EQAWarden knowledge base · Ferritin

Ferritin EQA results: why they fail and what to check.

What is known to go wrong with ferritin in external quality assurance, how each problem shows itself in the results, and the checks to make, with every source listed. This is the ferritin monograph EQAWarden ships with. First check: Compare with the Siemens Atellica ferritin group rather than the all-method mean (ferritin methods are not harmonised).

Technical content: Craig MacKenzie, medical scientist, chemical pathology, 27 years of experience.

At a glanceKnown EQA pitfallsInvestigation checksPlainly answeredReferences
At a glance

The limits, units and methods behind every ferritin result.

RCPAQAP APS, General Serum Chemistry (GSC)±4 µg/L up to 27 µg/L, ±15% above
Unitsµg/L. 1 ng/mL = 1 µg/L
MethodsTwo-site sandwich chemiluminescent immunoassay (acridinium-labelled goat polyclonal detection antibody, mouse monoclonal capture antibody covalently coupled to paramagnetic particles, no streptavidin-biotin), direct RLU: Atellica IM Fer (confidence: High)[1]
Other sandwich immunoassays (chemiluminescence, electrochemiluminescence) and latex immunoturbidimetry on other platforms, calibrated to one of the WHO ferritin international standards (confidence: High)[4, 3]
StandardisationNo reference measurement procedure and no JCTLM-listed reference material found[6]. Successive WHO international standards exist (2nd IS 80/578, 3rd IS 94/572, newest 19/118), but ferritin isoform heterogeneity and antibody selectivity leave between-method differences: inter-assay CV 34% across Abbott Alinity, Beckman DxI, Roche Cobas and Siemens Atellica in 2026, falling to 8.2% after recalibration with a commutable material[4, 3]. The Atellica IM assay states traceability to the WHO 2nd IS 80/578 and reads about 3% above ADVIA Centaur FER (y = 1.03x - 0.6 ng/mL, n 106)[1]. The fair EQA comparator is the Atellica method group.
Expected error modeProportional (calibration, lot, shared CAL C); constant component only near the lower limit.
Biological variationWithin-subject CV 12.87%, from the EFLM database (meta-analysis, 2 studies).
Desirable performanceImprecision 6.43%, derived from biological variation (Fraser). Bias and total error are not calculated where the database holds no between-subject figure.

Platform details are for the Siemens Atellica, the platform EQAWarden's first release covers. Other platforms differ: check your own instructions for use.

Known EQA pitfalls

11 ways a ferritin result goes wrong in EQA, and how to tell them apart.

Method-group target: ferritin assays are not harmonised

Why it happens. Methods are calibrated to different WHO ferritin standards and recognise liver and spleen isoforms differently, so between-method differences of tens of percent persist; in a 2026 study four EQA materials behaved like clinical samples in all method pairs, so the differences seen in EQA largely reflect true method differences rather than material artefacts[3]. Blackmore found one method over-recovering the WHO standards by 24-55%[4].

Direction. method-specific

How it shows. method-group, persistent, proportional; the Atellica peer group as a whole sits away from the all-method target

Check. Compare the result with the Siemens Atellica ferritin group; if the laboratory agrees with its peers but not with the all-method target, record it as a method difference, not a laboratory fault.

Confidence. High[1, 3, 4, 8, 9]

CAL C is shared with vitamin B12

Why it happens. The same lyophilised Calibrator C (5.0 mL per vial) calibrates both Atellica IM ferritin and vitamin B12, with separate assigned values per analyte on one lot sheet. Once reconstituted it keeps 28 days at 2-8 C but only 4 hours at room temperature, and it has no frozen-storage claim. A reconstitution volume error, an over-age or warm calibrator, or the wrong value sheet would shift both analytes in the same direction on the analyser calibrated with it.

Direction. either

How it shows. step, proportional, instrument-specific; ferritin and vitamin B12 move together on one analyser from a dated calibration

Check. Check vitamin B12 on the same analyser and survey; open the CAL C lot, reconstitution date and time on board for the last ferritin and B12 calibrations; compare IQC for both before and after.

Confidence. High[1, 2, 10]

New reagent lot and its master curve (Fer)

Why it happens. Each Atellica IM reagent lot carries a lot-specific master curve read from 2D barcodes; a new lot needs a new two-point calibration. Intervals in the IFU: lot calibration 50 days, pack calibration 28 days, onboard stability 28 days. The pack interval and onboard stability are both 28 days, so packs are recalibrated often. A lot-to-lot difference that the two-point calibration does not correct shifts results on every analyser using that lot. The size of the shift can differ between patient samples and EQA or control material.

