In the context of blood ammonia evaluation, what substance should be avoided in the collection tube?

Lithium heparin is the substance that should be avoided in the collection tube when evaluating blood ammonia levels. This is because lithium can interfere with the enzymatic assays used to measure ammonia, potentially leading to falsely elevated or skewed results.

For accurate measurement, it's crucial to use collection tubes that do not contain any additives that can interact with the analyte or affect the results. Therefore, using a collection tube with alternatives such as EDTA, sodium citrate, or fluoride is generally preferable, as these substances do not have the same interference issues with ammonia assays.

In blood ammonia testing, minimizing interference is essential for obtaining reliable results, which is why choosing the right type of collection tube is critical in clinical chemistry practice.