The MIDAS test is not a diagnostic test for headache but rather it measures the severity of headaches. It is similar to the HIT-6 test, but it emphasizes duration and frequency. It was developed to measure response to medication.
The MIDAS test mainly measures duration -- it asks questions such as "On how many days in the last 3 months did you miss work or school because of your headaches? ". If you missed every day -- your score for this question alone would be 90 !. There are 5 questions like this, but some are "either or". According to Stewart et al, "The MIDAS score was the sum of missed work or school days, missed household chores days, missed non-work activity days, and days at work or school plus days of household chores where productivity was reduced by half or more in the last 3 months." OK -- lets see:
Items #1, 2,3 can add up to 270. Items in #4 cannot be double-counted with #1 or #2, so we still have a total of 270 after including #4 -- i.e. if you didn't miss work, but your productivity was reduced by half or more, this would count towards your total of 270.
The MIDAS test is graded as "Severe" for 21 days or more. Thus the MIDAS test has a gigantic space for "severe" -- between 21 to 270.
The MIDAS scores are not distributed in a "Bell curve" - -they are hugely skewed towards low values. This makes statistics very challenging.
This histogram is from a very large # of patients who were tested with dizziness in our practice in Chicago. Many of these patients may not have been complaining of headaches at all. Thus it is not surprising that the largest number of subjects have a score of 0. It is disturbing however, that the scores are not at all distributed "normally", or in other words, like a bell curve. Rather, they seem to be less and less people who endorse having many days of symptoms. There are many people who score above 21, which is the "severe" group, at least considering this sample which is pretty diverse.
Validity means that it is statistically robust. It does not mean it is necessarily useful. From our discussion above, it is clear that the MIDAS test is not a diagnostic tool -- it is a severity tool, best suited for quantifying response to treatment. Or in ther words, a high score on the MIDAS does not diagnose you with migraine. What it measures is days of bad headaches. Or to put this in another way, someone with a brain tumor might have a very high MIDAS score, but not have migraines at all. Or someone who has the kinds of migraines with visual auras for a few minutes, might not have a very high MIDAS score.