CO2 test
How reliable are CO2 vitality tests?
The more correct question would be how accurate CO2 tests are. When it comes to effectiveness of tests, reliability and accuracy are two very different things. Reliability is about if the tests were to be repeated what the chance of getting the same results would be i.e. repeatability. The higher the chance, the better the test.
Accuracy is the chance that the test results truly represent what actually happened (in the pulp in this case). Here we are talking about accuracy not reliability.
To understand accuracy, we need to understand how we measure effectiveness of diagnostic tests. There are several concepts, we need to clarify before we can understand the measurement.
To measure the effectiveness of tests, we need to compare test results with something.
Index tests are tests that we want to measure effectiveness. In this case, it’s the result of CO2 test i.e. either sensitive or not sensitive to CO2.
Reference tests are something against which we compare index test. This reference test is also called gold standard. It should be something that truly show the state of what we want to find out using tests. In this case, histological section would be ideal as it can definitely tell if the pulp is necrotic, inflamed or healthy. In clinical research settings, this is difficult. The next best thing is visual inspection of the pulp after access opening, if there’s bleeding=vital, if no bleeding=necrotic.
There are five numbers that tell us about effectiveness of diagnostic tests. (There’s more advanced concept of test effectiveness involving receiver operating characteristic (ROC) curve, which will not be discussed here as it would take too long.)
There are three numbers (out of five mentioned above) that tell us about the performance of the test. We use these numbers to compare different tests that look into the same thing and use it as a basis for selecting a test. For example, we use heat test, cold test and EPT (different tests) to test pulp vitality (the same thing). We use these numbers to tell which of the three is the best to tell us about pulp vitality.
1. sensitivity tells us about how good this test is at detecting necrotic pulp
2. specificity tells us about how good this test is at detecting vital pulp
3. accuracy is the overall performance, the combination of the above two. The correct result was given by what proportion of all tests?
The nature of all tests is that when the tests have high sensitivity, they will have low specificity and vice versa. We cannot have it both ways even that’s what we ultimately want.
There are two more numbers that tell us about patients. When we have done CO2 test, we’ve got the results (either sensitive or not sensitive to CO2), these number will tell us about the chance that the pulp is vital or necrotic. We use these number as a basis for our treatment decision (i.e. if endo treatment is needed).
1. positive predictive value tells us about the chance that there’s a disease when the test is positive. In other words, if the patient is not sensitive to CO2, what is the chance that the pulp is necrotic?
2. negative predictive value tells us about the chance that there’s no disease when the test is negative. In other words, if the patient is sensitive to CO2, what is the chance that the pulp is vital?
Based on these number we can tell that when patients are sensitive to cold test, there’s 90% chance that the pulp is vital and when patients are not sensitive to cold test, there’s 89% chance that the pulp is non-vital (Petersson’99). The chances are pretty high that the results of cold tests truly represent what actually happened in the pulp.
Is it too subjective considering patient’s response?
It is by nature, subjective as we rely on patient responses, but I don’t think it’s too subjective to the point that we can’t trust the results. 89-90% chance, that’s pretty high and more than adequate in clinical settings. As we have not yet had anything better so we still need to stick with it.
Is CO2 test definitive when it comes to endo treatment decision?
That’d be definite no… As you can see, cold test can get it wrong roughly 10% of the time. We can reduce this number by not relying on just one test to drive our treatment plan. We have other tests, radiographs, patients’ symptoms, history etc to give us more information. If all of these findings pointed to the same direction, if we could create a coherent narrative, then we would have more confidence that we get it right.
Reference:
Evaluation of the ability of thermal and electrical tests to register pulp vitality. Petersson K, Söderström C, Kiani-Anaraki M, Lévy G. Endod Dent Traumatol. 1999 Jun;15(3):127-31.