6 ms·

Yes, the positive predictive value of the test may be low, with the PPV being equal to the number of true positives divide by the number of true positives plus

by abcc8 6y ago

Yes, the positive predictive value of the test may be low, with the PPV being equal to the number of true positives divide by the number of true positives plus the number of false positives. Since this test is designed to identify cancer very early, a likely solution to the low PPV problem would be to re-test the individual, perhaps even multiple times. The likelihood of multiple consecutive FP test results, barring a biological reason for the FP, would at some point warrant a more accurate test, be it a screen or diagnostic test depending on tumor type. Also keep in mind that the PPV of the test would increase with a person's age - an 80 year old is at a greater risk of developing cancer than is a 30 year old. This test would also be useful for people with hereditary cancer syndromes. The cost benefit would be huge - imagine if everyone could identify their cancer 6 months or even years earlier. Mortality would greatly decrease, as would spending on treatments, etc.

mbreese 6y ago
The cost benefit for someone with a cancer diagnosis could be huge. But for the people with false positives, that could also be very harmful to people. A (false) cancer diagnosis would be incredibly stressful. It’s tricky to find that balance.
- abcc8 6y ago
  This test is a screening test and thus can never be used to diagnose someone. Only a diagnostic test can be used to diagnose someone. Screening tests like these will represent a paradigm shift in cancer surveillance and as such will require those administering the test, most likely a pcp, to properly educate each patient about what the test results mean and what steps should be taken next.
- taylorlunt 6y ago
  This seems like a failure in how test results are communicated to patients. Patients should just been told that they have been "flagged for further testing", rather than being told they have tested positive for cancer.
  - inimino 6y ago
    And how would that conversation go? "You've been flagged for further testing." "Why?" "Um..." "Didn't I just take a cancer screen? What was the result?" "Ah..." "Level with me, doc. Do I have cancer?" "Well, [brings out probability textbook] you see ..."
    - landryraccoon 6y ago
      So I have a real issue with not just telling the patient flat out that their test results indicate a high risk of having cancer and they need to be tested again. As a society, we're just getting too used to accommodating people's ignorance about statistics and science. Where does it end? Do we have to really have to give up promising therapeutics and diagnostic tests because we're afraid people's feelings will be hurt?
      - waterhouse 6y ago
        Yeah, I really hate it when people say what sounds like, "We recommend against even trying this test because people are incapable of responding appropriately to the answer." It shouldn't even be that hard to state. "X% of people your age [+ whatever other factors they're using for a reference class] have [type of] cancer. Based on that and your test results, there's a Y% chance you have that type of cancer." Forgive me if I'm too disconnected from the average person, but is that too difficult to understand?
      - bsder 6y ago
        > So I have a real issue with not just telling the patient flat out that their test results indicate a high risk of having cancer and they need to be tested again. The problem is that "tested again" has its own complications. This is not theoretical. Part of the reason for updating the breast cancer screening guidelines is the fact that biopsies to confirm were causing more problems than the "cancers" they were finding. Medical probabilities are tricky things.
        landryraccoon 6y ago
hcknwscommenter 6y ago
"barring a biological reason for the FP" I think that's an unwarranted assumption. Sure, another source of false positives is contamination or user error even, but other than that most FP are indeed due to a (usually unknown) biological reason. I'm reminded of a friend that was just positive she was not pregnant. She tested herself three times with three different pee stick brands. She was wrong. They all use the same chemistry, and for some reason something in her body interferes with those tests.
- deleted 6y ago
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by abcc8 6y ago

mbreese 6y ago

The cost benefit for someone with a cancer diagnosis could be huge. But for the people with false positives, that could also be very harmful to people. A (false) cancer diagnosis would be incredibly stressful. It’s tricky to find that balance.

abcc8 6y ago
This test is a screening test and thus can never be used to diagnose someone. Only a diagnostic test can be used to diagnose someone. Screening tests like these will represent a paradigm shift in cancer surveillance and as such will require those administering the test, most likely a pcp, to properly educate each patient about what the test results mean and what steps should be taken next.
taylorlunt 6y ago
This seems like a failure in how test results are communicated to patients. Patients should just been told that they have been "flagged for further testing", rather than being told they have tested positive for cancer.
- inimino 6y ago
  And how would that conversation go? "You've been flagged for further testing." "Why?" "Um..." "Didn't I just take a cancer screen? What was the result?" "Ah..." "Level with me, doc. Do I have cancer?" "Well, [brings out probability textbook] you see ..."
  - landryraccoon 6y ago
    So I have a real issue with not just telling the patient flat out that their test results indicate a high risk of having cancer and they need to be tested again. As a society, we're just getting too used to accommodating people's ignorance about statistics and science. Where does it end? Do we have to really have to give up promising therapeutics and diagnostic tests because we're afraid people's feelings will be hurt?
    - waterhouse 6y ago
      Yeah, I really hate it when people say what sounds like, "We recommend against even trying this test because people are incapable of responding appropriately to the answer." It shouldn't even be that hard to state. "X% of people your age [+ whatever other factors they're using for a reference class] have [type of] cancer. Based on that and your test results, there's a Y% chance you have that type of cancer." Forgive me if I'm too disconnected from the average person, but is that too difficult to understand?
    - bsder 6y ago
      > So I have a real issue with not just telling the patient flat out that their test results indicate a high risk of having cancer and they need to be tested again. The problem is that "tested again" has its own complications. This is not theoretical. Part of the reason for updating the breast cancer screening guidelines is the fact that biopsies to confirm were causing more problems than the "cancers" they were finding. Medical probabilities are tricky things.
      - landryraccoon 6y ago

hcknwscommenter 6y ago

"barring a biological reason for the FP" I think that's an unwarranted assumption. Sure, another source of false positives is contamination or user error even, but other than that most FP are indeed due to a (usually unknown) biological reason. I'm reminded of a friend that was just positive she was not pregnant. She tested herself three times with three different pee stick brands. She was wrong. They all use the same chemistry, and for some reason something in her body interferes with those tests.

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