What is a common sign of dehydration in a patient?

Increased hematocrit is a common sign of dehydration because when the body is dehydrated, the plasma volume decreases while the red blood cell mass remains relatively unchanged. This leads to a higher concentration of red blood cells in the blood, resulting in an elevated hematocrit level. Essentially, dehydration causes the blood to become more concentrated due to the loss of fluid, which is reflected in the hematocrit reading.

Dry mouth and decreased blood pressure can also be signs of dehydration, but they are not as direct or specific as an increase in hematocrit. Dry mouth is more of a subjective symptom that may vary in perception among individuals, while decreases in blood pressure can occur due to a variety of other factors beyond just dehydration. Increased urination is generally associated with overhydration or certain medical conditions, so it wouldn't be an indicator of dehydration. Therefore, increased hematocrit serves as a clearer indicator of the physiological changes that occur due to a fluid deficit in the body.