Veterinarians and veterinary technicians rely on clinical laboratory tests to screen for disease in seemingly healthy animals, to diagnose the causes of disease in sick patients and to monitor the response of patients to various types of treatment. The value of in vitro diagnostic laboratory tests is related to their reliability in measuring concentrations of measured analytes as they were in vivo. The accuracy, precision, analytical sensitivity and analytical specificity of the tests determine the validity of test results. However, factors such as diet, medications and sample handling might also influence laboratory test values.

Blood, plasma, serum and urine biochemical analytes that are affected by recent consumption of food might vary substantially throughout the day. Collection of fasting samples is often recommended to minimize this source of variation.

Fasting can mask the effects of diet in patients with nutrient sensitive diseases. The primary purpose of this Diagnote is to emphasize the influence fasting and diets can have on the composition of blood, plasma, serum and urine.

Because the liver is the primary site for gluconeogenesis during periods of food deprivation, serum bilirubin concentrations, alanine transaminase (ALT) activity, asparate transaminase (AST) activity and sulfobromothalein (BSP) retention time may increase. Therefore, when similar test results are observed in patients that have been anorexic for prolonged periods, appropriate caution must be used not to conclude that they have primary hepatic dysfunction.

Likewise, detection of normal plasma ammonia and serum bile acid concentrations in anorexic patients does not exclude the possibility of hepatic failure.

Serum urea nitrogen (SUN) and serum phosphorus concentrations decrease during periods of decreased dietary protein consumption. Therefore, care must be used not to underestimate the magnitude of renal dysfunction when evaluating SUN and phosphorus concentrations in anorexic patients with renal failure. In this situation, evaluation of serum creatinine concentration may provide a better index of the functional status of the kidneys.

Concentration of urea The concentration of urea in the renal medulla plays a key role in the countercurrent mechanism or urine concentration. Therefore, if reduction in the quantity of urea produced as a consequence of decreased dietary protein consumption is of sufficient magnitude to deplete renal medullary urea, formation of urine with decreased specific gravity values will occur even though the kidneys are normal. Caution must be used not to misdiagnose primary renal failure, especially in anorexic patients with concomitant prerenal azotemia.

Measurement of 24-hour urinary solute excretions may be different during food consumption compared to the composition of urine during fasting. For example, during food deprivation, aldosterone secretion increases and promotes renal tubular reabsorption of sodium and renal tubular excretion of potassium. As a consequence, plasma potassium decreases, urinary excretion of potassium increases, and urinary excretion of sodium and chloride decreases. Also, during fasting, excretion of calcium, magnesium, uric acid, ammonia, titratable acids and hydrogen ion decreases while urinary pH values rise.

During periods of hospitalization, the amount of water consumed by patients may decline. Voluntary reduction in water consumption would normally result in a compensatory increase in urine concentration. In this situation, the urine concentration of various solutes would also increase even though the rate of urinary excretion of the same solutes during a 24-hour period is similar to excretion rates when water consumption was not reduced. Therefore, when hospitalized patients consume less water than in the home environment, caution must be used in interpreting 24-hour excretion and 24-hour urine concentration of various solutes in the diagnosis and therapy of urolithiasis.

Influence of eating on laboratory test results After digestion of a meal, the liver metabolizes nutrients. In humans, serum bilirubin concentrations, ALT activity, AST activity and BSP retention time increase two hours after eating. There is also an increase in postprandial serum alkaline phosphatase (ALP). The increase in serum ALP activity is due to the intestinal ALP isoenzyme. However, since the serum half-life of canine and feline intestinal ALP isoenzyme is less than six minutes, intestinal ALP isoenzyme does not influence total serum ALP activity in these species. In dogs and cats, serum glucose concentration increases two hours to four hours after consumption of food.