Expert: Dr.Paul Skett - 10/29/2010

Question
Dear Dr, could you please tell me what is meant by the terms 'negative predictability' and 'positive predicability' in the context of testing blood to assess alcoholism? I no longer drink and recently have had a series of blood tests, eg LFT and CDT #a new one# to show the family court i am sober. My results have been 'good', as in they suggest I don't abuse alcohol, but for the benefit of the court i need to know a bit more. Many articles I have read about these blood tests speak of 'negative predictability' and 'positive predictability' in connection with how ACCURATE the tests are meant to be. So if for eg the CDT test has 'negative predictability' of 83%-93%, what exactly does this mean? Also if you could explain what is meant by the terms 'specificity' and 'sensitivity' in these contexts that would also be helpful.
yours sincerely, Cathleen Maslen  

Answer
Hi, Cathleen,

Thanks for your question.

Predictability is the measure of how good the test is of showing something - so a "negative predictability" of 93% means it is 93% certain of being negative if it shows negative (i.e. 7% of people are really positive when they show negative).

Same for "positive predictability".

"Specificity" is how specific the test is in showing what it is supposed to show - i.e. if measuring LFT - how good is the test at measuring LFT and nothing else.

"Sensitivity" is as it suggests - how sensitive the method is of detecting what it is supposed to detect. e.g. can it detect LFT at 1, 10 or 100 U/L and how does this relate to the normal levels.

Hope this helps and good luck.

Best wishes,

Paul