Why a normal ferritin can still mean low iron

Anaemia is the most common complication of inflammatory bowel disease that happens outside the bowel, and iron is usually at the centre of it. The European Crohn’s and Colitis Organisation published a consensus on diagnosing and managing it in the Journal of Crohn’s and Colitis in 2015. It turns on one awkward fact: the blood test normally used to judge iron stores behaves differently in someone who is inflamed.

Why a reassuring result can mislead

Ferritin is the standard measure of stored iron, and in most people a low result means low stores. It is also an acute phase protein, which means inflammation pushes it upwards regardless of how much iron is actually there. In inflammatory bowel disease those two effects can pull in opposite directions at the same time, and a result that reads as unremarkable can be concealing a deficiency underneath it.

The consensus deals with this by using two different thresholds depending on whether the disease is active. Where there is no active inflammation, a serum ferritin below 30 micrograms per litre is the criterion for iron deficiency. Where inflammation is present, a serum ferritin as high as 100 micrograms per litre may still be consistent with iron deficiency.

How often it is checked

The consensus sets the monitoring interval by disease activity as well. For people in remission or with mild disease activity, it puts the measurements at every six to twelve months. For people with active disease, at least every three months.

Tablets or a drip

The route is not a matter of preference either. The consensus sets out when each is appropriate:

There is also a ceiling on the oral dose. The consensus recommends no more than 100 mg of elemental iron per day in inflammatory bowel disease — a lower figure than doses commonly used outside it, and a detail worth checking against what is actually written on a prescription or a supplement label.

What kind of document this is

It is worth knowing what a consensus is. It is a group of clinicians agreeing a position on the evidence available to them, which is a different kind of thing from a single trial result. The thresholds exist so that practice is consistent, not because 30 and 100 micrograms per litre are exact biological boundaries. They are decision points, chosen to be useful, and they are the ones a gastroenterology team will generally be working to.

The practical consequence is that iron in inflammatory bowel disease is not something to settle with an over-the-counter supplement. The thresholds move with disease activity, the monitoring interval moves with it too, and the choice between tablets and an infusion rests on numbers that come from a blood test rather than on how someone feels.