The invention provides, inter alia, automated medical methods and apparatus that test PD 
effluent in a flow path (e.g., with an APD 
system or CAPD setup) to detect, for example, the onset of 
peritonitis, based on optical characteristics of the 
effluent resolved at cellular scales of distance. For example, according to one aspect of the invention, an APD 
machine includes, in an 
effluent flow path, apparatus for early stage 
peritonitis detection comprising an illumination source and a 
detector. The source is arranged to illuminate peritoneal effluent in a chamber that forms part of the flow path, and the 
detector is arranged to detect illuminant scattered by the effluent. The 
detector detects that reflected or scattered illuminant at a cellular scale of resolution, e.g., on a scale such that separate cellular-sized biological (or other) components in the effluent can be distinguished from one another based on scattering events detected by the detector. Other aspects of the invention provide automated 
medical testing methods and apparatus that detect the onset of 
peritonitis and other bodily conditions by testing fluids in the body 
in vivo, e.g., the patient's 
peritoneum. Such apparatus and methods utilize a first 
fiber optic bundle to carry illuminant from a source of the type described above into a bodily organ or cavity, and a second 
fiber optic bundle to carry illuminant scattered by fluid in that organ or cavity to a detector as described above.