Abstract
BACKGROUND: s-Adenosylmethionine (SAM)1 has been suggested as a diagnostic test and surrogate marker for Pneumocystis jirovecii pneumonia (PCP) in HIV-positive patients. In this study, we report a robust hydrophilic-interaction liquid chromatography tandem mass spectrometry (LC-MS/MS) assay that can be used to quantitate serum SAM in clinical laboratories. METHODS: Proteins in serum samples were precipitated using trichloroacetic acid. The supernatant was separated after centrifugation. D3d3-SAM was added as the internal standard. SAM and d3-SAM were extracted using a mixed-mode cation exchange column. Extracts were dried under nitrogen and reconstituted in H2O and acetonitrile (1:9, vol:vol). HPLC-tandem mass spectrometry analysis was performed with a silica column and multiple reaction monitoring for SAM and d3-SAM. RESULTS: The limit of quantitation (LOQ) for SAM was 10 ng/mL. The assay was linear between 10 and 500 ng/mL. Intra-assay coefficient of variation (CV) was 8% and inter-assay CV was 17% at the LOQ. Turnaround time for each specimen was approximately 1 h. Using this method, we found that serum SAM concentration was correlated with fasting status, especially methionine intake. We also measured acute and convalescent serum SAM levels of 8 HIV-positive patients with PCP and non-PCP pneumonia. SAM concentrations in convalescent samples were significantly increased compared to acute levels only in patients with PCP. CONCLUSIONS: The LC-MS/MS method had sufficient analytical sensitivity for detecting low levels of SAM found in HIV-infected patients and can be used for quantitative measurements in a clinical laboratory. This method facilitates research and possible clinical application of SAM as a marker for PCP.