Diagnosis is made by biopsy, usually by duodenal endoscopy, which reveals PAS-positive macrophages in the lamina propria containing nonacid-fast, Gram-positive bacilli. Immunohistochemical staining for antibodies against "T. whipplei" has been used to detect the organism in a variety of tissues, and a polymerase chain reaction-based assay is also available, which can be confirmatory if performed on blood, vitreous fluid, synovial fluid, heart valves, or cerebrospinal fluid. PCR of saliva, gastric or intestinal fluid, and stool specimens is highly sensitive, but not specific enough, indicating that healthy individuals can also harbor the causative bacterium without the manifestation of Whipple's disease, but that a negative PCR is most likely indicative of a healthy individual.