What influence do fasting, eating have on laboratory test results?

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.

Influence of anorexia, fasting on laboratory test results

When food is withheld, there is a decrease in blood glucose concentration, insulin secretion and a reciprocal increase in glucagon secretion to promote glycogen mobilization and gluconeogenesis. If food is withheld for longer than 24 hours to 48 hours, lipolysis occurs, resulting in production of ketones (acetone, acetoacetic acid and beta-hydroxybuteric acid). Although fasting ketogenesis occurs in dogs and cats, these species apparently use ketones so efficiently that ketoacidemia is uncommon. If ketoacidemia and ketoaciduria are detected in an anorexic patient, diabetes mellitus or hepatic failure should be ruled out as underlying causes.

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.

After a meal, the plasma concentration of total lipids increases. If large quantities of fat are contained in the diet, visible lipemia can occur. Unfortunately, lipemia enhances hemolysis in vitro. Hemolyzed serum, in turn, can alter several test results such as serum amylase, lipase, ALT, AST, calcium and phosphorus. Turbidity of serum due to lipemia can interfere with spectrophotometric determinations (e.g. serum glucose concentrations) and flame photometric determinations (e.g. serum sodium and potassium concentrations).

Also, lipemia results in false elevations of plasma protein concentrations measured by refractometry. To minimize diet associated lipemia, a patient may be fasted for a minimum of six hours to 12 hours prior to collection of blood samples. Persistence of lipemia beyond 24 hours suggests an underlying metabolic disorder.

Following consumption of a protein-rich meal, serum and urine concentrations of urea nitrogen, phosphorus and uric acid increase. Consumption of large quantities of cooked meat containing creatinine and noncreatinine chromogens results in increased serum creatinine concentrations. Apparently, prolonged heating of meat results in progressive degradation of creatine to creatinine. Consumption of large quantities of protein also is associated with increased glomerular filtration rate.

Following ingestion of a meal, particularly one that is not restricted in protein, plasma ammonia concentration and urinary ammonia excretion increase. This phenomenon should be considered when evaluating blood ammonia concentrations in patients suspected of having abnormalities in the hepatic urea cycle.

Provocative serum bile acid tests are used to detect hepatic dysfunction or hepatic perfusion abnormalities. Dietary fat, dietary protein and duodenal acidification by gastric juices cause numerous neurohumoral and hormonal factors to induce bile secretion and gallbladder contraction. However, gastric emptying may be delayed by consumption of a meal containing only fat. Therefore, normal serum bile acid concentrations may be observed in patients with abnormal hepatic function.

Consumption of food stimulates gastric secretion of hydrochloric acid. As a result, a decrease in plasma chloride concentration and increase in bicarbonate concentration occurs in venous blood draining the stomach. Serum total carbon dioxide concentration and plasma bicarbonate concentration also increase. The resulting metabolic alkalosis is commonly called the postprandial alkaline tide. Urine pH will increase unless acidifying substances are contained in the diet. In a study of healthy adult Beagles, eating was associated with increased urinary excretion of hydrogen ions, ammonia, sodium, potassium, calcium, magnesium and uric acid.

The form of metabolites in food (bioavailability) and their metabolism once absorbed from the gastrointestinal tract may affect their plasma and urine concentrations. For example, inorganic phosphorus is more readily absorbed from the intestines of cats than organic forms of phosphorus, resulting in substantial increases in plasma concentrations after consumption. Likewise, urinary phosphorus excretion is increased after consumption of inorganic phosphorus.

Laboratory results can be substantially affected by changes in diets fed in a home environment compared to different diets fed in a hospital environment.

For example, urine crystals that form while animals are eating at home might not form or can be different from urine crystals observed when they are fed different diets during periods of hospitalization. This factor should be considered when interpreting the significance of crystalluria. To determine the influence of home-fed diets on laboratory test results, consider asking clients to bring home-fed diets to use during periods of diagnostic hospitalization.

Diets are frequently reformulated as new information concerning nutrition and its relationship to health and various diseases is discovered. Therefore, effects of various diets on laboratory test results can change even when the same name brand of diet is consumed. For example, a diet formulated in 1985 to induce canine struvite urolith dissolution was associated with decreases in albumin concentrations and increases in serum ALT and ALP activities. Today, the same brand of diet formulated to contain a greater quantity of protein is not associated with these serum biochemical changes.

Recommendations

Consider recording information about feeding, such as the type of diet fed and the length of fasting, on laboratory submission forms or in patients' records.

If possible, hospitalized patients should be given diets fed at home to establish baseline laboratory data prior to dietary changes.

To evaluate the influence of dietary modification on a disease process, blood, plasma, serum and urine samples should be collected two hours to six hours after food consumption.

To minimize unwanted postprandial influence on laboratory test results, a patient should be fasted at least 12 hours prior to collection of blood, plasma and serum samples.

Diets substantially influence urine pH values and urine analyte concentrations. This fact should be considered when collecting urine samples for diagnostic purposes or for monitoring response to dietary therapy.

Following consumption of a meal, dietary metabolites may be excreted in urine for at least eight hours and sometimes longer. To determine the influence of fasting or eating while on urine solute concentration, first empty the urinary bladder approximately eight hours after beginning the fast or feeding and discard the urine. Then collect appropriate urine samples for laboratory evaluation.