CLL Support Association
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False Positives and False Negatives - why screening tests can sometimes be wrong

False Positives and False Negatives - why screening tests can sometimes be wrong

We've joined this community because we either have CLL or care for someone who has CLL.  Those of us with CLL should be aware that we have a higher risk of secondary cancers and I expect most of us take that concern seriously enough to undergo regular screening tests to check we aren't adding to our health worries with another cancer - or another health condition for that matter.  So I'm sure  we're all familiar with the worry that comes with a positive test result for a screened health condition.  IF we get a call from a doctor to make an appointment to discuss the results of our latest test, we immediately wonder what are our chances of having a 'false positive' - having our test result come back positive when we don't in fact have the condition. 

For years I've been reading about Bayes theorem in the context of testing emails for spam.  Recently I came across this website that explains how such screening tests work, why they can be wrong and how to work out how likely that might be.  I can highly recommend it to anyone wanting to understand why screening tests can be wrong, with the section Anatomy of a Test providing a good example of how breast cancer checks can sometimes come up with the wrong result:

Hopefully, having read that, you'll have a renewed appreciation of why more checks are needed when you return a positive test for a health condition - and why you sometimes get spam in your inbox and occasionally have to check your junk mail folder for emails you should have received.   Considering spam has accounted for up to 90% of all email - that's not a bad result.  Encouragingly, spam emails recently fell to around 50% of all email traffic due to continuing efforts to reduce it.  Per , 'Spammers collect email addresses from chatrooms, websites, customer lists, newsgroups, and viruses which harvest users' address books, and are sold to other spammers.'  So you can do your part by being careful where you disclose your email and using Private Messaging facilities such as that provided by HealthUnlocked:

Now if only it was that easy to avoid cancers...


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