Being unwell pushes ferritin up, so a normal result can mislead
Among unwell people with no iron left in their marrow, ferritin averaged 82 micrograms per litre, above the 70 seen in healthy people who still had some.
What the study found
Inflammation raises ferritin, so a normal reading can hide deficiency
One number can mean two things. Among unwell people with no iron left in their marrow, ferritin averaged 82 micrograms per litre, above the 70 in healthy people who still had marrow iron. US surveys adjusted for inflammation found 3 to 7 percentage points more deficiency.
The caveat · Adjusted by formula, not by marrow
Every study underneath these means was observational. The 70 micrograms per litre comparator rests on three studies and 151 people. The inflammation correction is a statistical adjustment to survey data rather than a check against bone marrow, and the two proteins it needs are not measured alongside an ordinary ferritin test.