Slightly Prolonged PTT and Mixing Studies

From “Tom,” Recently, I did some lab tests. I don’t have any family history of bleeding problems, just some red spot on my skin made me go to see a doctor. CBC is all good.

PTT: 38.5s (reference: 24.3–36.2s)
PTT 4:1 NP: 37.2s
after 1 hr incubation PTT 4:1 NP: 38.2
Factor VIII: 43%
von Willebrand factor: 55% (normal)

I am wondering whether there is an inhibitor of factor VIII to prolong the PTT or not. The doctor told me not to worry about this, since just a slightly prolonged. Thanks a lot!

Hello, Tom, and thank you for your question. I apologize in advance for my wordiness, but your lab test results bring up some interesting questions. First let me throw in my usual disclaimer that I don’t attempt to make diagnostic or treatment-related comments, as these are your physician’s call.

First, it is often futile to perform mixing studies when the PTT result is just a second or two longer than the upper limit of the reference interval. Referring to a Fritsma Factor Quick Question posted July, 2010, notice that we have no agreement on a value above which mixing studies are performed. My personal opinion is that you should only perform mixing studies when the PTT result exceeds the RI limit by five seconds, but most of us choose, like your laboratory, to run studies whenever the result exceeds the upper limit, no matter how slim the margin.

Second, the question “what constitutes correction?” has several answers, as illustrated in a Quick Question posted in June, 2010. Here, I advocate for using the criterion “within 10% of the pooled normal plasma (PNP) result,” although the majority of laboratories, like yours, choose “mixture PTT within the RI.” In your case, either criterion leads to the “no correction” conclusion.

Third, laboratory directors differ on the ratio of patient to PNP mix. Yours uses 4:1, most use a 1:1 mix (personal observation). Those who use 4:1 say that it enhances mix sensitivity for inhibitors, those who advocate for 1:1 assert that 4:1 is overly sensitive and leads to false positive conclusions. It would be interesting to see how your results would come out using a 1:1 mix.

Fourth, when performing the incubated mix, laboratories arbitrarily choose either a 1- or 2-hour 37°C incubation mainly on the strength of “expert opinion.” I have no opinion on incubation length, except to state that in either event, the RI must be adjusted to compensate for the effects of incubation. The original RI is likely to be too short and would lead to a false positive conclusion.

Fifth, you can’t conclude from your results that you possess a factor VIII inhibitor. Most laboratories only choose to work up a factor VIII inhibitor when the factor VIII assay result is less than 30%. Further, if you possess either a factor VIII inhibitor or a lupus anticoagulant, you can’t trust the result of your factor VIII assay, as the result is based on the PTT, which is being affected by the inhibitor. This problem is partially resolved by running the factor VIII assay at three or four dilutions of the patient plasma. Coagulometers do this automatically, so this may be the case with your laboratory. If, however, there is any kind of inhibitor present, the dilution results come out “non-parallel,” the instrument will withhold the report, and the laboratory scientists sets about to identify the inhibitor.

This message is even more long-winded than I anticipated, and we are likely to attract comments from our participants, as mixing studies always provoke discussion. In your case, I would tend to accept your physician’s conclusion, though because factor VIII inhibitors are dangerous, she may wish to use a conservative, cautious approach by scheduling a return visit with repeat laboratory testing  12 weeks after your previous visit. In this instance, she would ensure that the laboratory centrifuges your plasma to be platelet-free, and uses a platelet-free PNP such as CRYOcheck™ pooled normal plasma, or its equivalent.

Meanwhile, watch this space for additional comments from our participants, and please let me know how your situation progresses. Geo.