For a ammonia blood test, does the specimen have to be put on ice immediately? If not how long do you have to put in on ice?

Immediately on ice. The specimen should be drawn in a Heparin tube and make sure the tubes are inverted and mixed 8-10 times. The specimen must be put on ice immediately to prevent degradation. If the lab cannot test right away, then the specimen must be spun down and the plasma separated out and frozen at-70 degrees. The ammonia level would be stable and could be thawed and tested at a later time.
Immediately. This is totally basic. And if there must be a delay in the assay, the separated plasma must be frozen in the lab.
Ask a lab. People who drow your blood should know the answer.