Two valid methods for one factor — and in mild disease they can disagree.
Factor VIII activity can be measured two ways: a one-stage aPTT-based clotting assay, and a two-stage chromogenic assay. In a subset of mild haemophilia A the two give different answers from the same sample, and the one-stage result is usually the higher. The gap is not laboratory error. It reflects how a particular F8 variant behaves in each assay system. Which method you trust can decide whether a patient is classed as mild or as having normal Factor VIII, and which assay you use to confirm a concentrate's potency. Technoclone's Technochrom FVIII:C (chromogenic), the FVIII inhibitor (Bethesda) assay and FVIII deficient plasma cover this work — we advise on when each method is the right call.
Plot one method against the other and the discrepant cases drop below the line.
If the two assays agreed everywhere, every sample would sit on the line of identity. In the discrepant subset — typically non-severe disease — the chromogenic result reads lower than the one-stage, so those points fall below the line. A patient can read in the haemophilia range by one method and near-normal by the other.
Line of identity; ringed points in the non-severe subset fall below it (one-stage > chromogenic).
Illustrative — schematic, not pooled patient data
Where the assays agree, one-stage ≈ chromogenic (points on the line). In the discrepant subset the one-stage value is the higher of the two, so the point sits below the line of identity. Discrepancy frequency runs from roughly 1% to about half of a cohort depending on the definition used — see below.
How often the two methods disagree — and why it is a range, not a number.
The reported frequency of one-stage/chromogenic discrepancy depends entirely on how "discrepant" is defined and which cohort is studied. It is a range — roughly 1% to about half of a cohort, depending on definition — not a single headline figure.
~30 %
of mild haemophilia A cases show a marked one-stage versus chromogenic discrepancy — a substantial minority in whom the method changes the result.
20 %
discrepant across a cohort of 163, with the one-stage result usually the higher of the two — the same direction shown in the scatter above.
1 %
discrepant under the strict ISTH-SSC definition (a one-stage / chromogenic ratio above 2.0 or below 0.5) — showing how tightening the definition collapses the figure.
Ph. Eur.
The European Pharmacopoeia mandates the chromogenic assay as the reference method for Factor VIII concentrate potency. Chromogenic is also preferred for certain extended-half-life and modified products, where one-stage can mis-read.
The discrepancy percentage is definition- and cohort-dependent, so it is best read as a range of roughly 1% to about half of a cohort. The phenomenon is a property of the one-stage and chromogenic methods themselves; it is not Technoclone-specific. Both methods are acceptable for diagnosis. The question is which one answers the clinical question in front of you.
Set against the one-stage clotting assay, the long-standing comparator, neither method is universally "best". Chromogenic is the prescribed reference for potency and the safer choice for some modified products; one-stage remains in routine use, is widely standardised, and reads many patients identically. The value is in choosing deliberately, and running both where the two might diverge.
The reagents, the method, and where judgement comes in.
Technochrom FVIII:C — chromogenic
A two-stage chromogenic Factor VIII activity assay: activated Factor X is generated in proportion to the FVIII present, then read against a chromogenic substrate. This is the method the European Pharmacopoeia prescribes for concentrate potency, and the one we recommend where extended-half-life or modified products are involved.
FVIII inhibitor (Bethesda) & deficient plasma
The Bethesda assay quantifies a neutralising FVIII inhibitor in Bethesda units; FVIII deficient plasma provides the substrate for one-stage factor assays and the inhibitor work-up. Together they cover diagnosis, quantification and monitoring around Factor VIII.
- As UK partner for Technoclone, we represent the reagents above and advise on which read-out and panel fit your throughput and case mix.
- We advise on assay selection: when chromogenic should lead, when one-stage is sufficient, and when to run both and report the discrepancy.
- Where modelling of a specific F8 variant's assay behaviour would help interpret a discrepant result, that is a human-led advisory service we provide — not a product on the shelf.
The sources behind every figure on this page.
Trossaert M, et al. Mild haemophilia A: one-stage versus chromogenic discrepancy (~30%). Haemophilia 2014;20(4):550–558. PMID 24517184
Cid AR, et al. One-stage/chromogenic discrepancy (20%, n=163; one-stage usually higher). Haemophilia 2008;14(5):1049–1054. PMID 18540892
Zwagemaker A-F, et al. Discrepancy under the strict ISTH-SSC ratio criterion (1%). JTH 2023;21(4):850–861. PMID 36696222
Marlar RA, et al. Reported one-stage vs chromogenic discrepancy of 10–51% across non-severe haemophilia A cohorts — the definition-dependent range. Eur J Haematol 2020;104(1):3–14. PMID 31606899
Hubbard AR, et al. (ISTH SSC). Recommendations on the potency labelling of factor VIII and factor IX concentrates — chromogenic as the reference method for FVIII concentrate potency, aligned with the European Pharmacopoeia. JTH 2013;11(5):988–9. PMID 23406084
Pickering W, et al. FVIII chromogenic assays for potency labelling and monitoring of N8-GP (glycoPEGylated modified rFVIII); one-stage clot assays add variability for modified products, chromogenic less affected. JTH 2016;14(8):1579–87. PMID 27241682
Tell us which Factor VIII question you need answered.
Send a short note — a chromogenic versus one-stage discrepancy you are trying to resolve, an inhibitor work-up, or a reagent you need to specify. We will reply with how we would approach it and an honest quote, usually within two working days.