Validation of the i-STAT®1 Analyzer for Postmortem Vitreous Humor Electrolytes and Glucose Analysis

The analytical value of vitreous humor as a specimen in postmortem forensic toxicology has been known for some time. Numerous medical examiner laboratories outsource the analysis of this important specimen for electrolyte and glucose measurements. This can be both time-consuming and costly. The utility of the i-STAT®1 medical device to measure electrolytes and glucose in whole blood samples has been demonstrated for over two decades in a clinical setting through single-use disposable cartridges that introduce samples to the i-STAT®1. Different cartridge types allow for the analysis of various analytes including sodium, potassium, chloride, creatinine, urea nitrogen and glucose. With only 100 μL of sample, results are obtained in under 4 min. In this study, we utilized the i-STAT®1 using an alternative specimen matrix, postmortem vitreous humor and quantitatively determined the validity and reliability of the instrument for this purpose. Acceptable criterion was used for each test as suggested by the Scientific Working Group for Forensic Toxicology. All analytes of interest, except creatinine, demonstrated a percent error < ±10% for both accuracy and precision studies. Drug interference and stability studies were performed with many of the analytes demonstrating a percent error < ±20%. Throughout drug interference and stability studies, all analytes of interest were detectable except for potassium, which gave inconclusive results. Significant interference with commonly found drugs were shown for creatinine and chloride but must be evaluated carefully. Volume additions to ethanol spiked samples caused significant interference for all analytes and is considered a limitation for this method of analysis that requires additional studies. As vitreous humor continues to be used in forensic medicine to aid in diagnostic interpretation, the i-STAT®1 has the potential to give accurate results in a timely and cost-effective manner.