The use of infrared spectrophotometry for measuring body water spaces

Background: The conventional method of measuring total body water by the de uterium isotope dilution method uses gas isotope ratio mass spectrometry (I RMS), which is both expensive and time-consuming. We investigated an altern ative method, using Fourier transform infrared spectrophotometry (FTIR), wh ich uses less expensive instrumentation and requires little sample preparat ion. Method: Total body water measurements in human subjects were made by obtain ing plasma, saliva, and urine samples before and after oral dosing with 1.5 mol of deuterium oxide. The enrichments of the body fluids were determined from the FTIR spectra in the range 1800-2800 cm(-1), using a novel algorit hm for estimation of instrumental response, and by IRMS for comparison. Results: The CV (n = 5) for repeat determinations of deuterium oxide in bio logical fluids and calibrator solutions (400-1000 mu mol/mol) was found to be in the range 0.1-0.9%. The use of the novel algorithm instead of the int egration routines supplied with the instrument gave at least a threefold in crease in precision, and there was no significant difference between the re sults obtained with FTIR and those obtained with IRMS. Conclusion: This improved infrared method for measuring deuterium enrichmen t in plasma and saliva requires no sample preparation, is rapid, and has po tential value to the clinician. (C) 1999 American Association for Clinical Chemistry.