Dehydration	Signs
<5%	Not detectable on physical exam; history is suggestive of losses; acute body weight changes
5%	Subtle loss of skin elasticity
6–8%	Mild delay of skin tent, slight prolongation of CRT, dry mucous membranes
8–10%	Obvious delay of skin tent, slight prolongation of CRT, dry mucous membranes, eyes slightly sunken in orbits
10–12%	Severe prolongation of skin tent, eyes sunken in orbits, dry mucous membranes, signs of shock likely present (prolonged CRT, tachycardia, weak pulses etc.)
>12%	Moribund

^*CRT, capillary refill time.

History

History often leads the clinician to suspect dehydration and to assess its magnitude more accurately. Question the owner about volume of intake (adipsia, hypodipsia, polydipsia, or normal intake of water). Because volume of water intake may, in part, be a function stimulated by food intake, note also the presence or absence of anorexia. Abnormal losses of body fluid may be determined from owner responses to questions about vomiting, diarrhea, polyuria, panting, excessive salivation, or other bodily discharge. The duration of these historical signs and the magnitude of losses affect the magnitude of clinically detectable dehydration.

Physical examination provides general guidelines for detecting dehydration but is subjective (see Table 5-3). Signs of listlessness and depression may occur from dehydration but may be partially attributable to the underlying disease or to concomitant electrolyte and acid-base abnormalities. As dehydration becomes more severe, decreased skin turgor, sunken eyes, dryness of mucous membranes, tachycardia, diminished capillary refill, and signs of shock may occur. An accurate and recent body weight, when available, can be used for comparison to evaluate change in body weight as an indicator of body water change.

Key Point

An acute increase or decrease in an animal’s body weight often reflects acute gain or loss of body water. This is the most sensitive clinical tool for assessment of dehydration and rehydration. An acute loss or gain of 1 kg is the equivalent of losing or gaining 1 L of fluid.

Skin Turgor

Skin turgor assessment during physical examination is important for estimating the percentage of body weight loss due to dehydration. Skin turgor is evaluated by determining the time required for skin gently lifted from the body to return to its original position (referred to as the skin pinch or skin tent). Normal skin pliability (skin turgor) depends on hydration of the tissues in the area tested. Skin turgor is largely determined by hydration status of the interstitial tissues, although both vascular and intracellular hydration also contribute. Elastin and adipose within skin and subcutaneous tissues will also influence the apparent skin turgor. Choose skin from the trunk as a test area. Avoid dependent areas and skin from the neck. Normal skin returns immediately to its initial position when lifted a short distance and released. Dehydrated skin shows varying degrees of slow return to the original position. As dehydration progresses, the time required for the return of the skin pinch to its initial position becomes greater. The clinician assigns increasing percentages of dehydration to abnormal skin turgor of increasing severity (see Table 5-3).

Skin Turgor Artifacts

Many artifacts confuse interpretation of skin turgor.

• Skin turgor of obese animals may appear normal despite dehydration, owing to the large amounts of subcutaneous fat.

• The skin of an emaciated animal with normal hydration may fail to return to its normal position owing to a lack of subcutaneous fat and elastic tissue. Consequently, underestimating dehydration in obese animals and overestimating dehydration in emaciated animals can occur.

• Avoid testing cervical skin because redundant skin in the neck area confuses the result.

• Skin turgor changes in longhaired animals are more difficult to detect than those in shorthaired animals.

• Differences in turgor assessment can occur in the same animal in the standing versus recumbent positions.

Key Point

Dehydration may be as much as 5% of body weight loss in lean dogs and 10% or more in obese dogs before loss of skin turgor is detected.

Laboratory Assessment of Dehydration

Packed Cell Volume and Total Plasma Protein

Simple laboratory testing is helpful in evaluation of intravascular hydration. Packed cell volume (PCV) recorded in percentages (SI unit: L/L) and total plasma protein (TPP) recorded in gm/dl (SI unit: g/L) can be rapidly and inexpensively determined using microhematocrit tubes and a refractometer. These two tests require only a few drops of blood and can be taken by capillary action from a 25-gauge venipuncture. TPP concentration may be more helpful in the detection of dehydration than PCV. Increased TPP and PCV provide documentation for intravascular dehydration. Simultaneous evaluation of PCV and TPP is recommended in order to minimize interpretation errors due to pre-existing anemia or hypoproteinemia. (Table 5-4) Additional value is obtained when PCV and TPP are followed serially as increasing values identify progressive dehydration.

Table 5-4 INTERPRETATION OF CHANGES IN PACKED CELL VOLUME (PCV) AND TOTAL PLASMA PROTEIN (TP)

PCV	Total Protein	Possible Interpretation
↑	↑	Dehydration
↑	N or ↓	Splenic contraction
		Erythrocytosis
		Hypoproteinemia with dehydration
N	↑	Hyperproteinemia
		Anemia with dehydration
		Hypertonic dehydration (RBC shrinkage)
↓	↑	Anemia with dehydration
↓	↑	Anemia with pre-existing hyperproteinemia
↓	N	Non-blood-loss anemia, normal hydration
N	N	Normal hydration
		Dehydration, after secondary compartment shift
		Dehydration with pre-existing anemia + hypoproteinemia
		Acute hemorrhage
↓	↓	Blood loss anemia
↓	↓	Overhydration