This post is reprised from 2013:
From Courtney Jackson at Oschner in Louisiana: We use a commercial pooled normal platelet-free plasma (PNP) for our prothrombin time (PT) and partial thromboplastin time (PTT) mixing studies. When we mix it with saline for our controls, we don’t get a normal PTT until the ratio is 4 plasma to 1 saline. We get close to normal at a 1:1 mix, but not in normal range. Often we get no clot at all (>150 seconds) at a 4 saline to 1 pool mix. The pool control is within normal limits. A similar finding is occuring with our PT mixing study as well. I find this counterintuitive–the mix of saline and normal pool should be close to normal even at 20% saline, and normal at 50%. Any thoughts? Thank you for your input.
Hello, Courtney, and thank you for your question. I’d like to learn more about your procedure, as I’ve not known of a need to dilute the commercial PNP (plasma control). Also, I’d be curious to learn if you are using buffered saline (veronal buffer, for instance). A mix with an alkaline pH will prolong on the basis of the pH alone. Provided you are using a buffered saline, I would anticipate that a 1:1 mix would yield PT and PTT results near the upper limit of normal, depending on the factor sensitivity of the reagent, and that any further dilution would prolong both assays to greater than the upper limit. Most PTT reagents are calibrated to prolong to beyond the upper limit as coagulation factors VIII and IX drop to below 30% or 40% activity.
There is a form of the PTT mixing study, advocated in particular by Gordon Ens and Flo Newlin at the former Colorado Coagulation Consultants (now Esoterix) laboratory that involves a saline mix of the patient plasma. Plasma with a prolonged PTT is mixed 1:1 with PNP in one tube and with veronal buffered saline in a second tube. If the first tube demonstrates correction and the saline tube is dramatically prolonged, you may suspect a coagulation factor deficiency. This is particularly helpful when the deficiency is relatively mild.
I hope this helps, and meanwhile, please let me know about the purpose for diluting the PNP. Perhaps other participants are also using this method. Geo.