Even in the age of ultrasound, history and physical exam can be useful in assessing for ascites.

History is a good place to start. Of all the questions we often ask when we suspect ascites (eg, increasing abdominal girth, weight gain and ankle swelling), lack of report of ankle swelling is probably the most helpful in excluding ascites (negative likelihood ratio [LR-], 0.1 in a study involving men), followed by no increase in abdominal girth (LR-, 0.17). Conversely, patient reported ankle swelling or increasing abdominal girth may be helpful in suspecting ascites (LR+ 4.12 and 2.8, respectively). ¹

Of the various physical signs and maneuvers, absence of peripheral edema is highly associated with the lack of ascites, followed by lack of shifting dullness or fluid wave (LR-, 0.2, 0.3, 0.4, respectively). The presence of a fluid wave may be the most helpful in suspecting ascites, followed by peripheral edema, and shifting dullness (LR+ 6.0, 3.8, 2.7, respectively). ¹  Relatively high sensitivities have been reported for shifting dullness (83-88%), while relatively high specificities have been reported for the fluid wave test (82-90%).^2,3 An elevated INR may also improve the positive predictive value of shifting dullness and fluid waves.⁴

So if you don’t get a history of ankle edema and find no evidence of peripheral edema or shifting dullness on exam, the likelihood of ascites is pretty low. On the other hand, if you find a positive fluid wave, you can be pretty sure that the patient has ascites.

Of course, the actual likelihood of detecting ascites also depends on several other factors, including your pre-test probability and the volume of the ascites in the abdominal cavity, with at least ~500 ml of ascites necessary before it can be detected on exam (vs ~100 ml for ultrasound). ^2,5

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References

1. Williams JW, Simetl DL. Does this patient have ascites? How to divine fluid in the abdomen. JAMA 1992;267: 2645-48. https://jamanetwork.com/journals/jama/fullarticle/397285
2. Cattau EL, Benjamin SB, Knuff TE, et al The accuracy of the physical examination in the diagnosis of suspected ascites. JAMA 1982;247:1164-66. https://www.ncbi.nlm.nih.gov/pubmed/7057606
3. Cummings S, Papadakis M, Melnick J, et al. The predictive value of physical examinations for ascites. West J Med 1985;142:633-36. https://www.ncbi.nlm.nih.gov/pubmed/3892916
4. Fitzgerald FT. Physical diagnosis versus modern technology. A review. West J Med 1990;152:377-82. https://www.ncbi.nlm.nih.gov/pubmed/2190412
5. CDC. Assessment for ascites. https://www.cdc.gov/dengue/training/cme/ccm/Assess%20for%20Ascites_F.pdf. Accessed November 13, 2019.