Coagulation Mixing Study Interpreter
Coagulation Mixing Study Interpreter
Mix patient plasma 1:1 with normal plasma and repeat the aPTT. Correction means a factor is missing; failure to correct means something is inhibiting one. This page computes the Rosner index and the percent correction, and uses the incubated mix to separate a factor VIII inhibitor from a lupus anticoagulant.
Mixing study
Rosner index → correction or inhibitorPatient aPTT 62 s, normal pooled plasma 31 s, immediate 1:1 mix 55 s, incubated mix unchanged
The two indices
Percent correction = (patient − 1:1 mix) ÷ (patient − normal pooled plasma) × 100
- Rosner index
- also called the index of circulating anticoagulant. Classic cut-offs: below 12 is correction and suggests a factor deficiency, 12 to 15 is indeterminate, above 15 is failure to correct and suggests an inhibitor
- percent correction
- the Chang formula, expressing how far the mix moved from the patient's value back towards normal. A high percentage means correction; the cut-off is commonly placed around 70%, and is laboratory-specific
- the cut-offs are local
- both indices need a decision limit established by the reporting laboratory against its own reagent, analyser and normal pooled plasma. Some laboratories use 11 rather than 12 for the Rosner index. Published numbers are a starting point, not a standard
- why 1:1 works
- the mix leaves roughly 50% of normal factor activity, which is enough to normalise most clotting times. A deficiency is therefore masked and an inhibitor is not
Worked example
Patient aPTT 62 s, normal pooled plasma 31 s, immediate 1:1 mix 55 s, incubated mix unchanged
Rosner index = (55 − 31) ÷ 62 × 100 = 24 ÷ 62 × 100 = 38.7
38.7 is well above 15, so the mix has failed to correct
Percent correction = (62 − 55) ÷ (62 − 31) × 100 = 7 ÷ 31 × 100 = 22.6% — the same conclusion from the other formula
The incubated mix is unchanged, so the inhibition is immediate and non-progressive
That is the behaviour of a lupus anticoagulant, which acts on reagent phospholipid rather than on a factor
Exclude heparin with a thrombin time, then confirm with a dRVVT screen and confirm pair, repeated at twelve weeks
What the four patterns mean
| Immediate 1:1 mix | After 2 hours at 37 °C | Interpretation | Next test |
|---|---|---|---|
| Corrects | Still corrected | Factor deficiency | Individual factor assays guided by which screen is prolonged |
| Corrects | Prolongs | Time-dependent inhibitor — classically acquired factor VIII | Factor VIII level and Bethesda assay, urgently |
| Does not correct | Unchanged | Immediate inhibitor — usually a lupus anticoagulant | dRVVT screen and confirm, plus a lupus-sensitive aPTT, repeated at twelve weeks |
| Does not correct | Prolongs further | Inhibitor, time-dependent | Factor VIII level and Bethesda assay. Around 15% of lupus anticoagulants also behave this way |
Causes by pattern
| Pattern | Common causes | Bleeding risk |
|---|---|---|
| Corrects — factor deficiency | Factor VIII, IX or XI deficiency; factor XII deficiency; liver disease; vitamin K deficiency; warfarin | Depends on the factor. Factor XII deficiency prolongs the aPTT markedly and causes no bleeding whatever |
| No correction, immediate | Lupus anticoagulant; heparin in the sample | A lupus anticoagulant is a thrombotic risk, not a bleeding one, despite the prolonged clotting time |
| No correction, time-dependent | Acquired factor VIII inhibitor; alloantibody in treated haemophilia A | High. Acquired haemophilia A causes severe soft-tissue and mucosal bleeding and carries meaningful mortality |
| Anything, in a patient on an anticoagulant | Heparin, direct oral anticoagulants, warfarin | Establish the drug history before interpreting anything. A direct oral anticoagulant can produce a non-correcting mix and is not an inhibitor |
Half the factor, all of the inhibitor
The mixing study rests on one piece of arithmetic. Most clotting factors have to fall below about 40% of normal activity before the aPTT or the prothrombin time becomes prolonged, so mixing patient plasma with an equal volume of normal pooled plasma leaves roughly 50% activity, which is enough to bring the clotting time back into range. If the problem was a missing factor, the mix corrects. An inhibitor, by contrast, is not diluted out of existence: it acts on the normal plasma too, and the mix stays prolonged. Correction therefore means deficiency and failure to correct means inhibitor, and almost everything else about the test is detail on top of that.
