What Blood Tests Do You Need Before an Iron Infusion? What Changed in 2026

By Salim Miah, Clinical Director · Published October 26, 2026

Written by Salim Miah MPharm, Independent Prescriber · GPhC 2081653

The photo usually arrives on WhatsApp, slightly blurred, taken on a kitchen table. One line of a blood test result, cropped tight: Ferritin 11. Underneath it, the question. “Is this low enough for an iron drip? My GP said it’s only a bit low.”

I understand why people send it that way. It is the number they have been told about, and it is the one that has been sitting in their head for weeks. But as a prescribing pharmacist, a single cropped number is the one thing I cannot safely make a decision on. I need to see the rest of the report, and I need to see who ran it, when, and what the laboratory considers normal.

That can feel like fussiness from the other side of the screen. It is not. This year has brought two pieces of news that make the blood tests around an iron infusion more important, not less, and I want to explain them properly.

The short answer: before an iron infusion you need a recent full blood count and ferritin, ideally with transferrin saturation and a marker of inflammation such as CRP, all on the laboratory’s own report with its reference ranges. Depending on your history, a phosphate level may be needed too, and blood tests should be repeated around four to eight weeks afterwards, not sooner. New American guidance published in 2026 suggests ferritin cut-offs for iron deficiency are higher than many people have been told.

Why does one ferritin number not tell the whole story?

Ferritin is the protein your body uses to store iron, and a low level is the most specific sign that those stores are running out. The British Society of Gastroenterology’s 2021 guideline describes a ferritin below 15 µg/L as showing absent iron stores, and below 30 µg/L as generally indicating low stores.

The difficulty is the other direction. Ferritin also rises whenever the body is inflamed: after an infection, with a flare of arthritis or inflammatory bowel disease, with liver problems, sometimes with excess weight. So a ferritin of 60 can be perfectly healthy in one person and hide genuine iron deficiency in another. Without a CRP or a transferrin saturation alongside it, you are reading half a sentence.

The full blood count matters for a different reason. It tells us whether you are actually anaemic, how far your haemoglobin has fallen, and whether the red cells look the way iron deficiency usually makes them look. Small, pale cells (a low MCV) fit the picture. Large cells point somewhere else, often towards vitamin B12 or folate, and an iron drip would not fix that.

Haemoglobin also changes the dose. The official prescribing information for ferric carboxymaltose, the intravenous iron we use, calculates the total iron needed from your haemoglobin and your body weight. Lower haemoglobin and higher weight mean a larger total dose. That is why we ask for both, and why we weigh you again on the day.

What changed in 2026 for ferritin results?

For years, many laboratory reports have flagged ferritin as “normal” from somewhere around 15 to 30 µg/L. Plenty of people who have come to us were told their iron was fine because their number sat just above that line, while they were still exhausted, breathless on the stairs and losing hair in the shower.

In 2026 the American Society of Hematology published new guidelines on diagnosing iron deficiency, and they raise the bar. For most adults who are not pregnant, the panel suggests a ferritin of 30 or below as the cut-off for iron deficiency, rising to 50 or below where someone has symptoms or ongoing risk factors, such as heavy periods. For adults with inflammation, it suggests iron deficiency may be present if ferritin is below 100 or transferrin saturation is below 20 per cent.

Two caveats, because they matter. This is American guidance, and the panel itself rated much of the evidence behind the cut-offs as low certainty. UK laboratories and NHS pathways have not all moved, and the NICE clinical knowledge summary on iron deficiency anaemia remains the reference most GP practices work from. But the direction of travel is clear, and it lines up with something the BSG already said back in 2021: that a cut-off of 45 µg/L strikes the most useful balance between catching iron deficiency and over-diagnosing it.

In plain terms, a ferritin of 22 on a report marked “normal” deserves a second look, especially if you feel the way iron deficiency makes people feel. We have written more about that experience in why you can still feel unwell with iron deficiency.

Which blood tests do you need before an iron infusion?

