Folate deficiency

01OverviewDefinition, clinical context and the essential points that orientate the chapter.

Folate supplies one-carbon units for thymidylate and purine synthesis. When stores fall, rapidly dividing marrow precursors cannot replicate DNA normally, creating nuclear-cytoplasmic asynchrony, ineffective erythropoiesis and intramedullary cell death. The resulting blood film resembles vitamin B12 deficiency: macro-ovalocytes, anisopoikilocytosis and hypersegmented neutrophils. LDH and indirect bilirubin may rise, but the absolute reticulocyte response is inappropriately low. Severe disease can cause leucopenia and thrombocytopenia as well as anaemia.

Because body folate reserves are limited, deficiency can develop within months of inadequate intake or increased demand. History should cover restricted diet, alcohol, food access, frailty, pregnancy, chronic haemolysis, dialysis, coeliac disease, inflammatory bowel disease and bowel surgery. Methotrexate, trimethoprim and selected antiseizure medicines require careful indication-specific review rather than abrupt cessation. Serum folate can change with recent eating and supplementation, so results need the blood count, film, B12 and clinical cause alongside them.

Treatment is straightforward only after B12 safety is established. The BNF adult regimen for folate-deficient megaloblastic anaemia is folic acid 5 mg daily for 4 months, with longer treatment when the cause persists and higher dosing in some malabsorption settings. Replacement should produce an early reticulocyte response and then rising haemoglobin. Lack of improvement raises non-adherence, continuing demand, mixed iron or B12 deficiency, malabsorption, medicine antagonism, inflammation or marrow disease. The endpoint includes correction of the cause, not only a normal FBC.

Key points

Folate deficiency produces ineffective megaloblastic erythropoiesis, often with macro-ovalocytes, hypersegmented neutrophils and low reticulocytes, but mixed iron deficiency can leave MCV within range.
Assess diet, alcohol, coeliac and other small-bowel disease, pregnancy, chronic haemolysis, dialysis and medicines that antagonise folate metabolism or alter absorption.
Measure vitamin B12 before or alongside folate and assess neurological symptoms explicitly. Do not give folic acid alone when B12 deficiency remains plausible.
Serum folate reflects recent intake and should be interpreted with the clinical and blood-film phenotype. A low result does not explain why deficiency developed.
Usual adult treatment for folate-deficient megaloblastic anaemia is folic acid 5 mg by mouth daily for 4 months; longer treatment is needed when the cause persists.
Severe malabsorption may require a higher oral dose under the BNF and specialist plan. Apparent failure should prompt adherence, diagnosis, ongoing demand and intestinal disease review.
In chronic haemolytic states, prophylactic folate may be indicated through the disease-specific haematology pathway because sustained reticulocyte production increases requirement.
A reticulocyte and haemoglobin response supports effective replacement, but recurrence prevention requires dietary support, treatment of malabsorption and coordinated medicine review.

02AetiologyUnderlying causes, associations and risk factors, with why each one matters.

Inadequate intake

Restricted or poor-quality diets, food insecurity, frailty, alcohol dependence and prolonged cooking of food can reduce folate intake and deplete relatively limited stores.

Malabsorption

Coeliac disease, inflammatory small-bowel disease, extensive bowel surgery and severe gastrointestinal illness reduce proximal small-intestinal folate uptake.

Increased requirement

Pregnancy, chronic haemolysis, rapid growth, malignancy and recovery from major marrow stress increase nucleotide synthesis and can outstrip folate supply.

Medicines and losses

Methotrexate, trimethoprim, selected antiseizure medicines, dialysis and other treatment contexts can impair folate metabolism, absorption or retention.

03PathophysiologyThe causal sequence from the underlying abnormality to symptoms and harm.

1
Folate stores decline
Sustained inadequate delivery or increased use exhausts hepatic folate stores over months, faster than the usual time course of vitamin B12 depletion.
2
Thymidylate synthesis fails
Reduced tetrahydrofolate-dependent one-carbon transfer impairs DNA replication, particularly in rapidly dividing marrow and gastrointestinal epithelial cells.
3
Maturation becomes asynchronous
Nuclear development lags behind cytoplasmic growth, creating megaloblasts, macro-ovalocytes, hypersegmented neutrophils and ineffective intramedullary erythropoiesis within marrow.
4
Cytopenias and symptoms emerge
Precursor cell death lowers reticulocyte output and may reduce several lineages, while falling haemoglobin causes fatigue, breathlessness and cardiovascular compensation.

04Clinical features and red flagsSymptoms, examination findings, patterns of presentation and time-critical warnings.

Megaloblastic blood pattern

Macro-ovalocytes, hypersegmented neutrophils, low reticulocytes and possible pancytopenia support folate or B12-related ineffective erythropoiesis rather than simple regeneration.