Deciding whether a mix has corrected is less obvious than it sounds, because "back into the reference range" is a poor rule when the patient's starting value was very long. Two indices are in common use. The Rosner index, or index of circulating anticoagulant, is the mix minus the normal pooled plasma, divided by the patient's value, times a hundred: below 12 counts as correction, above 15 as failure to correct, and the gap between is indeterminate and should be repeated. The Chang percent correction expresses how far the mix travelled from the patient's value back towards the normal, with a cut-off commonly placed near 70%. Both are useful, both usually agree, and neither is a standard — every laboratory has to establish its own decision limit against its own reagent, analyser and normal pooled plasma, and some use a Rosner cut-off of 11 rather than 12.
The incubated mix is where the test earns its keep. A lupus anticoagulant interferes with the phospholipid in the reagent, which it does instantly, so the immediate mix fails to correct and two hours at 37 °C changes nothing. An autoantibody against factor VIII works differently: it has to bind and neutralise, which takes time and warmth. The immediate mix can therefore correct completely, looking for all the world like a factor deficiency, and only the incubated mix reveals the inhibitor. That is how acquired haemophilia A is missed — an older patient or a woman post partum with new soft-tissue bruising, a prolonged aPTT that corrects, and no further testing. Incubate the patient plasma and the normal plasma separately alongside the mix, because factors V and VIII decay in vitro and a prolongation that appears in the normal control is a storage artefact rather than an antibody.
Two things should be settled before any of this is interpreted. Heparin in the sample produces a perfect non-correcting pattern and is the commonest explanation for one on a general ward, usually from a line draw; a normal thrombin time excludes it in minutes. And a direct oral anticoagulant will also prevent correction without any inhibitor being present. Establish what the patient is taking and where the blood came from, then interpret the mix. Finally, a mixing study is a triage test and never a diagnosis: a suspected lupus anticoagulant needs a phospholipid-dependent screen and confirm pair and a repeat at twelve weeks, and a suspected factor inhibitor needs a factor level and a Bethesda titre.
Frequently asked questions
What does it mean if a mixing study corrects?
That the patient's plasma is short of a clotting factor and the normal plasma supplied it. Mixing 1:1 leaves roughly 50% of normal activity, which is enough to normalise most clotting times. Follow with individual factor assays guided by which screen was prolonged, and add an incubated mix, because a factor VIII inhibitor also corrects immediately.
How do you tell a lupus anticoagulant from a factor VIII inhibitor?
By incubating the mix for two hours at 37 °C. A factor VIII autoantibody is time- and temperature-dependent, so the clotting time gets worse after incubation. A lupus anticoagulant inhibits phospholipid immediately and does not progress. Around 15% of lupus anticoagulants are themselves time-dependent, so confirmatory testing is still required.
What is the Rosner index?
The index of circulating anticoagulant: the 1:1 mix clotting time minus the normal pooled plasma clotting time, divided by the patient's clotting time, times a hundred. Below 12 indicates correction and a factor deficiency, above 15 failure to correct and an inhibitor, and 12 to 15 is indeterminate. The decision limit is laboratory-specific.
Why does heparin cause a mixing study to fail to correct?
Because heparin is an inhibitor. It potentiates antithrombin, which inactivates thrombin and factor Xa in the mixed plasma just as it did in the patient's, so adding normal plasma does not help. Heparin contamination from a line is the commonest cause of an unexpected non-correcting mix; a normal thrombin time excludes it.
Can a mixing study diagnose antiphospholipid syndrome?
No. Failure to correct that persists after incubation is consistent with a lupus anticoagulant, but the diagnosis requires a phospholipid-dependent screening and confirmatory pair such as the dilute Russell's viper venom time, persistence on repeat testing at least twelve weeks later, and interpretation alongside the clinical history and solid-phase antibodies.
Related calculators
References
- Rosner E, Pauzner R, Lusky A, Modan M, Many A. Detection and quantitative evaluation of lupus circulating anticoagulant activity. Thromb Haemost. 1987;57(2):144–147.
- Chang SH, Tillema V, Scherr D. A "percent correction" formula for evaluation of mixing studies. Am J Clin Pathol. 2002;117(1):62–73.
- Kershaw G, Orellana D. Mixing tests: diagnostic aides in the investigation of prolonged prothrombin times and activated partial thromboplastin times. Semin Thromb Hemost. 2013;39(3):283–290.
- Devreese KMJ, de Groot PG, de Laat B, et al. Guidance from the Scientific and Standardization Committee for lupus anticoagulant/antiphospholipid antibodies of the ISTH. J Thromb Haemost. 2020;18(11):2828–2839.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.