There is no established correlation between serum THC level and impairment.  Some states set presumptive intoxication, arbitrarily, at 15 mcgm/ml, others, even more arbitrarily, at any detectable, and still others have no standard at all.  This is a consequence of the Republican-led federal ban on serious scientific research into the effects of cannabis.

A field sobriety test is a crude indicator that prosecutors hate to take to trial.  "Tell me, Mr. Head, how were you feeling when Officer Krupke stopped you that day?"  "I was very sick."  "In what way?"  I had a fever and a raging headache."  "Did you tell Officer Krupke how you felt?"  "I tried to, but he wouldn't listen."  "Did your illness give you trouble with the things the officer asked you to do?"  "I'll say: I fell over as soon as I got out of the car." "What caused that?"  "My fever.  I was so weak I couldn't stay on my feet."