Jane Bossart originally posted this question on MEDLAB. I’m posting it here with her permission:
Hello, I have a few questions about mixing studies. What do your labs use for normal plasma (PNP)? If you purchase it, have you had any problems with it? What are your criteria for correction for both PT and APTT?
Tri City Medical Center
Dana Haas provided this answer:
We purchase our normal plasma from Precision BioLogic. It’s a pool of multiple donors. You can also purchase it from George King. It has about a 2 year stability at -70°C. You absolutely DO NOT use lyophilized control for the PT/PTT testing. That product has been buffered and stabilized within an inch of its life. Manufacturers of normal pool should provide documentation about the pool, but we also ALWAYS run a PT/PTT on the pool before doing the mixing study.
If I recall correctly, you can make your own normal pool, but it has to have at least three people in it to account for variety of factor levels.
As to correction criteria, the mixing study has to fall within the reference range for the PT and/or PTT to be considered corrected.
St. Luke’s Episcopal Hospital
Houston, TX 77030
From Geo: Dana, thank you for your answer. The Rosner index, Rosner E, Pauzner R, Lusky A, Modan M, Many A. Detection and quantitative evaluation of lupus circulating anticoagulant activity. Thromb Haemost 1987;57:144–7, provides a similar definition for correction. Rosner uses this formula:
Rosner index = [(PTT1:1 mix /PTTPNP)/PTTPNP x 100]
An index of 10 or lower indicates correction, 15 and above indicates no correction, and 10-15 is equivocal and should be repeated.
Rosner’s work is confirmed by Devreese KM. Interpretation of normal plasma mixing studies in the laboratory diagnosis of lupus anticoagulants. Thromb Res 2007;119: 369-76. Devreese uses this formula:
% correction = [(PTTpatient plasma-PTT1:1mix)/(PTTpatient plasma-PTTPNP) x 100]
She applies ROC analysis to confirm that a result >70% indicates correction, and <70%, no correction. Her formula provides equivalent sensitivity and specificity to the Rosner index.
We use the Rosner index but make it simpler by saying that if the the 1:1 result is within 10% of the PNP result, we have correction, and anything greater than 10% is no correction. The no correction results are reflexed to our lupus anticoagulant profile.
It probably goes without saying, but if you are performing a 37°C incubated mix, the PNP must be incubated parallel to the mix and the incubated PNP result used in the formula.
I’d love to hear from others on this subject, as nothing can be more fun in the coag lab than the argument raised over defining correction (we lead quiet lives). Geo.
Here’s more, sent by Bonnie Jo Taylor on 1/30/09:
If you prepare your own pool, you must use 20 normal specimens and the pool must be within normal range. Have you done a factor sensitivity on your PT and PTT reagents to determine at what % activity you get abnormal results? You may be calling a specimen normal that actually has an abnormal % activity of the factor due to low sensitivity of the reagent. Starting at what % activity would the physician be concerned? Precision BioLogic has a good product with full documentation of factor levels and has good pricing. Correction times in mixing studies do not have to fall all the way back down to the normal range.
Bonnie Jo Taylor
North Caddo Medical Center