Mixing Study Rosner Index and Percent Correction

When a patient's activated partial thromboplastin time (APTT) comes back prolonged, the mixing study is the test that tells clinicians whether the cause is a missing clotting factor or an inhibitor blocking the clotting cascade. But "the APTT looks corrected" is not good enough for a result that guides treatment decisions. Two factors deficiency and an inhibitor can produce mixes that look similar to the eye, especially near the borderline. This is where the Rosner Index, also called the Index of Circulating Anticoagulant (ICA), and the Percent Correction method come in. Both are simple formulas that convert mixing study results into an objective number, removing guesswork from the interpretation. Instead of asking "does this look corrected?", the laboratory scientist calculates a value and compares it against a validated cutoff.

These formulas are used daily by hematology laboratory scientists, coagulation specialists, and hematologists reviewing results for patients with unexplained bleeding, unexplained clotting, or a prolonged APTT found incidentally before surgery.

How the Formulas Work

Both formulas use the same three values: the APTT of the patient plasma (PP), the APTT of the pooled normal plasma (NP), and the APTT of the 1:1 mix of the two.

Rosner Index (ICA)

Rosner Index / ICA= (1:1 Mix APTT−NP APTT)​ / PP APTT ×100

This formula measures how far the mix sits above the normal plasma baseline, scaled against how prolonged the patient's own plasma was to begin with. A bigger gap relative to the patient's baseline prolongation points toward an inhibitor.

Interpretation

• Below 12: Correction. Suggests a factor deficiency.
• 12 to 15: Borderline. Needs repeat testing or clinical correlation.
• Above 15: No correction. Suggests an inhibitor, commonly a lupus anticoagulant or a specific factor antibody.

Percent Correction

% Correction = (PP APTT - 1:1 Mix APTT) / (PP APTT - NP APTT) x 100

This formula asks a more direct question: of the total prolongation in the patient's plasma, how much did the normal plasma manage to fix?

Interpretation:

• Above 70%: Correction. Suggests a factor deficiency.
• Below 50%: No correction. Suggests an inhibitor.
• 50% to 70%: Borderline.

The two formulas usually agree, but not always. The 2024 ICSH recommendations suggest using one of these calculations as a first step, followed by a confirmatory subtraction method, rather than relying on either formula alone [1].

Mixing Study Calculator

Case Scenarios

Case 1: Suspected Factor Deficiency

A 19-year-old male presents with easy bruising and a prolonged APTT of 58 seconds. PT is normal.

• PP APTT = 58 s
• NP APTT = 29 s
• 1:1 Mix APTT = 31 s

Rosner Index = (31 − 29) / 58 × 100 = 3.4 → Correction

Percent Correction = (58 − 31) / (58 − 29) × 100 = 93% → Correction

Both formulas agree on correction. This pattern points toward a factor deficiency. The patient went on to have a Factor VIII assay, which confirmed mild hemophilia A.

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Case 2: Suspected Lupus Anticoagulant

A 34-year-old woman with a history of two miscarriages has a prolonged APTT of 52 seconds found on a pre-surgical screen.

• PP APTT = 52 s
• NP APTT = 28 s
• 1:1 Mix APTT = 44 s

Rosner Index = (44 − 28) / 52 × 100 = 30.8 → No correction

Percent Correction = (52 − 44) / (52 − 28) × 100 = 33% → No correction

Both formulas point firmly toward an inhibitor. Given her clinical history, lupus anticoagulant was suspected and confirmed with a phospholipid neutralization test and a dilute Russell viper venom time (dRVVT) [2].

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Case 3: A Borderline Result

A 58-year-old man on no anticoagulants has an incidentally prolonged APTT of 40 seconds.

• PP APTT = 40 s
• NP APTT = 28 s
• 1:1 Mix APTT = 33 s

Rosner Index = (33 − 28) / 40 × 100 = 12.5 → Borderline

Percent Correction = (40 − 33) / (40 − 28) × 100 = 58% → Borderline

Neither formula gives a clean answer. This is a common real-world scenario, and it is exactly why the ICSH recommends repeating the test or correlating with the clinical picture rather than forcing a result into "corrected" or "not corrected" [1]. A low-titer or time-dependent inhibitor can sit right at this borderline, so an incubated mix and a Bethesda assay were ordered as the next step.

Why It Matters

A patient mistakenly labeled as having a factor deficiency when they actually have a lupus anticoagulant could be denied appropriate anticoagulation, or worse, treated as if they need factor replacement when their real risk is thrombosis. Conversely, a missed factor deficiency might delay treatment for active bleeding. The Rosner Index and Percent Correction exist to take that decision out of the realm of "it looks corrected" and put it on solid, reproducible ground.

Disclaimer: This article is intended for educational and informational purposes only. It is not intended to be a substitute for informed professional medical advice, diagnosis, or treatment. While the information presented here is derived from credible medical sources and is believed to be accurate and up-to-date, it is not guaranteed to be complete or error-free. See additional information.

References

1. Adcock, D. M., Moore, G. W., Kershaw, G. W., Montalvao, S. A. L., & Gosselin, R. C. (2024). International Council for Standardization in Haematology (ICSH) recommendations for the performance and interpretation of activated partial thromboplastin time and prothrombin time mixing tests. International journal of laboratory hematology, 46(5), 777–788. https://doi.org/10.1111/ijlh.14344
2. Devreese, K. M. J., de Groot, P. G., de Laat, B., Erkan, D., Favaloro, E. J., Mackie, I., Martinuzzo, M., Ortel, T. L., Pengo, V., Rand, J. H., Tripodi, A., Wahl, D., & Cohen, H. (2020). Guidance from the Scientific and Standardization Committee for lupus anticoagulant/antiphospholipid antibodies of the International Society on Thrombosis and Haemostasis: Update of the guidelines for lupus anticoagulant detection and interpretation. Journal of thrombosis and haemostasis : JTH, 18(11), 2828–2839. https://doi.org/10.1111/jth.15047