There is provided a method of diagnosing a disease state in a subject. The disease state is caused or effected by pneumococcal infection. The method includes obtaining a biological test sample from the subject. The biological sample includes at least one metabolite selected from the group consisting of citrate, trigonelline, acetylcarnitine, acetone, myo-inositol, 3-hydroxybutyrate, glucose and carnitine. A respective concentration of each one of the at least one metabolite is compared with a predetermined concentration value associated with a corresponding one of the at least one metabolite. The predetermined concentration value is indicative of the disease state. For example, the biological sample includes any one of citrate, trigonelline, acetylcamitine, acetone, myoinositol, 3-hydroxybutyrate, glucose and carnitine. As a further example, the biological samples include any combination of citrate, trigonelline, acetylcamitine, acetone, myo-inositol, 3-hydroxybutyrate, glucose and carnitine. As a further example, the biological sample includes each one of citrate, trigonelline, acetylcamitine, acetone, myoinositol, 3-hydroxybutyrate, glucose and carnitine.