MEASUREMENT OF HOMOCYSTEINE
The sum of all homocysteine species in plasma (free plus protein bound) is referred to as total homocysteine (tHey or homocyst(e)ine). As a marked redistribution between free and bound homocysteine takes place after preparation of plasma, this gives an accurate measurement without the need for exceptionally cautious sample handling and storage. The day-to-day variation in fasting tHcy among healthy individuals is small (coefficient of variation 7%), so that a single measurement is viable. The range for total tHcy differs somewhat from one laboratory to another but values between 5 and 15 llmollL are usually considered normal.
The variability may be because of different methodologies, differences in sample processing, or the selection of subjects who are under the influence of various factors that affect concentration of homocysteine. Because of these differences, a reference range is difficult to define, as are cut-off points for mild and moderate hyperhomocysteinemia.
