Is a ferritin of 20 actually normal?

How it works

A ferritin of 20 sits in the gap between two different definitions of normal, and that gap is where most of the confusion around iron lives. The NICE guideline on iron deficiency, updated in August 2024, sets adult iron deficiency at a ferritin below 30 micrograms per litre. Many UK laboratory reports still only flag results below 15, a threshold that comes from older WHO reference data. So a ferritin of 20 is printed with no flag beside it on most reports, while the current UK guideline would class the same number as iron deficiency. This is not a rare edge case. It is one of the most common reasons people are told their bloods are fine while still feeling exhausted.

The same logic reads any number in this territory. A ferritin of 16 or 25 sits in the same band as 20. A ferritin of 35 or 45 clears the NICE threshold and still sits low within the range, which matters if symptoms are present, because stores that low leave little in reserve.

Guideline positions

NICE iron deficiency guideline (August 2024): adult iron deficiency at ferritin below 30 ug/L. WHO ferritin guidance: below 15 ug/L consistent with deficiency in apparently healthy adults, which is where many laboratory flags still sit, and above 500 ug/L in adults who are unwell may indicate iron overload or other disease. BSG 2021: transferrin saturation read alongside ferritin, with below 20 percent alongside a ferritin of 30 or more consistent with iron deficiency. Ferritin is a positive acute-phase reactant, so inflammation raises it.

Practical framework

A result below 30 suggests raising it with your GP, particularly if you have symptoms, since guidance looks for the cause behind low iron alongside correcting the level. Two other numbers change how any ferritin should be read. A raised CRP makes ferritin unreliable, because ferritin rises with inflammation independently of iron stores, so a reassuring number alongside a raised CRP can still sit over genuinely low stores. And a transferrin saturation below 20 percent alongside a ferritin of 30 or more is consistent with iron deficiency even though the ferritin looks acceptable.

You can read your own numbers against all of these thresholds, together, with the [free blood test result interpreter](/apps/learn/free-blood-test-result-interpreter) on this site. It runs entirely on your device, nothing is saved, and pasting your report fills the values in for you. A retest around three months after any change shows which way your stores are moving.