Laboratory Studies
The complete blood count (CBC) is an important laboratory test in the workup of the patient with short-bowel syndrome. The primary reason to order this test is to determine if the patient is anemic. The type of anemia can correlate with specific nutritional deficiencies. These include the hypochromic microcytic anemia typical of depleted iron stores and the megaloblastic anemia associated with vitamin B12 deficiency.
The plasma albumin level is an important indicator of overall nutritional status. This protein has a half-life of approximately 21 days. Evidence is accumulating that severely depressed albumin levels, especially below 2.5 g/dL, are associated with increased rates of major morbidity and mortality in surgical patients. In addition, albumin is a good indicator of hepatic protein synthesis. Note that during periods of stress or infection, the liver produces acute-phase reactants (eg, C-reactive protein [CRP]) in preference to albumin.
In contrast to the above, an abnormally elevated albumin level may be observed rarely and is consistent with dehydration.
Prealbumin is a good indicator of acute nutritional status. Its half-life is approximately 3-5 days. Many nutrition support practitioners use this protein to monitor the efficacy of nutrition support regimens in their patients. Because of the relatively short half-life, it is not a good nutritional screening tool; albumin is better for this purpose. Prealbumin levels can also be skewed by hydration status and renal function.
Hepatocellular enzymes (eg, aspartate aminotransferase [AST], alanine aminotransferase [ALT]) are important to monitor, especially in patients receiving long-term parenteral nutritional support. Many patients on long-term parenteral nutritional support have transient elevations of these enzymes that subsequently normalize, especially as they begin or increase oral food intake.
Concern should be raised when patients have persistent elevation of the enzymes, especially when they continue to increase. This is the group of patients that may progress to true histologic hepatocellular damage, cirrhosis, and liver failure.
Serum bilirubin is a good indicator of liver function, but its sensitivity for early liver damage probably is less than that of the hepatocellular enzymes.
Measure standard serum chemistries, including sodium, potassium, chloride, and carbon dioxide–combining power, frequently in patients on long-term parenteral nutrition. Total parenteral nutrition (TPN) is commonly associated with disturbances in these values, and simple adjustments in the concentration of these are usually sufficient to correct the problem.
Blood urea nitrogen (BUN) determinations are important because they provide an indication of renal reserve or function. More important, in this patient group, rising BUN levels may indicate that the patient is being overfed with protein. Alternately, if BUN levels are disproportionately elevated in relation to creatinine (>20:1), the patient may be dehydrated.
Serum creatinine is a good indicator of renal function. Rising creatinine should raise concern about deteriorating renal function and may necessitate changes in the nutrition support regimen.
The divalent cations calcium and magnesium and the anion phosphorus are important in several cellular processes. Calcium and magnesium facilitate functioning of many enzyme systems, regulate membrane stabilization and excitation, and serve important functions in cardiac conduction and elsewhere. Phosphorus (as phosphates) and proteins are the major intracellular anions. Phosphorus is also involved in the generation of adenosine triphosphate (ATP), the major energy substrate of aerobic cells. Suspect loss of these ions in patients with severe diarrhea, especially steatorrhea.
Calculation of nitrogen balance allows the clinician to investigate whether adequate amounts of protein are being supplied to a particular patient. To perform this test, a 24-hour urine collection is obtained, and the amount of urinary urea nitrogen (UUN) is measured. The amount of protein (Pr) the patient is being fed is a known variable (g Pr). These values are applied to the following equation:
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Nitrogen balance = g Pr/6.25 – (UUN + 4 g)
For every 6 g of protein, 1 g of nitrogen is present. The figure 4 g is for fecal losses. The fecal protein loss can be much higher in patients with short-bowel syndrome, malabsorption, and diarrhea.
Attaining positive nitrogen balance is important. It is associated with proper immune function, good wound healing, replenishment of lean body mass in previously catabolic patients, and growth in children.
Vitamin levels can be measured in serum. This is achieved best when a specific abnormality that can be attributed to a vitamin deficiency is suspected on clinical grounds. The findings associated with various vitamin deficiencies are discussed elsewhere (see Presentation). Treat vitamin deficiency by supplementation of that vitamin.
