Methylation B-vitamin pair
Vitamin B12 and Folate: how to read them together
B12 and folate are interpreted together because they share the methylation pathway, present with similar symptoms (macrocytic anaemia, fatigue, glossitis), and supplementing one without the other can mask the deficiency of the other.
Quick answer
Vitamin B12 and Folate are interpreted together because b12 and folate are interpreted together because they share the methylation pathway, present with similar symptoms (macrocytic anaemia, fatigue, glossitis), and supplementing one without the other can mask the deficiency of the other. UK reference: NICE NG239 (B12 deficiency, March 2024) / NICE CKS Anaemia (B12 and folate deficiency).
Why these markers are ordered together
Vitamin B12 and folate are both required for DNA synthesis and the conversion of homocysteine to methionine. Deficiency of either produces the same macrocytic (large red cell) anaemia and many of the same symptoms. Crucially, folate supplementation can correct the haematological picture of B12 deficiency (anaemia improves) while neurological B12 damage continues unchecked, this is why every B12 test should be paired with folate, and why supplementing folate without checking B12 is risky. NICE NG239 (March 2024) confirmed deficiency cut-offs at B12 below 180 ng/L (133 pmol/L) and folate below 7 nmol/L (NICE CKS).
Four scenarios, four interpretations
The 2x2 interpretation matrix our clinicians use when these markers come back together.
Low B12 + Low folate
What it likely means: Combined nutritional deficiency, often dietary. Common in vegan diets, in older adults, and in alcohol excess. Coeliac disease is a recognised cause of both.
What to do next: Coeliac screen (anti-tTG IgA + total IgA per NICE NG20). Treat both deficiencies (B12 IM hydroxocobalamin first if neurological symptoms, then folic acid 5 mg daily). Investigate dietary and absorption causes.
Low B12 + Normal folate
What it likely means: B12-specific deficiency. Pernicious anaemia (autoimmune intrinsic factor loss), long-term metformin or PPI use, post-gastric-surgery, or strict vegan diet.
What to do next: Intrinsic factor antibodies, gastric parietal cell antibodies. IM hydroxocobalamin replacement per NICE NG239. NEVER supplement folate alone, risk of masking B12 deficiency while neurological damage progresses.
Normal B12 + Low folate
What it likely means: Folate-specific deficiency. Dietary (low leafy green vegetable intake), pregnancy/breastfeeding (increased demand), alcohol excess, methotrexate or other antifolate medication, coeliac disease.
What to do next: Coeliac screen. Folic acid 5 mg daily for treatment (NICE CKS). Address underlying cause. In women planning pregnancy, the recommendation is 400 micrograms folic acid daily pre-conception and through the first trimester per NHS.
Low or borderline B12 + High folate
What it likely means: B12 deficiency partially masked by high folate. This pattern often appears in patients self-supplementing folate without checking B12. Anaemia may be absent or mild despite real B12 deficiency.
What to do next: Stop folate supplement. Confirm B12 deficiency with MMA (methylmalonic acid) or active B12 (holotranscobalamin), both unaffected by acute folate. IM hydroxocobalamin replacement. Restart folate only after B12 is replete.
When the pair is ambiguous
Recent oral B12 or B-complex supplementation falsely raises serum B12, hold supplements for several weeks before testing for a true baseline (private medicine convention; NICE NG239 advises clinicians to ask about supplement use and interpret with caution but does not specify a wash-out period). Recent folate supplementation similarly raises serum folate within hours. MMA and active B12 are more robust than serum B12 in this context.
UK reference
NICE NG239 (B12 deficiency, March 2024) / NICE CKS Anaemia (B12 and folate deficiency)
Individual markers
Read about each marker in isolation:
Get the pair tested together
Both markers in one blood draw at 134 Harley Street. Custom panels designed by a GMC-registered doctor from £275, or fixed panels including this pair from £249.