From Vilas Hiremath, United Labs:
I have a patient with puberty menorrhagia. Her prothrombin time (PT) is prolonged, but corrected with normal pooled plasma (NPP). Her partial thromboplastin (PTT), thrombin time (TT), fibrinogen, FVII, and FV are all normal and her thromboelastography (TEG) R-value is prolonged to 60 minutes. Her factor X assay result is < 1%. We assessed the factor X activity using a PT-based assay. Since her factor X is deficient, why is her PTT normal?
Dear Vilas Hiremath, I took this to Dave McGlasson, Wilford Hall USAF Medical Center, who has done extensive research using all commercial PT and PTT reagents. Most PTT reagents are formulated to become prolonged in deficiencies of factors VIII, IX, and XI, but he found many are insensitive to low factor X activity levels. He expands by saying, however, that any PTT reagent should give a prolonged result when factor X is , 1%, so this may not really be the answer. He and I came up with the following suggestions:
- Try another PTT reagent: perhaps yours is just insensitive to factor X deficiency
- Try another factor X-deficient plasma reagent for your factor X assay, maybe you are encountering an effect related to factor VII
Sorry, we’ve not been very conclusive—let’s see what some of our participants suggest. Geo.
As you know Russell viper venom (RVV) activ
As you know Russell viper venom (RVV) activates factor X. I have spiked this patient sample and performed thromboelastography (TEG). There is no correction; it is just a straight line suggestive of factor X deficiency. Since factor X is not available there is no clot formation.
I have repeated with Sawscale viper venom (Echis carinatus, SSV), TEG is corrected to 0.3 mins from 60 mins suggesting it does not require factor X to form clot.
Also, I have three diff PTT reagents, results unchanged. With regards, Vilas
From George: Thank you for the update, Vilas. Of course, as you know, the SSV triggers coagulation at the prothrombin level, thus all your results fit the puzzle except for the PTT results.
Given no prolongation of the PTT, I’m not sure this will help, but I echo Steve Duff’s suggestion to attempt a PTT-based factor X assay. Meanwhile, I’m sending this case to some experts for further suggestions.
Very interesting case. Perhaps adding a PTT-based FX assay
Very interesting case. Perhaps adding a PTT-based FX assay using your in-house APTT reagent may also be informative?