Direction. either (sign set by the lot difference)

How it shows. step, proportional, persistent; begins at the date the new reagent lot was first used and follows that lot

Check. Pull the reagent and calibrator lot history for the analysis date; compare IQC before and after the lot change and, if available, run the master curve material or a retained EQA sample on both lots.

Confidence. High[1, 11, 12]

Two-point recalibration or calibrator handling (Fer, CAL C)

Why it happens. Atellica IM two-point calibration adjusts the lot master curve with a low and a high calibrator. CAL C is reconstituted with 5.0 mL special reagent water by precision pipette and stood 15-20 minutes before mixing; 28 days at 2-8 C, 4 hours at room temperature, no frozen claim. A reconstitution volume error, an evaporated or out-of-date opened calibrator, or a calibrator from the wrong lot shifts every result after that calibration on that analyser.

Direction. either

How it shows. step, proportional, instrument-specific; starts at a dated calibration on one analyser

Check. Open the calibration record for the run (date, calibrator lot, reconstitution or opening date and time, calibration acceptance); compare IQC before and after; compare with a second analyser calibrated separately.

Confidence. High[1, 2, 11]

Chemiluminescent signal or wash fault in a sandwich assay

Why it happens. The assay is a two-site sandwich with a direct relationship between analyte and relative light units (RLU). A loss of signal (trigger reagents, luminometer, particle loss at washing) reads low, while poor washing of unbound label reads high; competitive assays on the same analyser move the opposite way. The assay chart lists a water wash for Fer, whereas TnIH, BNP and NT-proBNP use Atellica IM Wash, so a fault in the IM Wash fluid may not move ferritin with those assays (confidence: Medium, inferred from the chart).

Direction. either: negative for a loss of signal; positive for residual unbound label

How it shows. instrument-specific, survey-wide, analyte-group:im_sandwich; sandwich assays move one way and competitive assays the other on the same analyser

Check. Check whether other Atellica IM sandwich assays on the same analyser moved in the same direction and competitive assays in the opposite direction; review trigger reagent, wash fluid and probe wash onboard times, luminometer checks and maintenance logs.

Confidence. Medium[1, 11, 13, 7]

Plasma-serum difference and haemolysis

Why it happens. In the IFU, K2 EDTA and lithium heparin plasma read about 4-5% below serum (slopes 0.96 and 0.95). Gross haemolysis releases red-cell ferritin and raises the result, even though added haemoglobin up to 900 mg/dL changed results by 0 to -2%. RCPAQAP ferritin material is human serum, so plasma bias applies only if a laboratory compares a plasma-based internal target.

Direction. either: negative for plasma against serum; positive for a haemolysed native sample

How it shows. sample-specific, single

Check. Confirm the EQA material type from the RCPAQAP instructions; for a single positive outlier look for visible haemolysis in the reconstituted vial.

Confidence. Medium[1]

Turbid or lipaemic reconstituted material

Why it happens. Intralipid 2000 mg/dL raised Atellica ferritin by 6-7% in the IFU. Reconstituted lyophilised serum can be turbid, which could add a small positive bias to one sample.

Direction. positive (small)

How it shows. single, sample-specific

Check. Inspect the reconstituted vial for turbidity; check whether other IM results from the same vial are also high.

Confidence. Low[1, 14]

High ferritin samples: onboard dilution and hook

Why it happens. The measuring interval is 0.5-1650 µg/L; above that the sample is diluted onboard 1:2, 1:5 or 1:10 with Multi-Diluent 1, and dilution recovery averaged 94% (per-sample means 91-98%). No hook effect was seen up to 80,000 µg/L. RCPAQAP Liquid Serum Chemistry material comes from haemochromatosis venesection donors, so high ferritin samples are plausible.

Direction. negative (a few percent on diluted samples); either for a wrong manual dilution factor

How it shows. single, concentration-dependent, proportional; only samples above 1650 µg/L

Check. For a high sample, check the dilution flag and factor, whether a manual dilution was corrected, and the onboard age of Multi-Diluent 1.

Confidence. Medium[1, 15, 16]

Low ferritin samples and imprecision

Why it happens. Within-lab SD was 0.31 µg/L at 4.2 µg/L and CV 5.4% at 8.2 µg/L; LoQ 0.9 µg/L. Near the iron-deficiency range a constant offset of a few tenths of a µg/L is a large percentage.

Direction. either

How it shows. concentration-dependent, constant, single

Check. Judge a low sample against the absolute part of the APS; compare with IQC at a similar level.

Confidence. High[1]

Test definition update left operator-defined parameters unchanged

Why it happens. Siemens issues new Atellica IM test definition versions (Fer test definition 1.2 added alternate units with a default conversion factor of 2.2 pmol/L per ng/mL). Operator-defined parameters are not updated automatically when a test definition is installed, so a dilution point, unit or conversion factor set by the laboratory can differ from the new default.

