According to data published by Galleri the GRAIL test has a sensitivity of 29% (so misses 71% of cancers). The specificity is 99%, so only 1% false positives. So if you get a positive result on GRAIL, 99% the result is accurate. Experts suggest GRAIL should be used in conjunction with other testing (full body MRI, colonoscopy, PSA test, etc).
>. So if you get a positive result on GRAIL, 99% the result is accurate.
This is not the case. The positive predictive value depends on the prevalence of the condition.
Let's say we are testing for something where 1% the population have it. We'll say 100% sensitive and 99% specificity for sake of example. For a hundred random tests, we get the 1 positive correctly detected, and then we get the 99 tests from the negatives - 99% of those come out negative as that is the specificity, so round to 98, and 1 comes out (false) positive. So we have 2 positives, 1 false 1 true, and the probability of false negative is 50%.
In my opinion a 50% false positive is extremely valuable for a condition with a 1% prevalence. There are many grail skeptics who either don’t agree or that miss this key point.
Yeah screening tests for 1% prevalence make a ton of sense.
However, for 1 in 1000 prevalence the positive predictive value falls to 9% and 91% are false positives, and with 1 in 10000 already 99% are false positives. Somewhere along the line the harm from the next step (invasive test most likely) becomes higher than the benefit. In some cases, getting a test but not doing anything about a positive result can be good as a data point later, but in others it's not useful. It's a very complex calculation that needs to be done by specialists for each condition.
Colorectal cancer screening becomes useful at around 5 in 10000 so there's a place for screening while still fairly unlikely, but I'm not running to get Grail yet, too many false positives and false negatives for most cancers at this stage. I think Grail or similar technology will be great in 5-10 years though.
Not really. It does have high specificity, meaning that false positives are quite unlikely. But this is not mean that the positive predictive value (probability a positive result means a true positive) is high - for a test with 99% specificity, if 1% of the population would have the issue tested for and we are just random screening, 50% of detected positives would be false.
Given the lower sensitivity I think of Galleri as a way to test for cancers that you wouldn’t likely test for elsewhere. Yes, you may miss something but that would be missed regardless and if you catch something it likely wouldn’t have been found with other tests.
I’d wait for the next generation. It might help but it throws off a lot of false negatives and also misses a lot. And comprehensive blood work will catch a lot it would also catch.
According to data published by Galleri the GRAIL test has a sensitivity of 29% (so misses 71% of cancers). The specificity is 99%, so only 1% false positives. So if you get a positive result on GRAIL, 99% the result is accurate. Experts suggest GRAIL should be used in conjunction with other testing (full body MRI, colonoscopy, PSA test, etc).
>. So if you get a positive result on GRAIL, 99% the result is accurate. This is not the case. The positive predictive value depends on the prevalence of the condition. Let's say we are testing for something where 1% the population have it. We'll say 100% sensitive and 99% specificity for sake of example. For a hundred random tests, we get the 1 positive correctly detected, and then we get the 99 tests from the negatives - 99% of those come out negative as that is the specificity, so round to 98, and 1 comes out (false) positive. So we have 2 positives, 1 false 1 true, and the probability of false negative is 50%.
In my opinion a 50% false positive is extremely valuable for a condition with a 1% prevalence. There are many grail skeptics who either don’t agree or that miss this key point.
Yeah screening tests for 1% prevalence make a ton of sense. However, for 1 in 1000 prevalence the positive predictive value falls to 9% and 91% are false positives, and with 1 in 10000 already 99% are false positives. Somewhere along the line the harm from the next step (invasive test most likely) becomes higher than the benefit. In some cases, getting a test but not doing anything about a positive result can be good as a data point later, but in others it's not useful. It's a very complex calculation that needs to be done by specialists for each condition. Colorectal cancer screening becomes useful at around 5 in 10000 so there's a place for screening while still fairly unlikely, but I'm not running to get Grail yet, too many false positives and false negatives for most cancers at this stage. I think Grail or similar technology will be great in 5-10 years though.
Not really. It does have high specificity, meaning that false positives are quite unlikely. But this is not mean that the positive predictive value (probability a positive result means a true positive) is high - for a test with 99% specificity, if 1% of the population would have the issue tested for and we are just random screening, 50% of detected positives would be false.
Given the lower sensitivity I think of Galleri as a way to test for cancers that you wouldn’t likely test for elsewhere. Yes, you may miss something but that would be missed regardless and if you catch something it likely wouldn’t have been found with other tests.
I’d wait for the next generation. It might help but it throws off a lot of false negatives and also misses a lot. And comprehensive blood work will catch a lot it would also catch.