Serum B12 and folate levels

What the assays measure

Both assays measure the wrong thing. Serum B12 measures total cobalamin, most of it inert; serum folate measures the last meal
The exam question is almost always interpreting a borderline or discordant result, not a floridly low one

	What it measures	Fails because
Serum B12	Total = holohaptocorrin (~80%, inert) + holotranscobalamin (~20%, the usable fraction)	A normal total can hide a low active fraction
Holotranscobalamin	The bioavailable fraction	Falls earliest; not universally available
Serum folate	Circulating, hours-timescale	Rises after a single folate-rich meal or a hospital diet
Red cell folate	Preceding ~3 months (incorporated at erythropoiesis)	Falls in B12 deficiency (methyl-folate trap) - mimics folate deficiency
MMA	Functional B12 at tissue level	Also rises in renal impairment, SIBO, hypovolaemia
Homocysteine	Functional B12 or folate	Non-specific: renal failure, hypothyroidism, MTHFR variants, smoking

B12 deficiency ~5-10% over age 65; pernicious anaemia ~1-2% of older adults, F>M, higher in northern European and African ancestry
Folate deficiency now uncommon in Australia - mandatory folic acid fortification of bread-making flour since 2009
- Routine folate testing is a Choosing Wisely low-value test in an unselected population

Pernicious anaemia - the commonest cause
- Autoimmune destruction of gastric body mucosa -> loss of parietal cells -> no intrinsic factor + achlorhydria
Gastric: gastrectomy, bariatric surgery, atrophic gastritis, PPI, H. pylori
Ileal: Crohn disease, ileal resection, tropical sprue, radiation, cubam (cubilin/amnionless) receptor mutation - Imerslund-Grasbeck
Luminal competition: bacterial overgrowth, *fish tapeworm (Diphyllobothrium latum)*
Pancreatic insufficiency - haptocorrin not degraded, B12 never handed to IF
Dietary: strict vegan (animal products are the only source)
Drugs: metformin, PPI, H2RA, colchicine
Nitrous oxide - irreversibly oxidises cobalt, inactivates methionine synthase -> *functional deficiency with a normal serum B12*

Intake: alcohol, elderly, poverty, "tea and toast"
Malabsorption: coeliac disease, chronic enteritis, tropical sprue, jejunal resection
inc demand: pregnancy/lactation, chronic haemolysis, exfoliative dermatitis, dialysis, malignancy
Drugs: methotrexate and trimethoprim (DHFR inhibition), sulfasalazine (blocks absorption), phenytoin and phenobarbitone (reduced absorption), alcohol

Serum B12 <150 pmol/L = deficient; 150-220 pmol/L = indeterminate -> measure MMA or holotranscobalamin
A normal serum B12 does not exclude deficiency, and a low one does not prove it

Falsely LOW B12 (no true deficiency)

Pregnancy, oral contraceptive, folate deficiency itself, myeloma/paraprotein, transcobalamin I deficiency, high-dose vitamin C

Falsely NORMAL or HIGH B12 (deficiency present)

Liver disease, myeloproliferative neoplasms (inc haptocorrin), alcohol, active haemolysis, nitrous oxide
High-titre intrinsic factor antibody can interfere with competitive-binding assays and produce a spuriously normal result

	Sensitivity	Specificity
Anti-intrinsic factor	~50%	~95% - use it to rule IN
Anti-parietal cell	~80-90%	Poor - positive in ~15% of normal older women

Macrocytic anaemia, MCV >95-110 fL, with oval macrocytes and hypersegmented neutrophils (>5% with >=5 lobes)
Low reticulocytes despite haemolytic markers - inc LDH (often very high), inc unconjugated bilirubin, dec haptoglobin
- This is intramedullary haemolysis from ineffective erythropoiesis, not peripheral destruction
Severe deficiency -> pancytopenia
Marrow: megaloblastic change, giant metamyelocytes
*A very high LDH with a macrocytic anaemia and low retics is B12 deficiency until proven otherwise* - the usual misdiagnosis is haemolytic anaemia or acute leukaemia

B12: glossitis, angular stomatitis, increased skin melanin/hyperpigmentation, mild jaundice ("lemon-yellow"), subacute combined degeneration, dementia, optic atrophy, infertility
Folate: general anaemia features + irritability, chronic diarrhoea, weight loss/poor weight gain; no neurological disease
Neural tube defects in pregnancy - both deficiencies

	B12	Folate
MMA	Raised	Normal
Homocysteine	Raised	Raised
Neurology	Yes	No
Onset after deficient intake	Years	Months

*Replace B12 first. Always.*
- High-dose folate given before B12 accelerates haematopoiesis, consumes remaining B12 and precipitates or worsens subacute combined degeneration, while partly correcting the blood count and masking the diagnosis
If both are deficient, or the result is not back in a sick patient -> give both, B12 first

Hydroxocobalamin 1000 microgram IM alternate days x5-6 doses, then 1000 microgram 3-monthly for life where the cause is irreversible (pernicious anaemia, ileal resection, gastrectomy)
Oral 1000-2000 microgram daily is effective even in pernicious anaemia - ~1% crosses by passive diffusion, independent of intrinsic factor
Dietary or reversible cause -> oral, and correct the cause

Reticulocytosis by day 3-5, peaks day 5-10; Hb normalises by ~8 weeks
*Check K+ - brisk erythropoiesis causes hypokalaemia in the first days*
Iron deficiency is commonly unmasked as stores are consumed - recheck ferritin
No response -> the diagnosis is wrong, or there is coexisting iron deficiency, thalassaemia, MDS or hypothyroidism

Autoimmune polyglandular clustering with pernicious anaemia: autoimmune thyroid disease, T1DM, vitiligo, Addison disease, hypoparathyroidism
Coeliac disease - low folate and low iron together
*Atrophic gastritis, H. pylori, PPI, metformin*
Alcohol use disorder - low folate, direct marrow toxicity, and macrocytosis without deficiency
Hypothyroidism, MDS, liver disease, reticulocytosis, drugs (hydroxyurea, azathioprine, zidovudine) - macrocytosis without either deficiency
Recreational nitrous oxide use in young adults

Haematological recovery is complete and predictable; neurological recovery is not
- SACD may be irreversible if replacement is delayed beyond ~6-12 months
- Neurological disease can occur with a normal Hb and MCV - do not require anaemia before treating
Pernicious anaemia: ~2-3x risk of gastric adenocarcinoma and gastric carcinoid (hypergastrinaemia -> ECL cell hyperplasia)
- Investigate new upper GI symptoms; no routine surveillance endoscopy programme
Untreated folate deficiency in pregnancy -> neural tube defects
Hyperhomocysteinaemia from either deficiency is associated with vascular disease, but supplementation lowers homocysteine without reducing cardiovascular events

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