On patients in whom ascites is thought to be present, a special percussion test for shifting dullness may be performed. While the patient is lying on the back, the examiner determines the borders of tympany and dullness. The area of tympany is present above the area of dullness and is caused by gas in the bowel that is floating on top of the ascites. The patient is then asked to turn on the side, and the examiner again determines the borders of the percussion notes. If ascites is present, dullness ''shifts'' to the more dependent position;the area around the umbilicus that was initially tympanic becomes dull. Shifting dullness has a sensitivity of 83% to 88% and a specificity of 56%. The test for shifting dullness is illustrated in Figure 17-18.
An additional test for ascites is the presence of a fluid wave. Another examiner's hand or the patient's own hand is placed in the middle of the patient's abdomen. Indenting the abdominal wall stops transmission of an impulse by the subcutaneous adipose tissue. The examiner then taps one flank while palpating the other side. The appearance of a fluid wave is suggestive of ascites. This technique is illustrated in Figure 17-19. The presence of a prominent fluid wave is the most specific of all physical diagnostic tests for ascites and has a specificity of 82% to 92%, according to several studies. A false-positive result may be obtained in obese individuals, and a false-negative result may be obtained when the ascites is small to moderate.
Another physical finding with ascites is the presence of bulging flanks. This change occurs when the weight of free abdominal fluid is sufficient to push the flanks outward. It has a 93% sensitivity and a 54% specificity for detecting ascites.
The most sensitive sign for ascites is the presence of shifting dullness, whereas the most specific sign is the presence of a prominent fluid wave. For an individual patient, the examiner must know the prevalence of disease or the pretest probability to apply sensitivity and specificity. Several studies have reviewed the operating characteristics of the physical examination tests for ascites. Their pooled sensitivity, specificity, and likelihood ratios for the presence of ascites are summarized in Table 17-4. The presence of a prominent fluid wave or shifting dullness is associated with the highest likelihood of the presence of ascites; the absence of bulging flanks, flank dullness, or shifting dullness decreases the likelihood.
Among the more common historical items perceived by patients, an increase in abdominal girth or recent weight gain has the highest chance of being associated with ascites. An increase in abdominal girth has a positive likelihood ratio (LR+) of 4.16;recent weight gain has an LR+ of 3.20. Conversely, the absence of the subjective increase in abdominal girth has a negative likelihood ratio (LR—) of 0.17;the absence of subjective ankle swelling carries an LR— of 0.10. With regard to the physical examination, the presence of a fluid wave or shifting dullness has the highest LR+ (9.6 or 5.76, respectively). The absence of bulging flanks or edema makes the presence of ascites least likely, with an LR— of 0.12 or 0.17, respectively.
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