Olsen et al. report a fatality case in which postmortem measurement of carboxyhemoglobin was key evidence in a criminal prosecution. Although the increased carboxyhemoglobin concentration and the gross autopsy findings were consistent with carbon monoxide poisoning as a cause of death, questions of spectrophotometric method accuracy and reliability were raised by the defense.
In postmortem toxicology, spectrophotometry is an established and commonly used technique for measuring carboxyhemoglobin. The technique is based on Beer’s Law and takes advantage of the unique but overlapping wavelength spectra of blood oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin, and methemoglobin. Use of molar absorptivities at a number of selected wavelengths equal to or exceeding the number of hemoglobin forms allows the solution of a set of simultaneous equations and quantification of the hemoglobins. Although monitoring of at least 4 wavelengths is required, some current CO-oximeters monitor >100 wavelengths.
The analytical performance of the CO-oximeter is determined not only by the number of wavelengths monitored but also by applying sound forensic principles, including method validation. The authors of this case study show evidence of comparability of results for a CO-oximeter and manual spectrophotometric method and cite studies comparing results for postmortem blood carboxyhemoglobin measured by CO-oximetry and gas chromatography. It is important to recognize that CO-oximeters are developed primarily for clinical use. The effect of matrix interferences in postmortem blood (such as increased methemoglobin and variable total hemoglobin content) varies between CO-oximeters. In our experience, postmortem increases in methemoglobin can cause positive or negative carboxyhemoglobin interference, and pretreatment with sodium hydrosulfite can eliminate this effect. Validation of increased carboxyhemoglobin in medical examiner cases should be accompanied by confirmatory testing by a method with a different chemical principle. In forensic toxicology, confirmation testing is a recommended practice and adds to the reliability and forensic defensibility of toxicology findings.
As evident in this case, a postmortem toxicology report can, and often does, serve as a basis for medical examiner conclusions that translate into significant legal consequences. Accuracy and reliability of toxicology results remain, as always, grounded in our scientific methods and validation practices.
Author Contributions: All authors confirmed they have contributed to the intellectual content of this paper and have met the following 3 requirements: (a) significant contributions to the conception and design, acquisition of data, or analysis and interpretation of data; (b) drafting or revising the article for intellectual content; and (c) final approval of the published article.
Authors’ Disclosures of Potential Conflicts of Interest: No authors declared any potential conflicts of interest.
Role of Sponsor: The funding organizations played no role in the design of study, choice of enrolled patients, review and interpretation of data, or preparation or approval of manuscript.
- © 2010 The American Association for Clinical Chemistry