A sample of oral fluid is collected by an absorbent nonreactive collecting swab brought into contact with a favorable surface in the oral cavity. The fluid is interstitial transudate selectively collected from the vestibule region of the oral cavity at the junction of the superior labial mucous membrane and the superior gingivae between the upper canine teeth. The collecting swab is an open cell foam material with a fine (e.g., 0.3 mm diameter) cell size. The collecting swab is placed in a cylindrical concentrator and the fluid sample is squeezed through a small hole in the concentrator by pushing a plunger into the concentrator and squeezing the fluid from the collecting swab into a monitoring instrument located off site. A common electronic circuit in the instrument generates an electrical glucose representative readout for oral fluid or whole blood. A sheet of graph paper has rectangular coordinates one of which represents glucose level and the other which represents time of day. The first coordinate of the graph paper is mounted on a flat surface adjacent to a linear glucose level display. The user can record readings on the linear display by simply projecting such readings laterally on the graph to the applicable time of day and placing a mark on the graph. In the instrument, an enzyme electrode receives the sample from the concentrator. The electrode has a membrane that is wetted by a buffer solution when not in use.