Serologic Testing for Rubella
I think this question can be answered if we take a look at several factors that are key in properly interpreting results, but may be overlooked when a laboratory test is ordered. These factors are number one, the prevalence of the disease, which is simply the number of cases of disease at a given time in a certain population. The second factor is the positive predictive value (PPV) of the laboratory test, which is the proportion of patients with positive test results who are correctly diagnosed.
As this graph depicts, the PPV of a laboratory test is highly dependent on the prevalence of the disease. In other words, if the disease has a very low prevalence, the PPV of a laboratory test will also be very low. This is the case even if the laboratory test demonstrates excellent specificity.
So let’s focus in on the example of IgM testing for Rubella to drive this point home. The current estimated prevalence of Rubella is well below 1 case per 1 million population. Because of this, the PPV of testing is extremely low. This point is critical to the proper interpretation of test results, because given the situation, virtually all positive Rubella IgM results are false positive results.
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