Hepatitis - calculating discriminant factor?

[amandil]

I am trying to figure out how the Maddrey's discriminant factor/function is calculated for a patient with hepatitis. I am able to find the equation:

Discriminant function = (4.6 x [prothrombin time - control PT]) + (serum bilirubin)

My question is - what is the control PT? What if you are working up a patient who has never been seen before, and you only have the currently elevated PT lab value? Is it still possible to calculate the DF?

Thanks in advance for the assistance!

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[brc12]

By control PT, usually you use the upper limit of normal PT for your particular lab. PT can vary from lab to lab depending on what reagents they use (this is why we use INR for anticoagulation rather than PT) so you'd need to find out the value from your lab. Usually on any lab report it will report what their normal range is.

 

[amandil]

Thanks for the reply! That answer makes sense to me, but I'm looking through a patient's notes and a few days ago their DF was calculated to be 66.4 using a PT of 20.8. Our ref range is 11.4 - 13.0. So, I'm confused how the intern arrived at that answer. 😕

I guess I can just calculate my own DF and defend my choice of control PT time.