Differentiating True Vitamin B12 Excess from Macro-B12 Interference
When encountering unexpectedly elevated B12 levels (>554 pmol/L or >750 pg/mL) without supplementation, perform polyethylene glycol (PEG) precipitation to detect macro-B12, as this interference is present in 18-45% of cases with elevated B12 and can lead to unnecessary investigations if not identified. 1, 2
Initial Clinical Assessment
When faced with elevated B12 levels, first determine if the elevation is explained by:
- Active B12 supplementation or recent intramuscular injections - timing of blood draw relative to administration is critical 3, 4
- Hematologic malignancies - particularly myeloproliferative disorders with eosinophilia, where B12 elevation is common with PDGFRA fusion genes 5
- Liver disease - hepatic dysfunction causes release of stored B12 3
- Alcoholism - associated with elevated B12 independent of supplementation 3
- Critical illness - higher B12 values observed in critically ill patients, with highest levels in non-survivors 3
Diagnostic Algorithm for Unexplained Elevated B12
Step 1: Screen for Macro-B12 with PEG Precipitation
Perform PEG precipitation on all samples with B12 >554 pmol/L (>750 pg/mL) when no clear cause is identified. 1, 2
- Recovery <68% or post-PEG level <122 pmol/L indicates macro-B12 interference 1
- Macro-B12 prevalence is 18-30% in samples with elevated B12 1
- The lowest interfered B12 result reported was 661 pmol/L (895 pg/mL) 2
Step 2: Confirm with Specific Testing (if PEG suggests interference)
If macro-B12 is suspected:
- Size-exclusion chromatography (SEC) can confirm immunoglobulin-B12 complexes - this is the gold standard for identifying macro-B12 1
- Measure haptocorrin and transcobalamin (holotranscobalamin) immunologically - elevated levels of these binding proteins cause true B12 elevation, not interference 6
- Both Cobas and Architect immunoassay platforms are affected by macro-B12 1
Step 3: Evaluate for Underlying Pathology (if true elevation confirmed)
If PEG precipitation shows normal recovery (>68%) and post-PEG B12 remains elevated, pursue workup for true B12 excess: 1
- Complete blood count with differential - assess for eosinophilia, dysplasia, monocytosis, or circulating blasts 5
- Comprehensive metabolic panel with liver function tests - evaluate for hepatic disease 5
- Serum tryptase - often elevated alongside B12 in myeloproliferative disorders 5
- Peripheral blood smear review - look for morphologic abnormalities 5
Step 4: Advanced Workup for Persistent True Elevation
If hematologic malignancy is suspected:
- Bone marrow aspirate and biopsy with immunohistochemistry (CD117, CD25, tryptase, reticulin/collagen stains) 5
- Cytogenetic and molecular testing - FISH and/or RT-PCR for TK fusion gene rearrangements 5
- Periodic monitoring with CBC and liver function tests if initial workup negative but B12 persistently elevated 5
Critical Clinical Pitfalls
Do not confuse elevated B12 with B12 deficiency - levels >350 ng/258 pmol/L are not indicative of deficiency 5. The diagnostic approach for elevated B12 focuses on identifying underlying pathology, not on methylmalonic acid testing which is reserved for suspected deficiency 5.
Macro-B12 does not guarantee adequate B12 stores - patients with macro-B12 interference may actually be B12 deficient despite falsely elevated serum levels 7. If clinical suspicion for deficiency exists (neurologic symptoms, macrocytic anemia, glossitis), measure B12 after PEG precipitation to determine true status 7.
Avoid extensive cancer workups based solely on elevated B12 without first excluding macro-B12 - up to 45% of unexplained elevated B12 cases are due to immunocomplex interference, and failure to identify this leads to unnecessary investigations 2, 7.
Prognostic Significance
True B12 elevation (not macro-B12) carries prognostic implications - associated with increased mortality and cancer risk with risk ratios of 1.88-5.9, making thorough investigation essential when true elevation is confirmed 5.