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    Thursday, September 17, 2015

    The Grateful False Positive


    Diagnostic tests such as CAT scans are not perfect. A test can make two errors. It can call a diseased person healthy – a false negative. This is like acquitting a person guilty of a crime. Or a test can falsely call a healthy person diseased - a false positive. This is like convicting an innocent person of a crime that she did not commit. There is a trade-off (link is external) between false negatives and false positives. To achieve fewer false negatives we incur more false positives.

    Physicians do not want to be wrong. Since error is possible we must choose which side to err towards. That is we must choose between two wrongness (link is external). We have chosen to reduce false negatives at the expense of false positives. Why this is so is illustrated by screening mammography for breast cancer.

    A woman who has cancer which the mammogram picks up is thankful to her physician for picking up the cancer and, plausibly, saving her life.

    A woman who does not have cancer and whose mammogram is normal is also thankful to her physician. The doctor does not deserve to be thanked as she played no hand in the absence of the patient’s cancer. But instead of thanking genes or the cosmic lottery, the patient thanks the doctor.

    How about the false negative - the cancer missed on the mammogram? A common reason doctors get sued is missing cancer on mammography (link is external).  The false negative is not a statistic but a real person. We promised her early detection of cancer but we failed. It is not surprising that she sues us for breaking our promise.

    Now consider the false positive. She doesn’t have cancer. The mammogram flags a possible cancer because of a suspicious finding. Abnormalities on mammograms are seldom binary. There are shades of gray. Because the shade of gray is a suspicious shade, she has an ultrasound and then a biopsy. She is waiting for the results of the biopsy. Her heart is pounding with anxiety. The physician breaks the news to her “no cancer, your biopsy is negative.”

    Imagine her relief. Far from being angry with the doctor for taking her in to a rabbit hole she is grateful. That the possible abnormality in her mammogram was not ignored shows that her doctor cares. You can never care too much. You can never be too safe. Better safe than sorry.

    This reminds me of the Stockholm syndrome (link is external) – a curious phenomenon first described in a bank robbery. This is when hostages develop positive feelings for their captors, and have an exaggerated appreciation for acts of unexpected kindness. Is the gratitude of the false positive the medical variant of the Stockholm syndrome?

    Doctors are thanked by the false positives but can be sued by the false negatives. Our choice is a no-brainer. Better thanked than sued.

    Doctors haven’t stopped being wrong. We just make more tolerable mistakes. But we are not alone. We live in a society obsessed with safety. Precaution is the new morality. False positive is precaution by another name.

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     This topic brought to you from psychologytoday.com
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