Comparison of MALDI-TOF-MS and HPLC-ESI-MS/MS for Endopeptidase Activity-Based Quantification of Anthrax Lethal Factor in Serum

Diagnosing and treating anthrax at the earliest stage of disease is critical. We developed a method to diagnose anthrax at early stages of infection by detecting anthrax lethal factor (LF) at the attomol/mL level in plasma or serum. This method uses antibody capture and quantification of LF endoproteinase activity by isotope dilution matrix-assisted laser-desorption ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS). Many public health laboratories do not use MALDI-TOF-MS; thus, we have adapted the LF method for detection by electrospray ionization (ESI) tandem MS (MS/MS), which allowed comparison of both MS platforms for LF quantification. Calibration curves were linear from 0.05-2.5 ng/mL when measured after 2 h and from 0.005-1.0 ng/mL after 18 h incubation time. The limit of detection was 0.005 ng/mL using a 200 mu L sample. The coefficient of variation for quality control samples was 6-12% for both MS platforms. Samples used to perform cross-validation included 158 serum samples from a study in rabbits exposed to anthrax spores by inhalation. Some were treated with anthrax immune globulin before exposure. Concentrations measured by ESI-MS/MS matched those by MALDI-TOF-MS with p = 0.99 (r(2) = 0.997) and -0.25% mean relative difference (+/-9% standard deviation). This study shows that isotope dilution MALDI-TOF-MS is a robust and precise quantitative MS platform.