Reference values for oral iron absorption of bivalent iron in healthy volunteers. 
It is a challenge to establish the cause of iron deficiency in the absence of obvious reasons.  There has been no prospectively validated oral iron absorption test (OIAT) that could confidently confirm or exclude iron malabsorption.  The aim, therefore, was the establishment of an OIAT with defining reference values of plasma iron in healthy volunteers.  We included 49 healthy volunteers who received 200 mg of bivalent ferrous fumarate in fasting condition and measured plasma iron after 2 and 4 hours as well as after 3 and 5 days.  After a wash-out phase, 23 healthy volunteers proceeded to receive trivalent iron hydroxide polymaltose in fasting condition, and 11 participants were tested without iron.  Reference values of absolute and relative plasma iron after oral iron ingestion could be established.  There was a significant increase of plasma iron after bivalent iron intake within 4 hours after ingestion (absolute increase after 4h: from initially 17 ± 4 µmol/l to 35 ± 12 µmol/l in females and from 21 ± 7 µmol/l to 33 ± 11 µmol/l in males, relative plasma iron concentration (defined reference: 1.0) after 4 h: 2.1 ± 0.7 in females and 1.7 ± 0.6 in males).  There was no relevant increase with trivalent iron nor without iron.  Relative iron increase correlates to iron storage.  In general, females had lower ferritin levels and thus higher increases of plasma iron.  An OIAT in healthy volunteers could be established.  Its prospective validation using bivalent iron in iron deficiency is desirable.