I am a MLT and I am currently working on my MT. I have a few questions about mixing studies. I have listened to several of your modules on your website but I am still having trouble with understanding factor deficiencies in certain scenarios. If a patient has a prolonged PT and PTT that corrects with normal pooled plasma but has an abnormal Russell Viper venom test and all other screening tests are normal, what factor deficiency could this patient have? I think it might be factor X deficiency but I am not exactly sure.
Another question I have is the same scenario as above, the prolonged PT, PTT is corrected by pooled plasma, all other screening tests are normal including the Russell Viper venom test. Does this sound like a factor deficiency such as V, II X or a vitamin K deficiency?
Thank You for reading and any help understanding this would be greatly appreciated.
Hi, Vanessa. Thank you for your question. When testing for a congenital single factor deficiency, a prolonged PT and PTT indicates a deficiency of one of the common factors, X, V, prothrombin (II), and fibrinogen when fibrinogen is less than 100 mg/dL. Mixing study correction confirms a deficiency and rules out an inhibitor. The Russell Viper venom (RVV) assay, which is obsolete, doesn’t really help, as it activates at the level of factor X. Thus a prolonged RVV also indicates deficiency of X, V, II, or fibrinogen (when less than 100 mg/dL). The only difference between the PT reagent and RVV reagent is that PT activates at the level of VII, RVV at X, so prolongation of the RVV when the PT and PTT are prolonged has no added significance. While the RVV is obsolete, the dilute Russell Viper venom time (DRVVT) is used regularly to detect lupus anticoagulant.
Vitamin K deficiency, which is common, works differently, as it causes deficiency of factors VII, X, IX, and prothrombin (II). Vitamin K deficiency usually prolongs the PT before the PTT because it first affects the activity of factor VII, whose turnover is the most rapid. So a prolonged PT with a normal PTT may indicate vitamin K deficiency. Prolonged severe vitamin K deficiency, however, may affect both the PT and PTT as factors X, IX, and II become deficient. In either case, the mixing study using pooled normal plasma corrects the PT and PTT.
Once you have determined there is a deficiency of X, V, II, or fibrinogen, you reflex to single factor assays to track now the culprit. I hope this helps, and please feel free to ask more questions as they arise. Geo.