Rosner Index Cutoff Values for aPTT Mixing Studies
The Rosner index cutoff value to determine if aPTT is corrected in a mixing study is approximately 15% or less, with values above 15-15.7% suggesting the presence of an inhibitor rather than factor deficiency. 1, 2
Understanding the Rosner Index
The Rosner index is calculated to differentiate between factor deficiencies (which correct with mixing) and inhibitors (which do not correct) when evaluating prolonged aPTT. The formula assesses the degree of correction when patient plasma is mixed 1:1 with normal pooled plasma.
Specific Cutoff Values from Evidence
15.7% cutoff: A 2012 study analyzing 525 specimens with positive lupus anticoagulant screening tests identified 15.7 as the optimal Rosner index cutoff value for dRVVT-based testing, with values above this threshold indicating inhibitor presence 2
General threshold of 15%: The Rosner index uses approximately 15% as the discriminatory value, where correction below this percentage suggests factor deficiency, while values exceeding this suggest an inhibitor 1
Comparison with Other Interpretation Methods
The estimated factor correction (EFC) method has demonstrated superior accuracy compared to the Rosner index in recent research, correctly identifying 38 of 39 known inhibitors and factor deficiencies 1. However, the Rosner index remains widely used and shows comparable thrombotic risk prediction to percentage correction methods 2.
Clinical Application
When interpreting mixing studies:
- Rosner index ≤15-15.7%: Suggests factor deficiency with adequate correction 1, 2
- Rosner index >15-15.7%: Suggests presence of an inhibitor (such as lupus anticoagulant or specific factor inhibitor) 1, 2
Important Caveats
The mixing-positive group (Rosner index >15.7%) demonstrates higher lupus anticoagulant titers and increased thrombotic risk compared to mixing-negative groups 2
Even mixing-negative results (Rosner index ≤15.7%) can be associated with substantial thrombotic risk, so clinical correlation remains essential 2
Different aPTT reagents show considerable variation in heparin responsiveness, which can affect mixing study interpretation 3, 4
The presence of heparin, direct thrombin inhibitors, or factor Xa inhibitors can interfere with mixing study results and should be excluded before interpretation 5
Integration with Modern Testing Approaches
Updated international guidelines for lupus anticoagulant detection suggest that integrated test systems (using screening and confirmation at different phospholipid concentrations) may not require the mixing test step 5, 2. The two-step procedure without mixing has shown higher thrombotic risk prediction than traditional three-step procedures including mixing 2.