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PLoS One. 2017 Dec 11;12(12):e0188332. doi: 10.1371/journal.pone.0188332. eCollection 2017.

Stability of serum ferritin measured by immunoturbidimetric assay after storage at -80°C for several years.

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INSERM UMR1153, Centre de Recherche en Épidémiologie et Statistique Sorbonne Paris Cité (CRESS), équipe Épidémiologie Périnatale, Obstétricale et Pédiatrique (ÉPOPé); Labex GR-Ex; Université Paris Descartes; Paris, France.
Department of General Pediatrics and Pediatric Infectious Diseases, Necker-Enfants malades hospital, AP-HP; Université Paris Descartes, Paris, France.
Université Paris Diderot, Sorbonne Paris Cité, Paris, France.
EPIUnit-Instituto de Saúde Pública, Universidade do Porto, Porto, Portugal.
Centre de recherche sur l'inflammation, INSERM UMR 1149, Université Paris Diderot; ERL CNRS 8252, Faculté de Médecine site Bichat, Labex GR-Ex, Paris, France.



Iron deficiency (ID) may impair long-term neurological development when it occurs in young infants. In cohort studies, it is sometimes necessary to evaluate ID with sera kept frozen for several years. To assess ID, learned societies recommend measuring serum ferritin (SF) level combined with C-reactive protein level. The long-term stability of C-reactive protein in frozen samples is well established but not ferritin.


We measured SF level (immunoturbidimetric assay; in micrograms per liter) immediately after collection from 53 young adults recruited and followed-up in Porto, Portugal, from 2011 to 2013 (SF1), and then, in 2016 in two aliquots kept frozen at- 80°C for 3 to 5 years: one without (SF2A) and one with (SF2B) intermediate thawing in 2014. We compared SF1 to SF2A then SF2B; statistical agreement was evaluated by the Bland and Altman method and the effect of intermediate thawing by regression modelling.


Mean SF2A-SF1 and SF2B-SF1 differences were -2.1 (SD 7.0) and 48.9 (SD 66.9). Values for Bland and Altman 95% limits of agreement were higher for the comparison of SF2B and SF1 than SF2A and SF1: -82.2 to 179.9 and -15.8 to 11.8, respectively; the effect of thawing was highly significant (p <0.001).


Agreement between SF values before and after 3 to 5 years of constant freezing at -80°C was in a generally accepted range, which supports the hypothesis of ferritin's stability at this temperature for a long period. In long-term storage by freezing, intermediate thawing induced a major increase in values.

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