Homocysteine

Most UK laboratories print 5 to 15 umol/L; it is a little higher in men and in older people in the general population, and some labs print a lower upper limit of 12.

Homocysteine is an amino acid, one of the building blocks of protein, made in the body as a by-product of processing another amino acid, methionine. The body clears it using vitamin B12, folate and vitamin B6, so the level rises when any of these is short. The test measures the total amount in the blood.

Doctors request it when looking at vitamin B12 and folate status in more detail, in some people with unexplained blood clots or early heart disease, and in rare inherited conditions of amino acid handling. It is also offered by private testing services.

• Low folate, vitamin B12 or vitamin B6
• Kidney disease
• An underactive thyroid
• Older age and being male, in the general population
• Smoking, heavy coffee drinking and lack of exercise
• Some medicines, including methotrexate, some epilepsy medicines and nitrous oxide
• Rare inherited conditions of amino acid handling

• Folate, vitamin B12 and B6 supplements or a diet rich in them
• Treatment of an underactive thyroid
• Pregnancy, in the general population

• The sample must be separated and chilled promptly, because red cells keep releasing homocysteine in the tube and the figure climbs falsely if it is delayed
• Not fasting raises it slightly
• Heavy coffee drinking and smoking push it up

Usually a fasting sample, and the tube must be put on ice and separated within an hour, so it cannot be collected everywhere. Read alongside B12 and folate from the same sample.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.