Serum levels of zinc, chromium, selenium, and other important minerals and trace elements can also be measured. Most of these elements serve as cofactors in various metalloenzyme systems. Their depletion leads to degradation in enzyme function and, sometimes, serious clinical sequelae, some of which have been described (see Presentation). Treat a deficiency in one of these elements by replenishment, especially if a related clinical disorder (eg, glucose intolerance and chromium deficiency) is present.
Hepatic synthesis of the proteins of the coagulation cascade is a highly conserved function. Deficient hepatic production of coagulation factors is usually a sign of advanced liver disease. Assess the international normalized ratio (INR), prothrombin time (PT), and activated partial thromboplastin time (aPTT) in all patients who are considered candidates for surgery, especially those with any evidence of liver dysfunction. Identification of a defect should lead to replacement therapy (eg, vitamin K, fresh frozen plasma [FFP]).
Imaging Studies
Obtain a chest radiograph routinely in all patients who undergo placement of a temporary or durable central venous catheter for hyperalimentation or other purposes. The chest radiograph is obtained to ensure that no complications (eg, pneumothorax) have occurred. In addition, it allows documentation of proper placement of the catheter tip (ie, in the vicinity of the superior vena cava–right atrial junction).
A plain abdominal radiograph allows a preliminary assessment of bowel status. Signs of ileus or obstruction, such as greatly dilated bowel, can be looked for.
Contrast studies are a more sensitive imaging choice than the plain radiograph. An upper gastrointestinal (GI) series with small bowel follow-through can be useful. The small bowel should appear somewhat dilated because this is one of the major mechanisms of small-bowel adaptation. Areas of stricturing appear as significant narrowing. Look for these especially at areas of known previous anastomoses. Overall, the bowel mucosal pattern should remain relatively unchanged.
Abdominal computed tomography (CT) with contrast can be used to identify enteric problems, such as bowel obstruction. It also is useful for imaging the liver and can demonstrate changes consistent with cirrhosis. Other earlier signs of liver dysfunction, such as fatty change, can be demonstrated as well.
Many patients with short-bowel syndrome develop biliary sludge or gallstones. Symptoms consistent with biliary colic or cholelithiasis can be investigated with abdominal ultrasonography. This study provides important information, such as indicating the presence or absence of stones, gall bladder wall thickness, and common bile duct diameter. Choledocholithiasis and fatty change of the liver may be demonstrated as well.
Other Tests
Patients with short-bowel syndrome, especially those on prolonged courses of TPN, can develop metabolic bone disease. The major mechanism is calcium and vitamin D malabsorption. Bone can become decalcified (less dense) and more prone to fracture. In this situation it is useful to obtain an estimate of bone density.
Bone density is estimated by dual radiographic absorptiometry. Bone mineral density is measured in terms of g/cm2. The patient's bone density is measured and compared to reference values. A determination is made as to whether or not the patient is osteopenic. Patients deemed osteopenic could be treated with estrogen; calcitonin; bisphosphonates; or supplementation of calcium, vitamin D, and magnesium. Patients may be advised to increase their activity level as well.
Diagnostic Procedures
In patients with liver dysfunction that is suggested by biochemical or radiologic modalities, procurement of a tissue specimen may be advisable. Liver biopsies can be performed percutaneously under the guidance of ultrasonography or CT.
Histologic Findings
Hepatic histology is most important in short-bowel syndrome. Many therapeutic decisions, including the decision to perform transplantation, are based on demonstrated alterations in liver histology.
The type of transplant performed also depends upon the condition of the liver. Those patients with hepatic cirrhosis require a liver–small bowel transplant. Those without cirrhosis do well with an isolated intestinal transplant.
Examples of diagnoses that can be made based on findings from histologic examination of liver biopsy specimens include cholestasis, fatty change, nonalcoholic steatohepatitis, and cirrhosis.
Features of intrahepatic cholestasis include bile plugs in dilated bile canaliculi. In fatty change, a large fat globule that crowds the nucleus to the cell periphery is observed in affected liver cells. Nonalcoholic steatohepatitis has elements of fatty change, with the addition of inflammatory cell infiltration and fibrotic changes. In hepatic cirrhosis, fibrotic material is deposited in large amounts in the area of the portal triads.
