vitamin B12. Low levels can affect brain and nervous system function, but there are no scientifically established links between S-holotranscobalamin and neuropsychiatric conditions such as autism.