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52 CHRONIC VOMITING IN CATS PEER REVIEWED significant proteinuria. If proteinuria is present, thorough evaluation for systemic disease is generally warranted before pursuing primary GI disease. Heartworm and Retroviral Testing Geographic location dictates prioritization of heartworm testing, although it should be considered a first-tier test in areas where heartworms are endemic. Serum antigen testing is generally considered more specific but less sensitive than antibody testing 18 because it assesses the presence of worm antigen versus historical exposure. However, a recent study found serum antigen testing to be more sensitive and specific than serum antibody testing. 19 Combination testing has the highest sensitivity, although false-negative results still occur. 20 Retroviral status should be confirmed as part of initial diagnostic screening, even for indoor-only cats. 21 Confirmatory testing is recommended for cats that are positive on initial screening. Care should be taken to ascertain FIV vaccination status because not all point-of-care antibody tests can accurately differentiate between prior vaccination and infection. 22 A positive FIV result from an antibody test with unknown differentiation ability should be confirmed via Western blot or polymerase chain reaction (PCR). Thyroid Hormone Quantitation Screening for hyperthyroidism is warranted regardless of age, given reports of clinical hyperthyroidism in cats as young as 8 months. 23 Measurement of total thyroxine (T4) concentration is diagnostic for hyperthyroidism in approximately 95% of cases. Free T4 concentration determination is recommended for cats suspected of hyperthyroidism that have total T4 concentrations in the upper half of the reference range. Elevated free T4 in the presence of total T4 measurements in the upper half of the reference range suggests hyperthyroidism. 24 SECOND-TIER DIAGNOSTIC TESTS If the cause of vomiting remains unknown after completion of the preceding tests, second-tier diagnostics are warranted. Abdominal Imaging To optimize diagnostic accuracy, abdominal radiography and ultrasonography should be performed after a routine fast. Inadequate fasting particularly complicates assessment for motility disorders. Radiographs are evaluated for alterations in organ size, the presence of mass effects and foreign bodies, GI abnormalities suggestive of obstruction or motility disorders, and peritoneal effusion. Such findings are generally specific, although their absence does not rule out significant disease. Advantages of ultrasonography in evaluation of patients with GI signs include the ability to detect homogenous and heterogenous changes to the parenchyma of various viscera; visualize the biliary tree and its path through the pancreas; differentiate between the GI lumen and various layers of the wall, as well as localize and quantitate any thickening present; assess intestinal motility, corrugation, and plication; identify mesenteric lymphadenomegaly, foreign bodies, and masses; and guide noninvasive tissue and fluid sampling. Care should be taken in differentiating between dilatation and simple dilation of the common bile duct (an aging-related change in cats). Assuming hyperthyroidism has been ruled out, changes in GI wall layering or thickness suggest primary GI disease (eg, FRD, IBD, neoplasia, histoplasmosis). It is generally accepted that small intestinal wall thickening of 0.28 cm or greater is clinically significant; such thickening has been associated with both lymphoma and IBD. 1,2,25,26 Wall thickening can be focal, diffuse, or segmental. Although focal thickening can suggest neoplasia, ulceration and IBD also can be associated with focal defects and warrant consideration. Segmental thickening has been found with multiple abnormalities, such as concurrent IBD and lymphoma. 26 Thickening can affect all wall layers or specific layers ( Figure 1 ). Thickening of the muscularis propria (greater than half the thickness of the submucosa) is more likely in cats with lymphocytic lymphoma, although it FIGURE 1. Diffuse muscularis thickening (arrows) in a cat with chronic vomiting diagnosed with hyperthyroidism.