Direction. either

How it shows. step, instrument-specific; starts at the software or test definition update

Check. Compare the installed test definition version, reporting unit and conversion factor on each analyser with the current assay chart and the enhancement notice; check the date of the last test definition installation.

Confidence. Medium[1, 17, 7]

Heterophile or human anti-animal antibodies

Why it happens. The IFU says the assay is designed to minimise heterophilic antibody interference but that such antibodies can still give falsely high or low results. These antibodies belong to individual donors; a pooled EQA material is unlikely to carry them at an effective level unless it comes from a single donor.

Direction. either

How it shows. single, sample-specific; one sample reads off in one method group and not in others

Check. If a single-donor EQA sample is off in the Atellica group only, ask RCPAQAP about the material source; retest after a heterophile blocking step or on another platform if the laboratory keeps the sample.

Confidence. Low[1, 18, 19]

Investigation checks

The checks, in order.

  1. Compare with the Siemens Atellica ferritin group rather than the all-method mean (ferritin methods are not harmonised).
  2. Check vitamin B12 on the same analyser: a joint shift points to the shared CAL C calibration.
  3. Review calibration and lot dates (lot 50 days, pack 28 days) and IQC across them.
  4. For a high sample, check dilution factor and recovery; for a low sample, use the APS absolute limit.
  5. Inspect the vial for haemolysis or turbidity; check units (µg/L and ng/mL are equal; pmol/L = µg/L x 2.2).

Unit traps

  • µg/L and ng/mL are numerically identical.
  • pmol/L = µg/L x 2.2 (Atellica alternate unit); a ratio near 2.2 suggests pmol/L entered as µg/L.

EQAWarden runs these checks as rules on every survey, ranks the likely causes with their evidence, and the reviewer decides. All analytes · The 58 rules · The whole knowledge base · How an EQA investigation runs

Plainly answered

Questions about ferritin EQA.

What is the RCPAQAP allowable performance specification for ferritin?

In the General Serum Chemistry (GSC) programme it is ±4 µg/L up to 27 µg/L, ±15% above, evaluated at the target. RCPAQAP can change its specifications, so confirm the current table at each enrolment year.

What is a common cause of a failed ferritin EQA result?

Method-group target: ferritin assays are not harmonised. Methods are calibrated to different WHO ferritin standards and recognise liver and spleen isoforms differently, so between-method differences of tens of percent persist; in a 2026 study four EQA materials behaved like clinical samples in all method pairs, so the differences seen in EQA largely reflect true method differences rather than material artefacts. Blackmore found one method over-recovering the WHO standards by 24-55%.

What should be checked first when a ferritin EQA result fails?

Compare with the Siemens Atellica ferritin group rather than the all-method mean (ferritin methods are not harmonised).

How are ferritin units converted?

1 ng/mL = 1 µg/L.

References

Where every fact on this page came from.

