Of the common problems found in nursing homes, dehydration occupies a special place in the hearts of long-term care staff, a place of fear, and a profound sense of helplessness. To families and residents, the word brings forth concern that the most basic of human survival requirements—water— was withheld to the detriment of the resident. In many cases, the staff of the nursing home know that efforts to provide for this basic need were consistent and often the fluid intake unchanged. Why then, do they often see their residents return from the hospital with the label of “dehydration” attached? Diagnostic confusion first arises from a growing tendency to use the terms dehydration and volume depletion interchangeably, despite the extensive work done to differentiate these two conditions as separate and distinct. 2 The general definition of dehydration is a large loss of water from the body. 3 Significant dehydration is accompanied by an elevation in the serum osmolality. Many who receive the label of dehydration in acute care settings have no such abnormality, 4 and are not suffering from a loss of total body fluid volume. The discussion that follows will discuss the physiology of the laboratory and clinical findings of dehydration and illustrate common clinical problems in which these findings occur either not due to dehydration at all, or unrelated to the diligence of staff in providing sustenance. A basic physiologic principle critical to understanding dehydration is recognition that only 8% of the total body fluid volume is found within the vascular space. Important bedside tools often proposed to diagnose dehydration such as tachycardia, low supine blood pressure, and orthostatic change are related directly to intravascular volume, rather than body water. Clinical findings of common age-associated changes of dry mouth, tenting skin, and sunken eyes are often considered confirmatory, despite evidence that they are not at all specific for dehydration. 5 Mistakes in diagnosis can lead to mistakes in management: true dehydration is corrected by extra free water, while volume depletion needs treatment with isotonic saline solutions. The clinical diagnosis of dehydration is also often applied when the laboratory studies ordered show an elevation of the blood urea nitrogen (BUN) and the BUN to creatinine (BUN/Cr) ratio. For reasons to be discussed below, this ratio is not specific for dehydration, and is abnormal when illness effects vascular volume as perceived by the kidney, regardless of the status of total body water. Although significant losses of body water will generally produce significant intravascular volume depletion, the reverse is not true. The elevation of the BUN/Cr ratio can occur due to three basic physiologic mechanisms: overproduction of urea, decreased elimination of urea relative to creatinine, and diminished levels of serum creatinine from decreased production.
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