INTRODUCTION: Ammonia levels are used to assess hepatic encephalopathy, but their levels are highly variable in clinical practice. METHODS: We studied factors associated with variation in ammonia values in cirrhotic patients without previous hepatic encephalopathy and healthy volunteers (HVs). RESULTS: Ammonia increased by 12% and 18% at 1 and 2 hour, respectively, after a protein meal in 64 cirrhotic patients (P < 0.001). In 237 HVs, ammonia levels varied significantly between sites (P < 0.0001). New site-specific ammonia upper limits based on HV levels using a strict analysis protocol differed from routinely used values. Correlation between paired fresh samples was high (r = 0.83) but modest between fresh and frozen samples (r = 0.62). DISCUSSION: Sample handling, processing, and protein intake impact ammonia levels across sites.
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