Dietary risk

Restricted food intake, alcohol dependence, frailty, food insecurity or a diet lacking fortified grains and green vegetables increases deficiency likelihood.

Malabsorptive risk

Chronic diarrhoea, weight loss, coeliac disease, inflammatory bowel disease or extensive small-bowel surgery suggests impaired folate uptake and possible mixed deficiency.

High-demand state

Pregnancy, chronic haemolysis, rapid growth or active malignancy can outstrip intake and may require a longer cause-specific prevention plan.

Possible concurrent B12 diseaseRed flag

Paraesthesia, proprioceptive loss, gait disturbance, cognitive change, gastric surgery or autoimmune history makes folate-only treatment unsafe and B12 assessment urgent.

05InvestigationsWhat to request, why it matters and how to interpret it.

Investigation order

Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.

01
FBC, film and absolute reticulocytesFirst step
Why
Define severity and recognise ineffective megaloblastic production.
Interpretation and limitations
Macrocytosis and hypersegmented neutrophils support the diagnosis but can be absent with mixed iron deficiency. Low reticulocytes before treatment distinguish ineffective production from bleeding or haemolysis.
02
Serum folate
Why
Identify biochemical folate depletion in the appropriate phenotype.
Interpretation and limitations
Interpret against the local assay, recent intake and supplementation. A low result supports deficiency but does not identify diet, demand, medicine effect or malabsorption as the cause.
03
Vitamin B12 with functional pathway
Why
Prevent missed cobalamin-related neurological injury before folate treatment.
Interpretation and limitations
Use NICE NG239 thresholds and methylmalonic acid or homocysteine where indicated. Neurological suspicion warrants prompt B12 replacement even if macrocytosis is absent.
04
Ferritin and transferrin saturation
Why
Find concurrent iron deficiency that can conceal macrocytosis and limit response.
Interpretation and limitations
Low ferritin supports depleted stores; inflammation can preserve ferritin. Mixed iron and folate deficiency may produce normal MCV with high RDW and a dimorphic film.
05
Coeliac serology and malabsorption assessment
Why
Identify a treatable intestinal cause when diet does not explain deficiency.
Interpretation and limitations
Follow NICE coeliac testing with total immunoglobulin context and review bowel disease, surgery and weight loss. Avoid starting a gluten-free diet before diagnostic assessment.
06
Liver, thyroid and haemolysis tests
Why
Evaluate alternative macrocytosis and increased folate demand.
Interpretation and limitations
Alcohol and liver disease may be non-megaloblastic; hypothyroidism can raise MCV; chronic haemolysis produces reticulocytosis and sustained folate use, sometimes alongside deficiency.

06Differential diagnosisRealistic alternatives and the features that help distinguish them.

Vitamin B12 deficiency

An identical megaloblastic blood pattern plus neurological disease, malabsorption risks or indeterminate cobalamin testing demands prompt B12 assessment and replacement.

Alcohol or liver disease

Exposure history, round rather than oval macrocytes, target cells and abnormal liver tests support non-megaloblastic macrocytosis, although nutritional deficiency can coexist.

Myelodysplastic syndrome

Persistent macrocytosis, dysplastic neutrophils or platelets, additional cytopenias and failure to respond to adequate haematinic replacement raises clonal marrow disease.

Reticulocytosis

Recent bleeding, haemolysis or recovery after treatment increases larger young red cells; a raised absolute reticulocyte count distinguishes regeneration from ineffective megaloblastosis.

Hypothyroidism and medicines

Thyroid failure, hydroxycarbamide, antiretroviral treatment and other medicines produce macrocytosis without folate depletion and require cause-specific interpretation.

07ManagementImmediate care, first-line treatment, alternatives and escalation.

01Confirm safelyEstablish deficiency and protect against B12 harmFirst stepMacrocytosis, megaloblastic film change, cytopenia or risk factors suggest folate deficiency.

1Take FBC, film, reticulocytes, serum folate, B12, ferritin, liver and thyroid tests, and ask specifically about neurological symptoms, diet, alcohol, pregnancy, bowel disease and medicines.
2PreferredAlternativeThe preferred interpretation requires biochemical deficiency plus a compatible phenotype and cause; an alternative mixed-deficiency pathway uses RDW, film and all haematinics when MCV is normal.
3EscalationEscalate neurological findings for prompt B12 treatment and severe symptomatic anaemia, bleeding, infection or pancytopenia for urgent clinical and haematology assessment.

02ReplaceGive folic acid and verify responseFolate deficiency is supported and vitamin B12 deficiency has been addressed safely.

1First lineFirst-line adult treatment is folic acid 5 mg by mouth daily for 4 months; continue longer while the cause persists and use any higher malabsorption regimen only through current BNF advice.
2Give dietary and medicine-interaction advice, treat coexisting iron or B12 deficiency, and coordinate methotrexate or antiseizure treatment changes with the responsible specialty rather than stopping them abruptly.
3EscalationCheck early reticulocyte and subsequent haemoglobin response; escalate failure to adherence, malabsorption, continued demand, mixed deficiency and marrow-disease review.

