Automated immunoassay methods for ferritin: recovery studies to assess traceability to an international standard.
BACKGROUND
Ferritin standardisation is problematical due to the heterogeneity of ferritin isoforms and the antibodies used in its immunoassay, and the lack of a reference measurement procedure. We investigated the performance of the 1st (liver), 2nd (spleen) and 3rd (recombinant) International Standards (ISs) for ferritin in major assays.


METHODS
The ferritin in a serum pool 'spiked' with either the 2nd or 3rd IS for ferritin was measured by 52 laboratories using five automated methods and the recovery of the target values calculated. A smaller serum pool was 'spiked' with the 1st IS for a limited recovery exercise. The ferritin values of five serum samples were also measured and recalculated relative to the ISs.


RESULTS
Recoveries of each of the 2nd and 3rd ISs were 90%-110% for four of five methods; recoveries of the 1st IS were 104% and 111% for two of three methods claiming traceability to this IS. One method significantly over-recovered each of the IS (124%-155%). Recalculating the ferritin values of the serum samples relative to the IS reduced the overall inter-method agreement, largely because of the anomalous over-recovery of the IS by one method.


CONCLUSIONS
The use of the 3rd IS to standardise assays will minimise assay drift due to manufacturers adopting a 'harmonisation' approach in which the calibration is adjusted to conform to overall mean values. Standardisation against the current IS also ensures compliance with the European Union In-Vitro Diagnostic Directive which requires traceability of assay calibrators to reference materials of a higher order. Assay drift may result in poor sensitivity and specificity in the diagnosis of iron status, and would require laboratories to continually re-evaluate reference intervals.
Blackmore
Sheena
S
UK NEQAS Scheme for Haematinics, Heart of England Foundation Trust, Good Hope Hospital, Sutton Coldfield, West Midlands, UK.
Hamilton
Malcolm
M
Lee
Anne
A
Worwood
Mark
M
Brierley
Matthew
M
Heath
Alan
A
Thorpe
Susan J
SJ
D001331:Automation; D005293:Ferritins / Q000032:analysis*; D006801:Humans; D007118:Immunoassay / Q000379:methods* / Q000592:standards*; D038622:Internationality; D012015:Reference Standards; D015203:Reproducibility of Results
D005293:Ferritins