Why is the specificity of a biomarker important? PSA for prostate cancer as an example.

I’ve described what biomakers are here and how they are discovered here. I’ve spent so much time discussing biomarkers because this is one of the aspects of personalized medicine that you may have already encountered in your doctor’s office or will encounter soon.  Similar to how we understand risk, it’s important to understand biomarkers because many healthcare decisions will be based on the results of tests that look at the presence, absence, or quantity of biomarkers.

So how do you know what the results of a biomarker test mean or whether or not a biomarker is good or not?  Scientists have created two measurements that can quickly tell you how good a biomarker test is: sensitivity and specificity.  Before we talk about what those two measurements measure, let’s first talk about the different scenarios for a patient after getting the result of a test using a biomarker.

  1. The test is positive and the patient has the disease. This is a good scenario because then the patient can move forward with the appropriate treatment.
  2. The test is positive but the patient doesn’t have the disease.  This is what we call a “false positive” because the test is incorrectly showing up as positive.  This can cause huge issues because a patient will receive a diagnosis, follow-up tests or treatment even though they don’t have a disease.
  3. The test is negative and the patient doesn’t have the disease.  Again, this is a good scenario because the patient is a-okay.
  4. The test is negative but the patient has the disease.  This is a “false negative” because the test is falsely showing that the patient doesn’t have a disease when they actually do.  This can also cause issues because then a patient won’t be treated even though they should be.

sensitivity_specificitySensitivity and specificity measure the best case scenarios – sensitivity measures when the test is positive and the patient has a disease and specificity measures when the test is negative and the person doesn’t have a disease.  The ideal test has 100% sensitivity (all sick people are tested as being sick) and 100% specificity (all healthy people have a negative test).  But this ideal situation is difficult to achieve.  Let’s use Prostate Specific Antigen (PSA) as a biomarker test for prostate cancer as an example.

There are 241,740 new cases of prostate cancer each year, and it is the most common malignancy in men (29% of all male cancers).  PSA levels are screened in men over 50 for increased expression and over $3 billion per year is spent for this screening.   What is PSA? It’s a protein produced by the prostate gland and can be elevated in men with prostate cancer, which is why it has been used as a biomarker for prostate cancer.  However, PSA may also be elevated in men with other conditions such as prostatitis (inflammation of the prostate), benign prostatic hyperplasia (enlargement of the prostate), or urinary tract infections.  Because of this, the PSA test is highly susceptible to false positives and false negatives.  Typically PSA greater than 4 ng/mL (this means that there is 4 nanograms of PSA protein in 1 milliliter of urine) is considered a positive test result for prostate cancer.  The sensitivity at this level is 21%, which means 21% of patients that have prostate cancer have PSA levels  greater than 4 ng/mL.  Specificity of the test is 91%, which means that 91% of patients who test negative do not have prostate cancer. This is good – there aren’t many patients who don’t have prostate having follow-up test or biopsy because of a false positive (because of the high specificity).

Let’s see what happens if a lower concentration of PSA are used as a cut off to try to detect more patients with cancer.  If you look at patients with greater than 1.1 ng/mL, the sensitivity increases significantly to 83%, which means that more people with cancer are being detected (great news!).  The trade off is a specificity of 39%, which means that a huge number of patients will be incorrectly diagnosed as having cancer (high false positives).  This will result in follow-up tests and biopsies.  The effect of these tests and biopsies are both psychological (thinking you have cancer when you don’t) and physical (an increased risk of complications and side effects caused by the biopsy).

psa test

So is PSA a good test or a bad test?  For the patients who have received a positive PSA test and have aggressive prostate cancer, this test saves lives.  However,because this is a common screening test, it does cost a lot of money.  Because the sensitivity of the test at higher PSA concentrations is so low, some cancers get missed.  And if the cut off is decreased to catch more of these cancers, there’s a much higher number of false positives in men who do not have prostate cancer resulting in costly, stressful test that may have added complications.

What’s the solution?  In the case of PSA, doctors have started measuring PSA levels over time.  Increases in PSA levels over time are found more often in men with prostate cancer.  PSA also exists in a few different forms. PSA can either be attached to other molecules or not.  The form that isn’t bound to other molecules is called “free-PSA” and scientists have found that the amount of free-PSA compared to the total amount of PSA is reduced in men with prostate cancer.  These improvements have decreased false negatives and false positives, making it a much better test.

Overall, biomarkers have the potential to revolutionize medicine, and in so many cases they already have.  But for you as a patient, understanding the challenges and pitfalls of these tests will help you be a more empowered patient with the knowledge to ask key questions when you receive the results from one of these new tests.


3 thoughts on “Why is the specificity of a biomarker important? PSA for prostate cancer as an example.

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