Scientists investigating autism have found that people on the autism spectrum have different levels of various proteins in their saliva from the levels of the same proteins in ‘normal’ people’s saliva. They are hoping to develop a simple saliva test to diagnose autism.
Scientists investigating anorexia have found that anorexics have lower body weight than ‘normal’ people. They are hoping to develop a simple weight test to diagnose anorexia.
OK I made the second one up, but the first is real, and contains, I believe, the same confusion of cause and effect.