03Prevent recurrenceCorrect the continuing causeThe blood count is responding and a longer-term plan is required.

1Treat coeliac or other intestinal disease, improve sustainable dietary intake and address harmful alcohol use with appropriate support while maintaining replacement through recovery.
2PreferredFor chronic haemolysis or ongoing dialysis-related loss, the preferred prevention schedule comes from the disease-specific specialist pathway; pregnancy supplementation follows current maternity guidance.
3AlternativeStop therapeutic folic acid only after the planned course when the cause is corrected, with an alternative long-term regimen or monitoring plan documented for persistent risk.

Key medicines and prescribing safety2 treatments · regimens, roles and cautions

Replenishes folate for DNA synthesis, allowing effective marrow maturation and correction of deficiency-related cytopenias.

Folic acid

For folate-deficient megaloblastic anaemia, give 5 mg by mouth once daily for 4 months; continue longer while the underlying cause remains active.

Exclude or treat vitamin B12 deficiency first because folate can improve the blood count while neurological injury progresses. Review methotrexate, trimethoprim and antiseizure interactions with the responsible clinician.

Provides a larger oral amount when intestinal uptake is reduced and standard treatment has a documented risk of inadequate delivery.

Folic acid in severe malabsorption

The BNF allows up to 15 mg by mouth daily in malabsorption; select the dose with specialist review and monitor clinical and haematological response.

Do not escalate solely for a slow FBC without revisiting adherence, diagnosis, B12 and iron status, continued demand and intestinal disease. Long-term high dosing needs a clear indication.

08ComplicationsImportant consequences, why they occur and why they matter clinically.

Symptomatic anaemia

Progressive ineffective erythropoiesis reduces oxygen carriage, causing fatigue, exertional breathlessness, palpitations and decompensation in people with limited cardiorespiratory reserve.

Multilineage cytopenia

Severe megaloblastosis can reduce white cells and platelets as well as red cells, increasing infection or bleeding risk and mimicking marrow failure.

Pregnancy-related harm

Insufficient folate around conception increases fetal neural-tube-defect risk, while maternal deficiency can contribute to anaemia and adverse pregnancy outcomes.

Masked B12 disease

Folic acid may correct megaloblastic haematology while untreated B12-related neurological injury continues, delaying recognition until disability is established.

Recurrent deficiency

Replacement without correcting poor intake, malabsorption, increased demand or medicine interactions leads to renewed depletion after the treatment course ends.

09Monitoring and follow-upTreatment response, safety checks and longer-term review.

Check reticulocyte and haemoglobin response at a clinically appropriate early interval, using the trajectory rather than serum folate alone to confirm effective marrow recovery.
Repeat the FBC until anaemia and any additional cytopenias resolve; urgent review is needed for worsening counts, infection or bleeding.
Review B12 and neurological symptoms if response is incomplete or new paraesthesia, gait change, cognition or visual symptoms develop.
Confirm adherence, dietary improvement and completion of coeliac, gastrointestinal or other cause-directed assessment before ending treatment.
For persistent increased demand or malabsorption, document whether replacement is continuing, at what dose, under which specialty and when it will be reviewed.
Coordinate monitoring with methotrexate, antiseizure treatment, dialysis, pregnancy or chronic haemolysis pathways when these create the ongoing risk.

10Special situationsVariants, exceptions and circumstances that change the usual approach.

Folate stores are relatively small

Deficiency can emerge over months, so a recent dietary, alcohol, gastrointestinal or medicine change may be directly relevant.

The film cannot separate B12

Macro-ovalocytes and hypersegmented neutrophils occur in both folate and B12 deficiency. Neurology, biochemical testing and cause assessment provide the distinction.

LDH can be very high

Intramedullary death of megaloblasts releases LDH and bilirubin despite low reticulocytes, creating a misleading resemblance to peripheral haemolysis.

Normal MCV can hide deficiency

Concurrent iron restriction produces smaller cells and may balance folate-related macrocytosis. RDW and film reveal the mixed population.

Replacement is not prevention

Four months of folic acid corrects stores only if poor intake, malabsorption, medicine antagonism or increased demand is also addressed.

11Common pitfallsFrequent interpretation and management errors.

01
Giving folic acid before assessing B12 and neurological symptoms.
02
Diagnosing folate deficiency from macrocytosis without a folate result and cause assessment.
03
Ignoring mixed iron deficiency because the MCV is normal.
04
Stopping methotrexate or antiseizure treatment without specialist coordination.
05
Extending folic acid indefinitely without explaining continued demand or malabsorption.
06
Assuming a normalised FBC proves the underlying cause has resolved.
07
Missing marrow disease when cytopenias fail to respond to adequate replacement.