Corrected end-tidal carbon monoxide closely correlates with the corrected reticulocyte count in coombs' test-positive term neonates.

Objective: To evaluate clinical usefulness of corrected end-tidal carbon monoxide (ETCOc) measurements in healthy, term, Coombs' test-positive neonates and correlate it to the corrected reticulocyte count (RC).

Methods: ETCOc and RC were determined (at 36 +/- 12 hours of age) in 50 Coombs' test-positive neonates and compared with the ETCOc values of 50 Coombs' test-negative neonates.

Results: Fifty percent of Coombs' test-positive infants had RCs <5% (within a normal range for a healthy newborn) and ETCOc = 1.8 +/- 0.34 parts per million (ppm) and likely did not exhibit hemolysis. Among infants with elevated RCs, 72% had RCs between 5% and 8% and ETCOc = 2.77 +/- 0.68 ppm, and 28% had RCs >8% and ETCOc = 4.52 +/- 0.83 ppm. There was an almost linear correlation (r = 0.86) between the RC and the ETCOc among Coombs' test-positive infants. The 50 Coombs' test-negative infants had ETCOc = 1.6 +/- 0.45 ppm. Serial ETCOc measurements were performed in 14 Coombs' test-positive infants: in all but 1 infant ETCOc values declined over time.

Conclusions: There is a good correlation between ETCOc and RC in Coombs' test-positive infants. ETCOc >2.5 ppm predicts a significant elevation of RC in 90% of Coombs' test-positive infants.