Laboratory tests in the analysis of states of dehydration

In an otherwise healthy child with acute dehydration known to be due to diarrhea or vomiting, the amount of deficit can best be estimated by accurate weight. Total serum protein and hematocrit provide a rough estimate of reduction in circulating blood volume, but calculation from these data will usually underestimate the deficit. Determination of urea nitrogen concentration helps to detect reduced glomerular filtration rate. Acid-base disturbances, most commonly metabolic acidosis, are detected by measuring pH and CO2 content (or base excess). Blood glucose should be measured to rule out diabetes mellitus, even in the absence of a suggestive history. Determination of potassium in serum is most important in the diagnosis of adrenal or renal insufficiency and in the post-acidotic phase after dehydration. Osmolality of body fluids is estimated by measuring [Na+]. Since osmolality of body fluids is normally maintained at the expense of fluid volume by the kidney and the hormones governing renal excretion of water and sodium, abnormal osmolality indicates a serious condition which has to be interpreted with the help of clinical data. In cases of abnormal renal function or of abnormal losses, as with removal of gastrointestinal fluids by suction, or excessive and prolonged diarrhea, measuring volume and composition of excreta may be essential. Because of cumulative deficits, patients with prolonged losses or inability to regulate oral intake by thirst cannot be treated without continual careful interpretation of the reports from a good laboratory.