This is the list our prescriber works from when reviewing an iron suitability assessment. It is not exotic. Most of it will already be on a standard GP or hospital blood test.

• Full blood count, including haemoglobin, MCV and platelets.
• Ferritin, ideally with transferrin saturation or a full iron study.
• CRP or another inflammation marker, so a ferritin result can be interpreted honestly.
• Kidney and liver function, where available or where your history suggests it is relevant.
• Phosphate, if you have had intravenous iron before, have had a repeat course recently, or have a risk factor such as low vitamin D or inflammatory bowel disease.
• B12 and folate, where the red cell size or your symptoms suggest something other than iron is going on.

We ask for results from the last six weeks, and we may accept up to three months depending on the picture. Older than that and the number may no longer describe the person sitting in front of us. Iron levels can fall quickly with heavy bleeding, and inflammation can come and go.

Why do we insist on the full laboratory report?

Because a typed number is not a result. A result is a number, a unit, a reference range, a date and the name of the laboratory that produced it. Ferritin can be reported in µg/L or ng/mL (the same value, as it happens), but haemoglobin turns up as both g/L and g/dL, and a 10 can mean two very different things. Reference ranges also differ between laboratories.

There is a quieter reason too. Seeing the whole report lets us notice what you have not mentioned: a raised platelet count, a white cell count that needs a GP’s attention, a kidney result that changes the plan. A good assessment is partly about the question you asked and partly about the ones you did not know to ask. A screenshot from the NHS App is fine, as long as the range and date are visible.

Why does phosphate matter after an iron infusion?

This is the part of the story I most want people to understand, because it is rarely explained at the point of booking.

Ferric carboxymaltose is one of the intravenous irons used across NHS hospital infusion units, and it works well. It also commonly lowers the level of phosphate in the blood. In two randomised trials published in JAMA in 2020, around three quarters of people given ferric carboxymaltose developed low phosphate within five weeks, compared with roughly 8 per cent given a different intravenous iron. For most, it is temporary and passes unnoticed.

For a small number it does not. In 2020 the UK’s medicines regulator issued a MHRA Drug Safety Update warning that symptomatic low phosphate can lead to softening of the bones and fractures. It advised monitoring phosphate in people having repeated higher doses or long-term treatment, and in anyone with risk factors such as vitamin D deficiency or inflammatory bowel disease. A note added in September 2026 confirms that the warning applies to every ferric carboxymaltose product on the UK market.

That same month, the US regulator went further. On 1 September 2026 the FDA added a boxed warning, its most prominent form of safety alert, to ferric carboxymaltose. It recommends correcting low phosphate before treatment and checking phosphate before a repeat course if someone is at risk, or if the new course falls within three months of the last. The FDA also noted, strikingly, that phosphate was checked in fewer than one in five administrations in the US data it reviewed.

I do not share this to frighten anyone. I share it because the symptoms of low phosphate, new muscle aches, bone pain and worsening tiredness, overlap almost exactly with the symptoms of iron deficiency. Someone who feels worse a few weeks after an infusion can easily assume the iron “did not work” and ask for more. That is precisely the wrong move if phosphate is the problem. So where our clinical policy calls for it, we arrange phosphate monitoring, and we tell everyone what to look out for afterwards.

When should you have your bloods checked again?

Not straight away. The prescribing information for ferric carboxymaltose advises reassessing haemoglobin and iron levels no earlier than four weeks after the final dose. Ferritin in particular shoots up after an infusion, because the iron has just been delivered into storage, so an early test can look reassuring when your body has not yet turned that iron into new red cells.

We normally repeat blood tests around six weeks after the final treatment. That gives a fair picture of how your haemoglobin has responded and whether your stores are where they should be. It also tells us whether the underlying cause is still pulling iron out faster than you can replace it.

The question underneath the question: why is your iron low?

An infusion refills the tank. It does not fix the leak.

