An apparatus for analyzing the composition of bodily fluid. The apparatus comprises a fluid handling network including a patient end configured to maintain fluid communication with a bodily fluid in a patient. The apparatus further comprises a pump unit in operative engagement with the fluid handling network. The pump unit has an infusion mode in which the pump unit is operable to deliver infusion fluid to the patient through the patient end, and a sample draw mode in which the pump unit is operable to draw a sample of the bodily fluid from the patient through the patient end. The apparatus further comprises a spectroscopic analyzer configured to obtain spectra from at least a portion of the sample, and a processor in communication with or incorporated into the spectroscopic analyzer. The apparatus further comprises stored program instructions executable by the processor to (a) identify one or more interferents to the measurement of one or more analytes in the bodily fluid as being possibly present in the portion, and (b) reduce errors in the measurement due to the presence of the determined possible interferents. The pump unit can comprise a single dual-mode or reversible pump operable in both the infusion and sample draw modes; or two pumps, one of which is operable in the infusion mode but not in the sample draw mode, and another of which is operable in the sample draw mode but not in the infusion mode.