Malabsorption results from premucosal, mucosal, and postmucosal diseases (Table 6.26). In premucosal diseases, defective digestion and absorption results from pancreatic or other systemic diseases and reduced bile salt concentrations. Mucosal defects result from anatomic or biochemical epithelial alterations, the presence of microorganisms, and inflammatory or infiltrative processes. Postmucosal diseases include malabsorption due to lymphatic obstruction, vascular disease, or congestive heart failure. Mucosal diseases are the most common disorders that a surgical pathologist is likely to encounter. Malabsorption syndromes affect patients
of all ages and the age incidence depends on the etiology. Celiac disease, the most common cause of malabsorption in developed countries, is detected at almost any age. Infectious disease, lactase deficiency, and nutrient deficiencies are the most common causes of malabsorption in the developing world. In an ideal world, the pathologist should be supplied with relevant clinical information, including results of laboratory or serologic tests, before attempting to arrive at a specific diagnosis. Information that allows the pathologist to make the most useful interpretation of the biopsy specimens is listed in Table 6.27.

Small intestinal biopsies can be obtained by either a suction capsule or by forceps after endoscopic visualization. The suction capsule method requires radiographic guidance and is more expensive than the more commonly used forceps biopsy. Focal or patchy lesions can be visualized and biopsied by the endoscopic method. This technique also permits visualization of the gastrointestinal tract and avoids the radiation exposure associated with suction biopsy. However, capsule biopsies are still preferred over endoscopic biopsies by some gastroenterologists, particularly in children younger than 2 years of age.

Either the duodenum or the jejunum is an appropriate site for biopsy, but the biopsy should be procured no more proximal than the second part of the duodenum to avoid artifacts due to prominent Brunner glands or the nonspecific or peptic duodenitis commonly seen in the bulb and proximal duodenum. Unfortunately, more often than not the biopsies derive from the proximal duodenum, a site that is not ideal for the interpretation of villous architecture. It is important to note that the presence of a normal small intestinal biopsy does not exclude many malabsorptive conditions. Conditions with malabsorption and a normal small bowel villous architecture are listed in Tables 6.28 and 6.29.

Certain factors optimize the diagnostic accuracy of the biopsy interpretation, including (a) careful biopsy handling and orientation, (b) provision of accurate clinical information
to the pathologist, (c) an understanding by the pathologist of the spectrum of normal intestinal histology, and (d) familiarity with the spectrum of small intestinal diseases that may present as malabsorption.