In Bradford, the commonest reasons we see are heavy periods, pregnancy and the months after it, and diets that are low in well-absorbed iron. But the BSG guideline is clear that for men and for women after the menopause, new iron deficiency anaemia should usually lead to investigation of the gut, typically with gastroscopy and colonoscopy, because bleeding from the bowel and conditions such as coeliac disease need ruling out. Coeliac disease turns up in 3 to 5 per cent of people with iron deficiency anaemia, according to the same guideline.

So if your blood test shows iron deficiency and nobody has asked why, that conversation belongs with your GP, and an infusion should sit alongside it rather than instead of it. If we review your results and think something needs investigating first, we will say so plainly and put it in writing for your GP. The NHS guide to iron deficiency anaemia is a sensible place to start reading about causes.

How does this work at our Bradford clinic?

We built our iron deficiency treatment service around the blood tests rather than around the appointment. You complete a free online assessment and send your full report and today’s weight. A prescriber reviews it remotely, and we usually reply within two working days. If intravenous iron is suitable, you are sent a booking link. If it is not, you have not paid for a consultation to find that out, and we explain why.

On the day, at our clinic in Listerhills Science Park, a short walk from the University of Bradford, we confirm your identity, reweigh you, run final safety checks and reconfirm consent. Most visits take 60 to 75 minutes, including at least 30 minutes of observation afterwards. Parking is free and on site, which matters more than you would think when people are travelling in from Shipley, Keighley, Halifax or across from Leeds.

Not everyone needs an infusion. Iron tablets, taken properly, work for a great many people, and we have written an honest comparison of iron infusions and iron tablets if you are weighing the two. Intravenous iron is a prescription-only medicine, not a wellness drip, and it is offered only after a prescriber has reviewed your results and decided it is appropriate for you.

Which brings me back to the cropped photo on the kitchen table. Ferritin 11 is low. On almost any guideline, old or new, that is iron deficiency. But whether an infusion is the right answer, what dose, and what else needs checking first, lives in the rest of the report. Send us the whole thing. It is the quickest way to a safe answer.

Frequently asked questions

Can I have an iron infusion without a blood test?

No. A recent full blood count and ferritin are needed to confirm iron deficiency and to calculate your dose safely. We ask for the full laboratory report, normally from the last six weeks.

What ferritin level counts as iron deficiency?

Below 15 µg/L shows absent iron stores and below 30 µg/L generally indicates low stores. New 2026 American guidance suggests 30 or below for most adults, and up to 50 with symptoms or risk factors. Inflammation can push ferritin up and hide a deficiency.

Do I need a phosphate blood test before an iron infusion?

Not always. It is recommended if you have had intravenous iron before, especially within the last three months, or have risk factors such as low vitamin D or inflammatory bowel disease. Your prescriber will advise.

How soon after an iron infusion should I have a blood test?

No earlier than four weeks after the final dose. Ferritin rises sharply straight after an infusion, so testing too early can be misleading. We usually repeat bloods around six weeks afterwards.

Why do I feel worse a few weeks after my iron infusion?

Mild flu-like symptoms in the first day or two are common. New muscle or bone pain, or worsening tiredness weeks later, can be a sign of low phosphate and should be reported to whoever gave your infusion.

Can I use my NHS blood test results?

Yes. A GP or hospital report, or a screenshot from the NHS App, is fine as long as it shows the laboratory, the date and the reference ranges next to each result.

Sources

• American Society of Hematology 2026 guidelines for the diagnosis of iron deficiency, Blood Advances
• Snook J et al. British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults, Gut 2021
• NICE Clinical Knowledge Summary: Anaemia, iron deficiency
• MHRA Drug Safety Update: Ferric carboxymaltose and risk of symptomatic hypophosphataemia (2020, updated note 2026)
• US FDA Drug Safety Communication: boxed warning for ferric carboxymaltose (September 2026)
• Wolf M et al. Effects of iron isomaltoside vs ferric carboxymaltose on hypophosphatemia in iron-deficiency anemia, JAMA 2020
• Summary of Product Characteristics: ferric carboxymaltose (electronic medicines compendium)
• NHS: Iron deficiency anaemia

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