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PSA test accuracy


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This analyzes the accuracy of the PSA test for prostate cancer screening

Published in: Health & Medicine
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PSA test accuracy

  1. 1. PSA Test accuracy Gaetan “Guy” Lion July 2010
  2. 2. Introduction <ul><li>The prostate specific antigen (PSA) test is a blood test for prostate cancer screening; </li></ul><ul><li>According to the American Medical Association, 3/4 th of US men older than 50 years old have been screened with this PSA test; </li></ul><ul><li>A PSA value >4.0ng/mL is a standard threshold for prostate cancer suggesting further diagnostic investigation (biopsies). </li></ul>
  3. 3. How accurate is this test? <ul><li>It depends on the age of the patient; </li></ul><ul><li>The older the patient, the more prevalent prostate cancer is and the more accurate the test is; </li></ul><ul><li>Age-specific prostate cancer prevalence statistics are not readily available. You have to construct them from age-specific incidence rate. </li></ul>
  4. 4. Prostate Cancer Incidence Rate Those incidence rates look very small. But, on a cumulative basis they translate into material prevalence rates as one ages as shown on the next slide.
  5. 5. Prostate Cancer Prevalence Rate At 45 years old very few men have prostate cancer. By 85, nearly a quarter of them do have it. The age specific prevalence rate of prostate cancer is a material driver of the accuracy of the PSA test. The higher the prevalence rate the more accurate the PSA test.
  6. 6. PSA Test Accuracy with high Prevalence The high end prevalence is 23.4% associated with 85 years old. In such a case, a positive test is 50% accurate.
  7. 7. PSA Test Accuracy across Prevalence Rates Using the same Bayesian logic as on the previous page, we can see how the test accuracy when positive decreases very quickly when the age-specific prevalence drops and vice versa.
  8. 8. PSA Test Accuracy vs Prevalence Rate Accuracy goes up as the age specific prevalence rate goes up. But, at the high end of prevalence rate (for 85 years old) a positive test is still no more accurate than a flip of the coin.
  9. 9. PSA Test Accuracy vs Age Bucket For anyone under 70 years old, the PSA test (positive result) is highly inaccurate (accuracy ranging from near 0% to only 22%). This will cause a lot of unnecessary biopsies.
  10. 10. The Data is not the whole story <ul><li>The data suggests one should wait till they reach their 80s to get tested as the accuracy of the test is so much higher at that stage. Unfortunately, this makes no sense. This is because prostate cancer at that age is often benign and has no lifespan implication. In the minority of cases, when such cancer at that age is not benign it is often one of several underlying diseases having lifespan implications. Thus, treating the cancer does not necessarily imply a longer lifespan; </li></ul><ul><li>How about taking the test at a younger age: better safe than sorry. The consequences of the frequent false positives can be dire. A positive PSA test can lead to biopsies which may lead to removal of the prostate. At each diagnostic steps, the medical establishment may use a “better safe than sorry” approach. Unfortunately the price of “safety” is very high. Removing the prostate often causes incontinence and impotence. And, the vast majority of prostate cancers are indolent and do not effect lifespan. </li></ul>
  11. 11. U.S. Preventive Services Task Force (USPSTF) Recommendation For men 75 years or older, the USPSTF, explicitly recommends no screening. For men under 75 years, the USPSTF makes no recommendation because “the evidence is inadequate to determine whether screening improves health outcomes.” Per the USPSTF, harms of treatment include erectile dysfunction, urinary incontinence, bowel dysfunction, and death. A proportion of those treated would never have developed cancer symptoms during their lifetime.