Methodologic considerations in the interpretation of postmortem carboxyhemoglobin concentrations

The interpretation of postmortem carboxyhemoglobin (COHb) concentrations can have major implications even when COHb is not the direct cause of death. Much litigation may hinge on the length of time an individual was alive during the fire. Therefore, the reliability of an analytical method to measure COHb, ranging from low to sub-lethal levels, is critical to the proper interpretation of results. This study used 40 blood specimens from fire deaths and 15 blood specimens from non-fire deaths to compare COHb levels obtained from a spectrophotometric method (IL 482 CO-Oximeter; CO-Ox) with a reference gas chromatographic method (GC). Since spectrophotometric methods are influenced by the amount of total hemoglobin(Hb) present in the blood, multiple saline dilutions of specimens were performed to yield Hb as low as lg/dl, For GC COHb concentrations < 5% and Hb less than or equal to 4g/dl, the average and median ratios of CO-Ox COHb to GC COHb concentrations were 2.8 and 2.6 respectively. These ratios were 8.0 and 6.8 respectively when Hb < 4g/dl. At GC COHb levels ranging from 5 to 40% and Hb greater than or equal to 4g/dl, the average and median ratios were 1.6 and 1.0 respectively. These ratios were correspondingly 2.1 and 1.9 when Hb < 4 g/dl. These data clearly indicate COHb can be influenced by the analytical methods used.