Spotting a folate deficiency: serum folate, red blood cell folate and what sets them apart

The short answer: there are two laboratory values, and they answer different questions. Serum folate shows what has been eaten over the past few days — it rises after a folate-rich meal and falls when intake is briefly tight, without the stores being empty. Red blood cell folate reflects supply over roughly three months and is therefore the more robust value for the question of a deficiency. And neither should ever be assessed without vitamin B12. Anyone who measures serum folate alone is most likely measuring breakfast.

The reason for this duplication lies in the biology of the red blood cells: folate is built into them while they are forming in the bone marrow — after that, their content no longer changes. A red blood cell lives around 120 days. Its folate content is therefore a store level, not a snapshot.

The two values side by side

	Serum folate	Red blood cell folate
Period covered	days	around three months
Influenced by the last meal	yes, markedly	no
Fasting sample needed	yes	no
What it says	current intake	supply status
Responds to supplementation	within days	within weeks to months

From this follows the practical rule: a low serum folate on its own does not establish a deficiency — it can reflect a temporarily tight intake. A low red blood cell folate points to inadequate supply over a longer period. Conversely, a normal serum folate can mask an already emptied store if supplementation or a folate-rich meal came shortly before the blood draw.

That is exactly why the timing of the blood draw matters: anyone who has been taking a preparation for two weeks and then has serum folate determined is essentially measuring the preparation.

The usual thresholds

Reference ranges differ between laboratories because different measurement methods are in use. The report from your own laboratory is therefore what counts. As orientation, the thresholds of the World Health Organization serve:

• Serum folate below 7 nmol/l (around 3 ng/ml) counts as a deficiency
• Red blood cell folate below 305 nmol/l (around 140 ng/ml) counts as a deficiency

For the question of the risk of neural tube defects, a considerably higher target value is named in addition: a red blood cell folate above 906 nmol/l. That is not a deficiency threshold but a target range for the time before and at the beginning of a pregnancy — and it lies far above what counts as "not deficient". What follows from this in practice is set out in folic acid in pregnancy.

Homocysteine as a third value

Alongside the two direct values there is a functional one: homocysteine. This amino acid is broken down with the involvement of folate, and in folate deficiency it rises.

The value has one advantage and one disadvantage. The advantage: it shows whether the metabolism is actually impaired — not merely whether a number is low. The disadvantage: it is non-specific. Homocysteine also rises with a deficiency of vitamin B12 and B6, with impaired kidney function and with age. An elevated value says that something is not right — not what.

Why B12 always belongs alongside

This is the most important point of this article, and it is not a detail.

Folate deficiency and vitamin B12 deficiency produce the same pattern in the blood count: red blood cells that are too large, an elevated MCV, a megaloblastic anaemia. From the blood count alone the two cannot be told apart.

If in this situation only folic acid is given, the blood count normalises — even when the cause was a B12 deficiency. The conspicuous finding disappears, the neurological damage from the B12 deficiency carries on and can leave lasting consequences.

That is why the determination of vitamin B12 status belongs with every suspicion of folate deficiency, and why treatment does not begin with folic acid alone. Why this constellation occurs more often precisely with plant-based diets and in older age is set out in folic acid and vitamin B12; for assessing B12 status itself there is a text of its own: spotting a vitamin B12 deficiency.

How else a deficiency shows itself

The signs are non-specific, and none of them proves anything. Described are tiredness and reduced performance, pallor, inflammation of the mucous membranes of the mouth and tongue, and digestive complaints. In the blood count an elevated MCV stands out, often before the haemoglobin values fall.

The order is revealing: first serum folate falls, then red blood cell folate, then homocysteine rises, then the blood count changes, and only at the end does anaemia develop. An abnormal blood count is therefore a late sign — not the first one.

Who is more often affected

A deficiency rarely arises from a single cause. More often several come together:

• Low intake — little vegetable matter, heavily processed food, long cooking times. How much is lost in cooking is set out in folate-rich foods
• Increased requirement — pregnancy and breastfeeding
• Regular alcohol consumption — impairs absorption and utilisation
• Diseases of the small intestine — coeliac disease or chronic inflammatory bowel diseases, for example
• Certain medicines — among others methotrexate, some antiepileptic drugs and sulfasalazine

The last point expressly belongs in medical hands: with some of these medicines the effect on folate is part of the treatment goal. Supplementing on one's own initiative can influence the therapy.

When a test makes sense — and when it does not

As a screening examination in the absence of complaints, the determination is not intended. It becomes sensible when there is an occasion for it: an abnormal blood count, persistent complaints, one of the situations named above, or a concrete wish to have children.

For the general case the intake recommendation still applies, not the laboratory value — 300 µg folate equivalents a day for adults. How this figure relates to the 200 µg on the label and the 400 µg when planning a pregnancy is set out in folic acid daily requirement: 200, 300 or 400 µg?.

What is on the packaging

Folic acid 1000 µg carries 500 % of the nutrient reference value per tablet; further strengths and combinations with B12 are in the folic acid category. What to look at when comparing is set out in the folic acid buying guide.

Food supplements are not a substitute for a balanced and varied diet and a healthy lifestyle. This text does not replace a medical diagnosis; laboratory values belong in the assessment of the treating doctor.

For folate, eight claims are authorised in the EU under Regulation (EU) No 432/2012; these five describe what the vitamin is involved in:

Folate contributes to normal blood formation.

Folate contributes to normal amino acid synthesis.

Folate contributes to normal psychological function.

Folate contributes to the reduction of tiredness and fatigue.

Folate contributes to maternal tissue growth during pregnancy.

Frequently asked questions

Which value should I have determined? That is decided by the treating doctor on the basis of the question at hand. For the question "am I well supplied in the long term?" red blood cell folate is the more informative value; serum folate reflects above all the intake of the last few days. In both cases B12 status belongs alongside.

Do I have to fast for the blood draw? For serum folate yes — it rises measurably after a folate-rich meal. Red blood cell folate is independent of this, because the content of a finished cell no longer changes.

I have been taking a preparation for weeks. Is a test still informative now? For serum folate hardly — it then essentially reflects the preparation. Red blood cell folate responds more slowly and still shows the starting state for a while. If the question is whether a deficiency existed without the preparation, the time course belongs in the assessment.

My serum folate is low, but I feel fine. What now? A single low serum value is not a finding that would have to be acted upon — it can reflect a brief tight intake. More informative are red blood cell folate, the blood count and B12 status. Interpreting them belongs with the doctor, not in the self-assessment of a single value.