Why Lab Data Can Be Misleading in Hypovolemia

Editor’s note: In a recent issue (CONSULTANT, November 2009, page 676), Dr Gregory Rutecki made the case for diagnosing dehydration on the basis of physical findings rather than relying solely on elevated blood urea nitrogen (BUN) and creatinine levels.

Dr Rutecki’s thoughtful note, "Diagnosing Dehydration: What Would Osler Do?" resonated strongly with me. I would add 2 domains in which review of the laboratory data in isolation will be most misleading, if we utilize an elevated BUN:creatinine ratio as a marker of hypovolemia-related prerenal azotemia:

• If poor food intake has coexisted with poor fluid intake, there is inadequate substrate for urea synthesis, and hence the BUN is falsely lowered, and the ratio fails to reflect the degree of extracellular fluid depletion.

• If substantial GI bleeding has occurred, the BUN will be falsely and unduly elevated, and the patient may be adjudged more hypovolemic than he or she actually is. Of course, a rectal examination with a test of the stool for either macroscopic blood or melena, or fecal occult blood, will help determine whether this artifact is present or absent.

I’d also add that we need to be very circumspect in interpreting the absence of tachycardia (whether at baseline or orthostatic) in volume-depleted elders: so many aged persons either have autonomic dysfunction, whether endogenous or from α- and/or β-blockade, or have conduction system disease, that with them we cannot say, "The pulse is only 68 beats per minute, so they can’t be dry as a bone." Yes, they can.

— Henry Schneiderman, MD
Vice-President for Medical Services and Physician-in-Chief
Hebrew Health Care, West Hartford, Conn
Professor of Medicine (Geriatrics)
Associate Professor of Pathology
University of Connecticut Health Center, Farmington

When Dr Schneiderman agrees with me, I take it as a supreme compliment. As his series "What’s Your Diagnosis?" has consistently demonstrated, he is a clinician who always garners the most information possible from the history and physical examination. His points are well taken and are cognizant of an expanded differential diagnosis for a prerenal ratio. Notice that he approaches that differential through noninvasive, yet insightful tests. His caveat about pulse is also right on.

— Gregory W. Rutecki, MD
Professor of Medicine
University of South Alabama College of Medicine
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