  1. Siemens Healthineers. Atellica IM Ferritin (Fer) [OUS], Instructions for Use, part 10995343 Rev. 04, 2023-03. Siemens Document Library id 982939.
  2. Siemens Healthineers. Atellica IM and Atellica CI Calibrator C (CAL C), Instructions for Use, part 11204921 Rev. 03, 2024-01. Siemens Document Library id 1054821. https://content.doclib.siemens-healthineers.com/rest/v1/view?document-id=1054821
  3. Bioch MM, Schoeman RM, Weykamp CW, et al. Evaluating feasibility of ferritin assay harmonisation with commutable proof-of-concept reference materials and EQA-based monitoring. Clin Chem Lab Med 2026 (online 3 Sep 2026). Abbott Alinity, Beckman Access DxI, Roche Cobas and Siemens Atellica: inter-assay CV 34%, 8.2% after recalibration with a commutable material; four EQA materials fell within the 95% prediction interval of clinical samples; the newest WHO standard 19/118 showed poor commutability.
  4. Blackmore S, Hamilton M, Lee A, Worwood M, Brierley M, Heath A, Thorpe SJ. Automated immunoassay methods for ferritin: recovery studies to assess traceability to an international standard. Clin Chem Lab Med 2008;46:1450-1457. Recoveries of the 2nd and 3rd WHO IS were 90-110% for four of five methods and 124-155% for one; ferritin heterogeneity and no reference measurement procedure limit standardisation.
  5. Williams RD, Duff CJ, Yates A. Determination of in vitro stability of routine haematinics tests using EFLM standards and the CRESS checklist. Ann Clin Biochem 2023;60:367-373. On the Siemens Atellica platform ferritin (except gel-free tubes) was stable more than 5 days at room temperature and at 2-8 C.
  6. JCTLM Database of higher-order reference materials and reference measurement procedures. Keyword searches (troponin, troponin I, cardiac troponin, ferritin, prostate, PSA, natriuretic, BNP) through the public API https://www.jctlmdb.org/backend/api/jctlm/searchRm and searchRmp (multipart fields page_number, nb_results, keywords), 2026-09-29. Found only C1RM_P4, BCR-613 prostate specific antigen (purified PSA, EU-JRC, certified mass 70.8 ug per unit); none found for troponin I or T, ferritin, BNP or NT-proBNP. Absence in a keyword search is reported as 'none found', not as proof of non-listing.
  7. Siemens Healthineers. Atellica Solution IM Assay Chart, part 11314389 Rev. 05, 2025-12. Siemens Document Library id 1151608. https://content.doclib.siemens-healthineers.com/rest/v1/view?document-id=1151608
  8. Miller WG, Jones GRD, Horowitz GL, Weykamp C. Proficiency testing/external quality assessment: current challenges and future directions. Clin Chem 2011;57:1670-80. doi:10.1373/clinchem.2011.168641
  9. Badrick T, Miller WG, Panteghini M, Delatour V, Berghall H, MacKenzie F, Jones G. Interpreting EQA - understanding why commutability of materials matters. Clin Chem 2022;68:494-500. doi:10.1093/clinchem/hvac002
  10. Siemens Healthineers. Atellica IM Vitamin B12 (VB12), Instructions for Use, part 10995437 Rev. 03, 2023-03. Siemens Document Library id 981048. https://content.doclib.siemens-healthineers.com/rest/v1/view?document-id=981048
  11. Siemens Healthineers. Atellica Solution Operator's Guide, software version 1.31, part 11069101 Rev. 13, 2025-10.
  12. Miller WG, Erek A, Cunningham TD, Oladipo O, Scott MG, Johnson RE. Commutability limitations influence quality control results with different reagent lots. Clin Chem 2011;57:76-83. doi:10.1373/clinchem.2010.148106
  13. Wild D (ed.). The Immunoassay Handbook: Theory and Applications of Enzyme Immunoassay, Electrochemiluminescence and Related Techniques. 4th ed. Oxford: Elsevier; 2013. Cited as textbook knowledge for competitive versus sandwich signal-response relationships; not re-read in this review.
  14. RCPAQAP Chemical Pathology. QAP Commutability & Matrix Effects. 2013 (Wayback capture, 2014-2015 range; base URL not captured in the research document).
  15. RCPAQAP product pages for General Serum Chemistry, Liquid Serum Chemistry, Alcohol and Ammonia, Blood Gases, Trace Elements: Serum (rcpaqap.com.au/product/...). Direct fetch returned HTTP 403 in this session; content (sample type, volume, storage on receipt, LSC 'commutable, haemochromatosis venesection source') was taken from search-engine summaries of the pages. Re-read the live pages. https://rcpaqap.com.au/products/chemical/page/4
  16. Jassam N, Jones CM, Briscoe T, Horner JH. The hook effect: a need for constant vigilance. Ann Clin Biochem 2006;43:314-317. doi:10.1258/000456306777695726
  17. Siemens Healthineers. Atellica IM Analyzer Test Definition Enhancements, Customer Information, part 11314997 Rev. 17, 2026-05. Siemens Document Library id 1173692. https://content.doclib.siemens-healthineers.com/rest/v1/view?document-id=1173692
  18. Kricka LJ. Human anti-animal antibody interferences in immunological assays. Clin Chem 1999;45:942-956. doi:10.1093/clinchem/45.7.942
  19. Tate J, Ward G. Interferences in immunoassay. Clin Biochem Rev 2004;25:105-120 (AACB journal). https://pmc.ncbi.nlm.nih.gov/articles/PMC1904417/
  20. EFLM Biological Variation Database (Aarsand AK et al., EFLM WG-BV). https://biologicalvariation.eu/ - meta-analysis values read from https://biologicalvariation.eu/api/meta_calculations and individual studies from https://biologicalvariation.eu/api/bv_specifications, retrieved 2026-09-28. https://biologicalvariation.eu/
  21. Fraser CG. Biological Variation: From Principles to Practice. Washington DC: AACC Press; 2001 (textbook; formulae desirable CVa<=0.5CVi, bias<=0.25*sqrt(CVi^2+CVg^2), TEa=1.65*CVa+bias).
  22. RCPAQAP. Chemical Pathology Analytical Performance Specifications (web page; dateModified 2025-03-05). https://rcpaqap.com.au/resources/chemical-pathology-analytical-performance-specifications/

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