Before you watch this video you should really watch the previous video which covers Two by Two Tables, TP, TN, FP & FN. That video lays the foundation for this video so it may be tough to watch this one by itself.

Sensitivity & Specificity

Sensitivity (Sen) & Specificity (Spec) are used to evaluate the validity of laboratory tests (not results of the tests). Basically, you use sensitivity and specificity to determine whether or not to use a certain test or to determine what situations a certain test would work best in. It is important to note that Sen and Spec are fixed for a certain test as long as you don’t change the cutoff point. Therefore, Sensitivity & Specificity are not affected by changing prevalence. Both are given as a percentage ranging from 0% to 100%.

Sensitivity is the percentage of patients with the disease that receive a positive result or the percentage chance that the test will correctly identify a person who actually has the disease

 Sensitivity = TP / (TP + FN)

or

Sensitivity = TP / Diseased

Specificity is the Percentage of patients without the disease that receive a negative result

 Specificity= TN / (TN+FP)

or

Specificity = TN / Not Diseased

Imagine you have 2 very different guns. The first gun fires when you barely touch the trigger. A strong gust of wind could set it off. The first gun has high sensitivity and low specificity. It is sensitive to the smallest of signals to fire while not being very specific to an intentional pull of the trigger. You never miss a possible chance to shoot your gun (~ Low FN), but you often accidentally fire when you shouldn’t (~ High FP). The second gun only fires if you pull the trigger really hard. This gun has high specificity and low sensitivity. It is very specific to firing only when you intentionally pull the trigger (~Low FP), but it isn’t very sensitive to a weak pull of the trigger (~High FN).

In the real world you never have a test that is 100% Sen and 100% Spec. We are usually faced with a decision to use a test with high Sen (and lower spec) or high Spec (and lower Sen). Usually a test with high sensitivity is used as the Initial Screening Test. Those that receive a positive result on the first test will be given a second test with high specificity that is used as the Confirmatory Test. In these situations you need both tests to be positive to get a definitive diagnosis. Getting a single positive reading is not enough for a diagnosis as the individual tests have either a high chance of FP or a high chance of FN.  For example, HIV is diagnosed using 2 tests. First an ELISA screening test is used and then a confirmatory Western Blot is used if the first test is positive.

There are also specific situations where having a high specificity or sensitivity is really important. Consider that you are trying to screen donations to a blood bank for blood borne pathogens. In this situation you want a super high sensitivity, because the drawbacks of a false negative (spreading disease to a recipient) are way higher than the drawbacks of a false positive (throwing away 1 blood donation). Now consider you are testing a patient for the presence of a disease. This particular disease is treatable, but the treatment has very serious side effects. In this case you want a test that has high specificity, because there are major drawbacks to a false positive.

Now that you have finished this video you should check out the next video in the Biostats & Epidemiology section which covers the calculation of Predictive Value Positive & Negative (PPV & NPV). That video has some mnemonics and concepts that also apply to this video.