Is a serum vitamin B12 level of 397 considered borderline low?

Vitamin B12 Level of 397 ng/L is Not Borderline Low

A serum vitamin B12 level of 397 ng/L (293 pmol/L) is not considered borderline low and suggests vitamin B12 deficiency is unlikely according to current guidelines. 1

Interpretation of Vitamin B12 Levels

According to the 2024 NICE guidelines, vitamin B12 levels are interpreted as follows:

Total B12 concentrations	Likelihood of vitamin B12 deficiency
Less than 180 ng/L (133 pmol/L)	Confirmed vitamin B12 deficiency
180-350 ng/L (133-258 pmol/L)	Indeterminate test result – possible vitamin B12 deficiency
More than 350 ng/L (258 pmol/L)	Test result suggests vitamin B12 deficiency is unlikely

With a value of 397 ng/L, this falls clearly into the "vitamin B12 deficiency is unlikely" category, as it exceeds the upper threshold of 350 ng/L (258 pmol/L) 1.

Clinical Decision Algorithm

1. Initial Assessment:

   • A B12 level of 397 ng/L indicates B12 deficiency is unlikely
   • No further testing is required if the patient is asymptomatic

2. If Neurological Symptoms Are Present Despite Normal B12 Levels:

   • Consider measuring methylmalonic acid (MMA) and homocysteine levels as functional markers of B12 status
   • Neurological symptoms can sometimes occur even with B12 levels in the normal range 2, 3

3. Risk Factor Assessment:

   • Even with normal B12 levels, consider risk factors for deficiency:
     • Gastric or small intestine resections
     • Inflammatory bowel disease
     • Metformin use for more than 4 months
     • Proton pump inhibitor or H2 blocker use for more than 12 months
     • Vegan or strict vegetarian diet
     • Age over 75 years 4

Important Clinical Considerations

Functional B12 Deficiency

Some patients may have normal serum B12 levels but still have functional B12 deficiency. This can be identified by elevated methylmalonic acid (MMA) and homocysteine levels 3, 5. This is particularly important in patients with neurological symptoms despite normal B12 levels.

Neurological Manifestations

Neurological symptoms may be the earliest and sometimes the only manifestation of B12 deficiency, even when serum levels appear normal 2. B12-responsive neuropathy can present as:

• Sensorimotor axonal polyneuropathy (predominantly sensory)
• Sensory neuronopathy
• Cognitive difficulties or "brain fog"
• Balance issues and impaired proprioception 1, 2

Common Pitfalls to Avoid

1. Relying solely on serum B12 levels: While your level of 397 ng/L is not borderline low, if you have neurological symptoms, additional testing with MMA and homocysteine may be warranted 3, 6.

2. Overlooking B12 deficiency in elderly patients: Symptoms may be attributed to aging or comorbidities rather than B12 deficiency 5.

3. Inadequate treatment of neurological B12 deficiency: Patients with neurological manifestations of B12 deficiency often require parenteral (injectable) B12 supplementation, even if serum levels appear normal 3.

In summary, a vitamin B12 level of 397 ng/L is not borderline low according to current guidelines. However, if you have neurological symptoms or significant risk factors for B12 deficiency, further evaluation with MMA and homocysteine levels may be appropriate.