Total-body-water measurement with 18O-labeled water in short-bowel patients with an ileostomy

The aim of this study was to evaluate the use of H218O for total-body-water (TBW) determination in patients with short bowel because ileostomy losses of labeled water can falsify TBW. Thirteen adult short-bowel patients were studied after an overnight fast. Saliva, blood, urine and ileostomy output were collected before and 30, 60, 90, 120, 240, and 360 min after oral ingestion of 0.88 g/kg of 2.5% H218O to measure 18O abundance by isotope-ratio mass spectrometry. TBW was calculated from the dilution of the isotopic water in saliva and plasma. The quantity of labeled water lost in the ileostomy was calculated from the isotopic enrichment of the ileostomy output from T0 to T360. The values obtained from saliva or plasma with (corrected) and without (uncorrected) considering H218O lost in the ileostomy output were compared with a paired t test. Agreement was evaluated using the Bland-Altman method. From T0 to T360, the ileostomy output and the percentage of lost labeled water were 490 ± 314 mL and 6.38 ± 8.52%, respectively. TBW calculated from plasma or saliva isotopic enrichment was different, and a significant difference was also observed between corrected and uncorrected TBW values (saliva: uncorrected TBW = 32.35 ± 7.52 L, corrected TBW = 30.29 ± 6.09 L; plasma; uncorrected TBW = 30.80 ± 7.29 L, corrected TBW = 28.79 ± 5.79 L). The agreement between the values obtained from the two dilution spaces or between the two calculation methods was poor. Because of the large discrepancies between calculation methods, determination of TBW from oral ingestion of 18O-labeled water in patients with short bowel should be calculated only from plasma 18O space dilution and should consider ileostomy losses.