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


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.


Source: DVM360 MAGAZINE,
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