Last week, I went on a bit of a “poor me” rant. This week, I am back with a little cheese to go along with my whining. This week’s blog plugs in some others’ thoughts on the Optokinetic tracking test, and the purpose of screening tests in general. Forgive the liberal use of cutting and pasting this week, but this is not my forte.
First, what makes a good screening test? The American Medical Association Council on Scientific Affairs describes the desired attributes:
The test for the disease must
• be capable of detecting a high proportion of disease in its preclinical state
• be safe to administer
• be reasonable in cost
• lead to demonstrated improved health outcomes
• be widely available, as must the interventions that follow a positive result
Well, the Optokinetic tracking test meets at least some of these criteria. Let’s look at the first one. How sensitive is the test? Is it different when using a light bar as opposed to full field stimulation? Believe it or not, someone actually took the time to evaluate this. Kveton et al, (1999) report that when using a light bar, the sensitivity of finding a neurologic disorder is 87%. However, the specificity is quite low, only 17%. This means that you won’t miss many people with an associated disorder, but most of the abnormal tests will be false positives. With full-field optokinetic stimulation, the sensitivity is quite low (47%), but the specificity is quite high (93%). This means that when the test is abnormal, there is a high likelihood that an associated neurologic disorder is actually present, but that more than half of people with said disorder will be missed.
Here again, is the American Medical Association take on this:
The usefulness of the screening test is evaluated by its sensitivity and specificity. Sensitivity is the true positive rate; that is, the probability that a patient with a positive test result has the disease. As sensitivity increases, the number of patients with preclinical disease not diagnosed by the test decreases. Specificity is the true negative rate; the probability that a patient with a negative test result does not have the disease. A highly specific test produces a small percentage of erroneously positive results. Sensitivity is usually increased at the expense of specificity when the disease is serious and curable in its preclinical phase. However, high specificity may be desired over sensitivity when the costs or risks of further testing are significant, as they are, for example, with surgical biopsy. Patients must be informed that a negative screening result does not mean disease is not present, but rather the likelihood of disease is low. Since few tests have both high sensitivity and high specificity, multiple tests are often used to aid in detection of disease in the preclinical phase.
Many people argue that you are really just testing saccades and pursuit with a light bar. I don’t disagree, but I look at it rather from the perspective of screening versus diagnostic. I would prefer a screening to be highly sensitive and less specific. I would never make a diagnosis of central vestibular dysfunction simply based on an abnormal optokinetic test anyway, whether light bar or full field stimulation. I would love to hear from some “full field” advocates.
Can you believe we just spent three weeks discussing Optokinetic tracking?