Comparison of recoveries in breath carbon dioxide of (HCO3-)-C-13 and (HCO3-)-C-14 administered simultaneously by single 6 h constant unprimed intravenous infusion

The aim of this study was to assess the bioequivalence of H (CO3-)-C-13 and H (CO3-)-C-14, by administering both labels simultaneously by single infus ion and comparing their recovery in breath CO2 and urinary urea. Six health y male subjects (age range 24-41 years; weight 76 . 7 (sD, 18 . 6)kg; heigh t 1 . 79 (SD 0 . 05) m) were infused with unprimed solutions of HCO3- (110 . 0 mmol/kg) labelled with C-13 (0 . 76 mmol C-13/h) and C-14 (48 Bq/h) at a constant rate for 6 h, in a whole-body calorimeter (1400 litres) for meas urement of CO2 production. Samples of breath were collected hourly in a Dou glas bag and all urine was collected into two batches (0-4 h and 4-6 h) for estimating recovery of infused label by measurement of enrichment or speci fic activity. Recovery in breath CO2 of both labels increased from about 25 % for the first hour to 88 % and above for hours 3-4 onwards. Mean recover y of C-13 in breath CO2 was slightly higher than that of C-14 for all perio ds (mean difference always less than 1 % of infused label) but was signific ant only for the first 3 h (P < 0 . 05). Recovery of C-14 in urea was signi ficantly higher (P < 0 . 01) than C-13, but was confounded by substantial v ariability and uncertainties concerning (CO2)-C-13 background enrichments. These results suggest that there is no compelling need to alter factors cur rently used for recovery of C-14 in breath when using C-13 instead, and vic e versa.