The invention provides a method of detecting the presence, absence or severity of a disease in a patient wherein said disease is accompanied by decreased level of S-adenosylmethionine, or increased level of S-adenosylhomocysterine, or reduced methylation index comprising: identifying any individual or a patient that is suspected of having said disease or is at risk of having said disease; obtaining a biological sample from said patient; determining the level of SAM in said biological sample using an antibody derived from a hapten analog of SAM, SAH; and correlating the levels of SAM, SAH and MI in said biological sample with the presence, absence, or severity of said disease. The invention also provides methods for determining methylation index in biological fluids which is indicative of the health status of an individual. Additionally, the invention includes colloidal gold test strips and homogenous enzyme immunoassays which are useful for determining S-adenosylmethionine and S-adenosylhomocysteine.