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2012 February

The predictive values in rapid diagnostic tests


Diagnostic tests are now so routinely used that nobody can imagine diagnosing any pathology without processing a sample through a panel of different tests.

Most methods used are intended to help the practitioner in giving the appropriate diagnostic to administrate the most convenient treatment or to conduct other confirmatory testings.  Nevertheless, the main problem to face is still to consider if a test is convenient to detect a precise pathology. In other words, does the test of interest answer the main question regarding the pathology: is the sample positive or negative for it? And even, when a sample is detected as positive or negative, what about the confidence in the result ?

One must consider if a test is able to distinguish between diseased and healthy people or more scientifically speaking between true positives versus true negatives. Several parameters allow characterising a diagnostic test.  These are mainly the specificity, the sensitivity, and the positive (PPV) and negative (NPV) predictive values.

Specificity is defined as the proportion of true negative samples, as measured by a gold standard, which are so identified by the diagnostic test under study while sensitivity is defined as the proportion of true positive samples, as measured by a gold standard, which are identified as positive by the test under study. These two parameters are not linked to any prevalence of the disease since they are calculated on either negative sampling or positive sampling respectively which do not depend on the epidemiological status of the disease of interest.

Dependently of the disease, the test should be developed to be more sensitive or more specific and, by the way, to show either a high negative predictive value or a high positive predictive value. To this concern, Clostridium difficile diagnosis is a very good example. Most of the algorithms used in laboratories described a two steps methodology with a first test to identify the bacteria. This is to be carried out by the detection of the glutamate dehydrogenase enzyme, specific for C. difficile. If this test is positive, then toxins detection must be considered and performed either by immunological assays or, better, molecular assays.

In this case, because infected people are at high health risk and because any detected positive should give rise to other testings, both sensitivity and NPV should be as high as possible, to avoid useless testings. And the goal to reach for any test is to show 100 % NPV. By having such a high value, practioners know without any doubt that all the negatives are real negatives.

And what is claimed in hospital: “What we will be looking for is a rapid GDH test that has a high NPV to exclude those specimens where C.difficile is not present from the second more expensive and time consuming test for C.difficile toxin with the Vidas ».

And it is exactly what Coris is now proposing, indeed.

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