What is the appropriate workup for suspected vitamin B12 deficiency in an elderly (>60 years) patient with a vegetarian diet and chronic proton‑pump inhibitor (PPI) use?

Workup for Suspected Vitamin B12 Deficiency

Begin with either total serum B12 or active B12 (holotranscobalamin) as your initial test, then add methylmalonic acid (MMA) testing only when results fall in the indeterminate range. 1

Initial Laboratory Testing

Order total serum B12 as the first-line test because it costs approximately £2 with rapid turnaround in local laboratories, making it the most practical initial screen. 1 Active B12 (holotranscobalamin) is an acceptable alternative that measures the biologically active form available for cellular use, but costs £18 per test with longer processing times through external laboratories. 1

Interpretation of Total Serum B12

• < 180 pg/mL (< 133 pmol/L): Confirmed deficiency—initiate treatment immediately without further testing. 1
• 180–350 pg/mL (133–258 pmol/L): Indeterminate result—proceed to MMA measurement to confirm functional deficiency. 1
• > 350 pg/mL (> 258 pmol/L): Deficiency unlikely, but consider MMA testing if clinical suspicion remains high based on symptoms or risk factors. 1

Interpretation of Active B12 (if used instead)

• < 25 pmol/L: Confirmed deficiency—start treatment immediately. 1
• 25–70 pmol/L: Indeterminate—order MMA to clarify functional status. 1
• > 70 pmol/L: Deficiency unlikely. 1

Confirmatory Testing with Methylmalonic Acid

Measure serum MMA when initial B12 results are indeterminate (180–350 pg/mL for total B12 or 25–70 pmol/L for active B12). 1 MMA reflects actual cellular B12 status and detects an additional 5–10% of patients with functional deficiency who have low-normal serum B12 levels. 2

• MMA > 271 nmol/L: Confirms functional B12 deficiency with 98.4% sensitivity. 2
• MMA ≤ 271 nmol/L: Functional deficiency unlikely. 2

MMA testing costs £11–80 per test and requires specialized equipment, but cost-effectiveness analysis demonstrates it is worthwhile at £3,946 per quality-adjusted life year when used after indeterminate B12 results. 1 Using MMA as first-line screening is not cost-effective due to expense and processing delays. 1

Complete Blood Count

Order a complete blood count (CBC) concurrently with B12 testing to evaluate for megaloblastic anemia, though anemia may be absent in one-third of cases. 2 Macrocytosis (MCV > 98 fL) often appears before anemia develops and represents the earliest laboratory sign of B12 deficiency. 2

Additional Functional Markers (Optional)

Consider measuring homocysteine if MMA is unavailable or to support the diagnosis in borderline cases. 2 Homocysteine > 15 μmol/L supports B12 deficiency, though it is less specific than MMA because it also elevates in folate deficiency, renal impairment, and hypothyroidism. 2 The combination of elevated homocysteine plus elevated MMA confirms B12 deficiency, whereas elevated homocysteine with normal MMA suggests folate deficiency. 2

Risk Factor Assessment in This Patient

Your elderly patient has three major risk factors that substantially increase the likelihood of B12 deficiency:

• Age > 60 years: Metabolic B12 deficiency affects 18.1% of patients over 80 years, even with "normal" serum levels. 2 Atrophic gastritis affects up to 20% of older adults, causing food-bound B12 malabsorption. 2
• Vegetarian diet: Limited dietary B12 intake from lack of animal products (meat, fish, dairy, eggs) increases deficiency risk. 2
• Chronic PPI use: Proton pump inhibitors impair B12 absorption by reducing gastric acid needed to cleave B12 from food proteins, though recent meta-analysis shows conflicting evidence on whether this translates to clinically significant deficiency. 3, 4

Common Pitfalls to Avoid

Do not rely solely on serum B12 to rule out deficiency in elderly patients, because standard serum B12 testing misses functional deficiency in up to 50% of cases. 2 The Framingham Study demonstrated that while 12% had low serum B12, an additional 50% had elevated MMA indicating metabolic deficiency despite "normal" serum levels. 2

Do not order MMA as a first-line test because it is not cost-effective for screening and causes processing delays. 1 Reserve MMA for indeterminate B12 results or when clinical suspicion remains high despite normal B12 levels. 1

Interpret MMA and homocysteine cautiously in patients with renal insufficiency, hypothyroidism, or hypovolemia, as these conditions can cause false elevations independent of B12 status. 2

Practical Diagnostic Algorithm

1. Measure total serum B12 (or active B12) plus CBC. 1
2. If B12 < 180 pg/mL: Diagnose deficiency and treat immediately. 1
3. If B12 180–350 pg/mL: Order MMA; if MMA > 271 nmol/L, confirm functional deficiency and treat. 1, 2
4. If B12 > 350 pg/mL but high clinical suspicion persists (neurological symptoms, multiple risk factors): Consider MMA testing on a case-by-case basis. 1

Etiologic Workup (After Confirming Deficiency)

Once deficiency is confirmed, test for intrinsic factor antibodies if pernicious anemia is suspected, as this determines the need for lifelong intramuscular therapy. 2 Consider checking gastrin levels if pernicious anemia is likely, with markedly elevated levels (> 1000 pg/mL) supporting the diagnosis. 2

Screen for celiac disease with tissue transglutaminase (tTG) antibodies and total IgA in B12-deficient patients, particularly those with autoimmune conditions, as 13.3% of B12-deficient hypothyroid patients have positive celiac markers. 2