Sensitivity and specificity in cancer screening

There are two schools of thought when it comes to cancer screening. One school believes that increased screening creates risk for the patient.  The psychological damage of a false positive result – believing you may have cancer when in fact you do not – outweighs the benefit of more aggressive screening protocols.  Equally problematic is the comfort of a false negative result – you believe you are cancer-free, when in fact that’s not the case.  

This logic would suggest that too much knowledge can be dangerous, and is one of the reasons that status quo screening guidelines are designed to start looking for cancer once risk has exceeded a certain threshold. However, that risk threshold may not be sufficient. Today, less than half of all cancers are caught at the localized stage. The USPSTF only publishes screening protocols for 4 cancers (breast, colorectal, lung, cervical) which collectively account for just XX% of total cancer cases.

Here at Catch, we believe deeply that knowledge is power, and following an aggressive screening protocol is one of the best ways to reduce risk. However, it is important to place that knowledge in context, particularly when it comes to cancer screening and the precision of the data you’ll receive from each screening test.  Context is critical in order to make informed health decisions.

There are two important concepts to understand to help place cancer screening in context: test sensitivity and specificity. 

Sensitivity

Test sensitivity refers to the ability of a test to accurately identify individuals with a disease (true positive rate).  So for example, if 100 individuals with breast cancer went in for a screening and 87 tested positive, the test would have an 87% sensitivity rate.  This means that the test failed to detect 13 individuals who do in fact have the disease (false negative).  

Specificity

Test specificity refers to the ability of a test to accurately identify individuals who do not have a disease.  So using the example above, if we tested 100 individuals who we know do not have breast cancer, and 90 of them received a negative screening, the test would have a 90% specificity rate. This means that the test inaccurately returned positive results for 10 individuals who were actually cancer-free (false positive). 

Test sensitivity and specificity are inversely related, which means that as test sensitivity increases the specificity tends to decrease.  If you think about this in extremes, it makes sense.  If we told all 200 individuals in the examples above that they had breast cancer, the test sensitivity would be 100%, because we have correctly identified the 100 people who do in fact have breast cancer. However, we’ve simultaneously told 100 people without cancer that they received a positive result (false positives). 

Tests with low sensitivity (i.e. the test has a low ability to identify true positives) are important screening tools. However, tests with low sensitivity produce a number of false negatives, which can provide a false sense of security. That’s why for certain cancer types or risk profiles, we may recommend a combination of tests or higher frequency of screenings to ensure cancer is caught early. 

On the other hand, tests with low specificity (i.e. the test generates a lot of false positive results) can create a lot of anxiety. A false positive test result is scary, even when you know the underlying data on test accuracy.  It can lead to unnecessary testing that may carry health risks or financial burden.  That’s why it is valuable to understand test specificity in advance of any screening, and keep this in mind if you receive a positive result. In the example above, for a breast cancer screening with 90% specificity, that still means that 1 out of every 10 women who receive a positive result do not actually have cancer.  

There is a significant impact to a false positive or a false negative test result, which is why it’s important to understand the accuracy and the limitations of any screening you pursue.  Beyond test specificity and sensitivity, it’s also valuable to understand the positive predictive value for a particular screening - more on that here. 

It’s important to approach every healthcare decision armed with knowledge.  Our goal at Catch is to empower our members with that knowledge – the good and the bad – to enable you to make the smartest choice based on your personal risk factors, risk tolerance, and preferences.

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