Hi. I am Krishna. I would like to ask about the interpretation of an APTT (PTT) mixing study that we performed by calculating the Rosner index:
- Patient sample: 51.4 s
- Normal plasma (NP): 28.9 s
- Immediate 1:1 mix: 29.0 s
- Incubated 2 h, 37°C 1:1 mix: 86.6 s
Rosner index formula: (Mix PTT – NP PTT) / Patient PTT × 100
Rosner index calculation: 86.6 – 28.9/51.4 x 100 = 112
Rosner Index: 112 (>15)
<12: Correction; factor deficiency
12–15: Suspicious; re-test
>15: No correction; lupus anticoagulant (LA)? Inhibitor?
How do I interpret this result, do I need to give the value of the Rosner index of 112 or give the interpretation of an inhibitor and perform a test for LA? Please advise.
Hello, Krishna, and thank you for your question. Your PTT mixing study indicates correction in the immediate 1:1 mix and no correction in the incubated 1:1 mix. This pattern leads to the presumption that a specific inhibitor such as anti-FVIII is present, as most specific inhibitors are time and temperature dependent. In case the patient’s bleeding history is not available, your next step is to perform factor assays, for instance, FVIII, IX, and XI, and continue with an inhibitor assay when indicated. This presumes that the PT is normal. In most instances, LA is ruled out when the immediate mix PTT result indicates correction.
You may wish to update your mixing study procedure by incubating the NP along with the 1:1 mix, as incubation is likely to prolong the NP PTT. You report an unusually wide disparity between the immediate and incubated mix result, I would be curious to know the formulation of the PTT reagent you are using. The Rosner index decision limit may vary among laboratories, in some instances the ratio of 11 is used as the cutoff, though the values of 12–15 are the classic cutoffs. Laboratory directors often establish their own limits by testing a number of positive and negative samples.
Most laboratory directors use the Rosner index, percent correction, an absolute number, or the Chang index to establish correction versus non-correction, however the ratios we use to reach our conclusions are typically internal laboratory computations that lead to posting a narrative report.
- Chang index: Chang SH, Tillema V, Scherr D. A “percent correction” formula for evaluation of mixing studies. Am J Clin Pathol 2002;117:62–73.
- Rosner index: Rosner E, Pauzner R, Lusky A, Modan M, Many A. Detection and quantitative evaluation of lupus circulating anticoagulant activity. Thromb Haemost 1987; 57: 144-147.
- More on mixing studies: Devreese KM. Interpretation of normal plasma mixing studies in the laboratory diagnosis of lupus anticoagulants. Thromb Res. 2007;119:369–76.