After passing field sobriety tests, can a blood/pee test that shows positive for marijuana end up being a DUI?

Note I am not an Indiana attorney and that this is not legal advice but a general discussion of prevailing DUI-OVI law. One may be charged with OVI-Marijuana in Ohio, I suspect that Indiana is similar. It is regretable that the test for marijuana is based on a metabolite that lasts in the system weeks after the intoxicating effects have dissapated. Whether the police take action will depend on the threshold for THC metabolites present per liter in your blood, that your state has set to meet the definition of intoxication, If convicted you would likely go to a three day minimum intox school. I am not sure how to answer your last question as one cannot fight charges after convicted other than an appeal. If you meant to ask whether you could fight the charges if arrested or charged; you can, I suggest you get yourself an Indiana lawyer with a reputation for vigorously defending DUI-OVI cases. Marijuana defense is rather complex and depends on persuasively challenging the science behind detection then exposing the lack of causal connection between the presence of THC and the intoxicating effect.

Did you take a blood test on the scene or elsewhere? Did you take a urine test on the scene or elsewhere? I doubt that you could be successfully prosecuted for DUI if all they have is some MJ in your urine - that will only prove that you'd smoked in the past - it would be very hard to use that urine to prove that you were intoxicated at the time of the driving.

Whether or not you are summoned to court would likely depend on whether the police forwarded a police report to the District Attorney for prosecution. If they made you take a blood or urine test, they may have forwarded the reports. The District Attorney will likely review the reports to determine if they think they can prove you were under the influence of marijuana at the time of driving. If they decide to press formal charges against you I would suggest you consult with a DUI attorney who is familiar with marijuana DUIs and the courts in your county. The prosecution would have to prove you were under the influence, but an attorney, along with a DUI drug expert, can work together and prove that the marijuana metabolites in your system were not actively affecting your mental abilities--this depends on a review of the toxicology report you will be provided in the discovery if you fight your case and when you last ingested marijuana. If you were not "under the influence" you have a good shot at fighting the charges, if any are filed against you.