I have no idea. Mine was a blood test. My arigning and ornithine are normal,too. Citrulline under 20 is sometimes considered a biomarker for crohn's disease. Mine is 21. Threonine is 53, normal 73-261. Theronine is used to generate intestinal mucus. A low level causes leaky gut and fatty liver(enlarged liver).
Thanks for replying ... it's so frustrating to know that it means something - there's a ton of websites and papers discussing hypocitrullinemia - but no obvious definitions of what qualifies as hypocitrullinemia! I'll try wading through it and see what I can find.