Carboxyhemoglobin determined in neonatal blood with a CO-oximeter unaffected by fetal oxyhemoglobin.

Measurements of carboxyhemoglobin (COHb) for clinical purposes are routinely made with CO-oximeters. However, fetal hemoglobin (HbF) interferes with this spectrophotometric method. The manufacturer (Ciba Corning Diagnostics) of a new CO-oximeter (CCD 270) claims that COHb measurements with this instrument are insignificantly affected by HbF. We examined this claim through CO-oximeter analysis of cord blood/adult blood mixtures and compared the results with those obtained by gas chromatography (GC). We also studied the influence of oxygen and bilirubin concentrations. Measurements of COHb with CCD 270 were significantly lower than the GC measurements, but the differences were not clinically important. Linear regression analysis of HbF and COHb measurements (n = 68) showed significant correlation (P < 0.0001) between the two factors for the older CCD 2500 CO-oximeter (R2 = 0.56), but not for the CCD 270 (R2 = 0.06) or GC (R2 = 0.02). Bilirubin concentrations, which affected COHb measurements with CCD 2500, did not significantly affect CCD 270 measurements. We conclude that COHb measurements with CCD 270 CO-oximeter are not affected by HbF or bilirubin concentrations.