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Pierre-Hugues Carmichael

As I understand it, positive screening tests are usually followed by more rigorous diagnostic tests, that is to say tests that are more sensitive than screening tests, right ? Am I also right in assuming that screening tests are designed to have as high a specificity as possible ?

I agree that screening tests should only be administered to "sufficiently at risk" individuals, that is to say that there is probably no need to screen young men for prostate cancer. However, I am left wondering if the problem of false positives is purely economical (that is to say these false positives cost more to society because they have to undergo the more expensive diagnostic tests). Is past information taken into account when analysing the results of a screening test ? Say your first three screenings are negative and your fourth is positive, does that increase the chances that this last result is a false positive ? I would love to read your thoughts on the question.


PHC: Love the thoughtful comment.

I tried to find some numbers for you. It appears that the PSA test has sensitivity of 80%+ and specificity of 33%. That means, someone who is healthy has a 2/3 chance of testing positive. (This comes from a Hoffman, Gilliland, et. al. Albuquerque study, cited by Prof. Jeff Douglas at Illinois.) This is worse than randomly flipping a coin.

For the PSA test, a negative means something but a positive means nothing so if you had three negatives in a row, I wouldn't be too worried. But... I'm assuming these are tests taken in quick succession because if these tests are taken years apart, there is no reason to believe the health status of the patient has not changed.

Economics is an important consideration since money is a finite resource. But for prostate cancer, we don't need an economic argument. As Dr. Ablin pointed out, clinical trials showed that those receiving PSA screening had the same death rates as those who didn't (both very low).

So, to summarize, this test costs $3 billion a year, tells 2/3 of healthy people they have cancer, and does not help the identified sick people prolong their lives. Pretty damning, huh?

Jon Peltier

Positive screening tests (especially mammograms and PSA tests) are followed-up in one of two ways:

1. Wait-and-see: have a repeat test in 3 or 6 months.

2. Biopsy.

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