Whipple's Disease, Kinyoun Carbolfuchsin Acid-Fast Stain
The most important item on the differential diagnosis is mycobacterial infection in an immunocompromised (usually AIDS) patient. However, this patient was tested and found to be HIV-negative. This acid-fast (AFB) stain shows no organisms. In a mycobacterial infection, these enlarged histiocytes are stuffed with innumerable acid-fast bacilli. Here there are none.
With the negative AFB stain, it became clear that this might be a career-first case of Whipple's disease for me after almost 35 years in pathology!