Why is diagnosis difficult and slow? Why do you have to do so many tests?

For those who ask why diagnosis is taking so long and why I have to do so much testing:  

This article explains why hard it is to diagnose Cushing's: there is no one good test...all produce both false positives and false negatives, and thus you have to get multiple "high" test results in more than one area (and the higher the better!).  Each person's own body seems to show highs in certain tests and not in others as well.  For instance, I've had 2 high serum results, 1 high 24hr urinary free cortisol, and a second high 24hr urine 17 ohc level, but I have yet to get a high salivary result!

 

http://www.healio.com/endocrinology/adrenal/news/print/endocrine-today/%7B6224334E-508C-4B90-A1F1-D1D8CCD9E68D%7D/The-difficulties-of-Cushings-syndrome