Plasma total homocysteine (tHcy) determination is used in the diagnosi
s of homocystinuria, in cobalamin and folate deficiency and in cardiov
ascular risk assessment. However, determination of tHcy includes many
pitfalls which complicate the assessment of homocysteine status. In th
e present article, we review basic knowledge for a rational use of pla
sma tHcy in diagnostic as well as scientific work. The subjects dealt
with are procedures for sample handling and processing, the principles
of tHcy analyses, and genetic and acquired determinants of the plasma
tHcy concentration.