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Vitamin B12 - Active
Vitamin
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No Fasting Required
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Measures only the specific, biologically available portion of B12 (holotranscobalamin) that cells can actually absorb and use.
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Vitamin B12 - Active
- Why is it done?
- Measures the active (holotranscobalamin) form of vitamin B12, which is the biologically available portion actively utilized by cells for energy metabolism, DNA synthesis, and neurological function
- Detects B12 deficiency earlier than total B12 testing, as it reflects only the bioavailable portion of B12
- Evaluates patients with symptoms of B12 deficiency including fatigue, weakness, numbness, tingling, cognitive changes, or anemia
- Ordered to assess absorption disorders such as pernicious anemia, celiac disease, Crohn's disease, or post-gastrointestinal surgery
- Monitors patients on long-term medications affecting B12 absorption such as metformin, proton pump inhibitors, or H2 blockers
- Useful in vegetarians, vegans, or elderly patients at higher risk for B12 deficiency
- Normal Range
- Normal Active B12 (holotranscobalamin): >35 pmol/L (>47 pg/mL)
- Reference ranges may vary slightly by laboratory; specific ranges should always be verified from the testing facility
- Normal result: Active B12 levels within the normal range indicate adequate bioavailable B12 for cellular function
- Low result (<35 pmol/L): Indicates B12 deficiency affecting the actively available B12 pool; suggests need for supplementation or investigation of absorption issues
- Borderline values (25-35 pmol/L): May warrant careful clinical correlation and repeat testing or further workup
- Interpretation
- Elevated Active B12: Usually not clinically significant; rarely occurs naturally but may be seen in certain myeloproliferative disorders or B12 supplementation
- Decreased Active B12: Indicates clinically significant B12 deficiency; represents the bioavailable fraction directly involved in metabolic processes
- Greater sensitivity than total B12: Active B12 may be low while total B12 is normal, making it superior for detecting early or subtle deficiencies
- Factors affecting results include: adequate dietary B12 intake, intrinsic factor production, gastric acid secretion, terminal ileum absorption capacity, and B12 transport proteins
- Clinical significance: Low active B12 correlates with risk for megaloblastic anemia, subacute combined degeneration, peripheral neuropathy, and cognitive dysfunction if left untreated
- Discordance between active and total B12: Total B12 may be normal while active B12 is low due to increased inactive B12-binding proteins
- Associated Organs
- Gastrointestinal tract: Primary site of B12 absorption; disorders here directly impair B12 status (stomach, terminal ileum most critical)
- Nervous system: B12 essential for myelin formation and neurological function; deficiency causes peripheral neuropathy, paresthesias, weakness, ataxia, and cognitive changes
- Blood-forming organs (bone marrow): B12 deficiency impairs DNA synthesis leading to megaloblastic anemia with abnormal red and white blood cell production
- Liver: Stores B12; chronic deficiency may affect hepatic metabolism and vitamin storage capacity
- Common conditions associated with low active B12: pernicious anemia, intrinsic factor antibodies, atrophic gastritis, celiac disease, Crohn's disease, bacterial overgrowth, parasitic infections
- Post-gastrectomy or ileal resection patients at high risk due to loss of intrinsic factor or absorptive surface
- Potential complications of untreated deficiency: subacute combined degeneration, irreversible neurological damage, severe anemia, cardiac complications, cognitive decline
- Follow-up Tests
- Total B12 level: Complements active B12 testing; helpful when results are discordant to assess total body B12 stores
- Methylmalonic acid (MMA): Increased levels confirm functional B12 deficiency; sensitive marker of B12 deficiency at cellular level
- Homocysteine: Elevated in B12 deficiency; indicates impaired methionine metabolism and increased cardiovascular risk
- Complete blood count (CBC): Detects megaloblastic anemia with elevated MCV, abnormal RBC indices, and reduced RBC count
- Intrinsic factor antibodies: Confirms pernicious anemia as underlying cause; positive result indicates autoimmune B12 malabsorption
- Parietal cell antibodies: May be present in pernicious anemia; supports autoimmune etiology
- Schilling test: May be performed to assess B12 absorption and differentiate intrinsic factor deficiency from other causes (less commonly used now)
- Peripheral blood smear: Visualizes megaloblastic changes and hypersegmented neutrophils characteristic of B12 deficiency
- Monitoring frequency: After initiating B12 supplementation, recheck active B12 and CBC within 4-6 weeks; reassess annually if on maintenance therapy
- Folate level: B12 and folate metabolism are linked; deficiency in either can cause similar anemia; often ordered together
- Fasting Required?
- Fasting: No
- Food and drink: No fasting required; the test can be performed at any time without dietary restrictions
- Medications: May continue all regular medications including B12 supplements as directed; inform phlebotomist of current B12 therapy
- Timing note: If recently started on B12 supplementation, test results may be affected; optimal timing is usually before starting treatment or at least 5-7 days after last B12 injection
- Sample collection: Simple blood draw from arm vein; no special preparation needed
- Patient position: Can remain seated or lie down as comfortable during blood draw
- Stress factors: Minimal stress or strenuous activity before test is ideal but not mandatory
How our test process works!
