Omega-3 index determined by gas chromatography with electron impact mass spectrometry

Omega-3 index is a relatively new concept, defined as the sum of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) expressed as a percentage of the total fatty acids in red blood cell membranes. This index reflects medium to long-term intake of omega-3 polyunsaturated fatty acids and could be a useful tool in epidemiological studies. The standard technique used for fatty acid analysis and quantification has been gas chromatography (GC) with flame ionization detection. This method is robust and has good precision and sensitivity. However, a major disadvantage is inability to confirm spectrometrically the identity of fatty acids detected, which is important especially in complex biological samples. The current study measures omega-3 index in 12 healthy human volunteers using GC-mass spectrometry (MS). Both the intra-assay and day-to-day variations were well within 5% with linearity of response extending to a concentration of 250 μg/ml (830 μmol/L) of EPA. The limit of detection of EPA was 0.36 μg/ml (1.2 μmol/L). About 25 fatty acids were consistently detected in red blood cells from healthy volunteers including cis and trans isomers. The omega-3 index ranged from 2.4% to 6.2% among the 12 volunteers examined and there was no difference between samples taken in the fasting and postprandial states. EPA and DHA concentrations ranged from 3.53 to 105.89 μg/ml (11.7–350 μmol/L) and 12.19 to 214.42 μg/ml (37.1–652.7 μmol/L), respectively. Thus a GC–MS method has been developed for measuring the omega-3 index. Further studies are required to determine the role of this index as a predictor of disease.