Biliary sludge or microlithiasis: This condition is often considered the single most likely diagnosis for acalculous cholecystitis because it can cause inflammation of the gallbladder without the presence of visible gallstones on imaging. Biliary sludge is a mixture of particulate matter that can accumulate in the gallbladder and may cause symptoms similar to those of gallstones.

Gallbladder ischemia or infarction: Reduced blood flow to the gallbladder can lead to ischemia or infarction, presenting similarly to acalculous cholecystitis. This condition is more common in critically ill patients or those with vascular disease.
Infectious cholecystitis: Bacterial, viral, or parasitic infections can cause inflammation of the gallbladder without the presence of stones. This diagnosis should be considered, especially in patients with risk factors for infection.
Chemical cholecystitis: Exposure to certain chemicals or drugs can cause inflammation of the gallbladder. This is a less common cause but should be considered in the appropriate clinical context.

Gallbladder cancer: Although rare, gallbladder cancer can present with symptoms similar to acalculous cholecystitis. It is crucial to consider this diagnosis to avoid delayed treatment.
Sclerosing cholangitis: This chronic liver disease can cause inflammation and scarring in the bile ducts, leading to symptoms that might mimic acalculous cholecystitis. Early diagnosis is essential for proper management.
Mirizzi syndrome: This rare condition involves obstruction of the common bile duct due to a gallstone impacted in the cystic duct, which can cause symptoms similar to acalculous cholecystitis. It's a critical diagnosis to consider due to its potential for severe complications.

Xanthogranulomatous cholecystitis: A rare form of gallbladder inflammation characterized by the presence of xanthoma cells. It can mimic gallbladder cancer on imaging and is often diagnosed after surgical removal of the gallbladder.
Porcelain gallbladder: A condition where the gallbladder wall becomes calcified, which can increase the risk of gallbladder cancer. It's a rare finding but important to recognize due to its association with malignancy.
Cholesterolosis: A condition characterized by an abnormal accumulation of cholesterol esters and triglycerides in macrophages within the gallbladder wall, leading to polypoid or flat lesions. It's usually asymptomatic but can cause symptoms similar to acalculous cholecystitis in some cases.