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Low blood urea nitrogen is generally not a cause for concern. It may at times be seen in liver disorders, mal nutrition or in over hydration. Low creatinine levels are seen if there is muscle wasting.
If the levels are so low maybe a creatinine clearance test should be done to test the condition of the kidney and rule out diabetes insipidus. CPK or creatinine phosphokinase and liver enzymes should be done if there is muscle wasting.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!
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