What is the role of Methylmalonic Acid (MMA) in diagnosing vitamin B12 deficiency?

Methylmalonic Acid (MMA) in Diagnosing Vitamin B12 Deficiency

Methylmalonic acid (MMA) is a crucial functional marker for vitamin B12 deficiency that provides superior diagnostic accuracy compared to serum B12 levels alone, and should be used in combination with holotranscobalamin (holo-TC) for optimal diagnosis of B12 deficiency. 1

What is MMA?

Methylmalonic acid is a metabolic intermediate that accumulates in the blood when there is insufficient vitamin B12. In normal metabolism:

• Adenosylcobalamin (a form of B12) serves as a coenzyme for methylmalonyl-CoA mutase
• This enzyme converts methylmalonyl-CoA to succinyl-CoA
• When B12 is deficient, methylmalonyl-CoA accumulates and is hydrolyzed to MMA 2

Diagnostic Value of MMA

Advantages over standard B12 testing:

• MMA is a functional marker that reflects metabolic B12 deficiency
• Can detect B12 deficiency even when serum B12 levels appear normal
• MMA levels >271 nmol/L strongly suggest functional vitamin B12 deficiency 1
• Studies show significant percentages of patients with normal serum B12 but metabolic deficiency:
  • 50% in the Framingham Study
  • 15% in the NHANES III study 1

Interpretation of MMA results:

• Elevated MMA (>271 nmol/L) indicates functional B12 deficiency
• MMA has stronger associations with poor functional status and physical performance than serum B12 levels 3
• MMA concentrations are independently associated with mortality risk, beyond just reflecting B12 status 4

Optimal Diagnostic Approach

For accurate diagnosis of vitamin B12 deficiency:

1. Use a combination of biomarkers: At least two biomarkers (holo-TC and MMA) provide optimal assessment 2, 1
2. Interpretation guidelines:
   • Holotranscobalamin levels:
     • <25 pmol/L: Confirmed B12 deficiency
     • 25-70 pmol/L: Possible B12 deficiency (requires MMA testing)

     • 70 pmol/L: B12 deficiency unlikely 1

   • Vitamin B12 levels:
     • <180 ng/L: Confirmed B12 deficiency
     • 180-350 ng/L: Indeterminate (requires MMA testing)

     • 350 ng/L: B12 deficiency unlikely 1

When to Test MMA

MMA testing is recommended for:

• Patients with symptoms or signs of B12 deficiency
• Patients with indeterminate total B12 or active B12 results
• Patients presenting with anemia or isolated macrocytosis
• Patients with established diagnosis of polyneuropathies, neurodegenerative diseases, or psychosis 2, 1

Important Considerations

Factors affecting MMA levels:

• Renal function significantly impacts MMA levels (impaired renal function can elevate MMA)
• Only 22% of variation in MMA levels is explained by vitamin B12, eGFR, age, and sex 4
• Other factors affecting MMA include catabolism, dietary components, and gut microbial production

Technical aspects:

• MMA can be measured using:
  • Liquid chromatography-tandem mass spectrometry (LC-MS/MS)
  • Gas chromatography-mass spectrometry (GC-MS)
• LC-MS/MS offers advantages including time and cost savings, precision, and flexibility 5
• Modern methods require minimal sample volumes (as little as 25 μL of serum) 6

Common Pitfalls to Avoid

• Relying solely on serum B12: May miss functional deficiency
• Not considering renal function: Impaired renal function can elevate MMA levels
• Using MMA alone: Best used in combination with holo-TC
• Delaying treatment: In symptomatic patients, delaying treatment can lead to irreversible neurological damage 1
• Missing concurrent folate deficiency: Should be checked and treated if present, but B12 deficiency should always be addressed first 1

Monitoring

• Recheck holo-TC and MMA levels after 1 month of treatment
• Long-term monitoring should be performed at least annually 2, 1
• Monitor clinical symptoms for improvement

MMA testing, while more expensive than standard B12 testing, may be cost-effective compared to not testing, with an incremental cost-effectiveness ratio of £3946 per quality-adjusted life year 1.