Nick Reiner — who was arrested on suspicion of killing his parents Rob and Michele Reiner — will NOT appear in court today for the first time since their grisly murders … he doesn’t have medical clearance.

Prosecutors were expected to decide whether and how to officially charge Nick with killing his parents … but that will have to wait, because Nick’s defense attorney, Alan Jackson, says his client is not medically cleared to appear in court.

As we first reported … Nick is on suicide watch at Men’s Central Jail in L.A., where he’s being held without bail. Jackson said this morning he has not received any filing yet from prosecutors … and he doesn’t believe the court has received any filing either.

TMZ broke the story … sources told us Nick was acting very strange Saturday night at Conan O’Brien‘s Christmas party … not interacting with guests, and his attire was out of place … it was a pretty ritzy event, but Nick was rolling around in a sweatshirt and seemed high on something. We’re told Nick got into a loud argument with Rob at the party, and Rob left with Michele shortly after.

Nick lived at home with his parents, but he opted to check into a hotel at around 4 AM Sunday, using his credit card. The reservation was made for just a day, but Nick never formally checked out.

When the staff came into his room later Sunday morning, they found the shower “full of blood” and blood on the bed.