What further work up do you recommend for patients with a chronic high anion gap metabolic acidosis with normal eGFR who have been ruled out for lactic acidosis, ketosis, toxic alcohols, and other usual culprits of high anion gap metabolic acidosis?

I take a good history for Tylenol for pyroglutamic acid, as that may not be a test you can do.

If there is ANY GI history of a D-lactate level. Might as well get a salicylate level, it can be hidden in wintergreen oil and a few other things. 

Make sure it isn't pseudohypobicarbonatemia, seen with hyperlipidemia, and paraproteinemias (Get simultaneous ABG with BMP).

If in hospital think of propylene glycol if on drips.

Other causes of high anion gap metabolic acidosis include pyroglutamic acidemia, inborn errors of metabolism (methylmalonic, propionic, and isovaleric acidemia), paraldehyde poisoning, iron poisoning, salicylate, diethylene glycol and propylene glycol (D-lactic acid). Of note, the standard enzymatic laboratory assay for lactate will not detect D-lactate, and special enzymatic assays that use D-lactate dehydrogenase should be performed.

Agree with Dr. @Dr. First Last. In up to 30% or more of cases like these, we never find the culprit anion and defining area for further study.