'This will be fun," I say to Gus, my husband, as I prepare to puncture his skin with a lancet to see if he is harbouring an evil chemical that might make him peg out with a heart attack before his time. "I'll be Angelina and you be Billy Bob." Except instead of wearing phials containing each other's blood to declare our undying love, we'll send ours to a lab to see if our homocysteine and cholesterol levels are safe.

Homocysteine is a substance made by the body; if you have too much of it, you increase your chances of heart disease and about 50 other medical conditions. To lower homocysteine, you need to consume less fatty meat, coffee, salt and alcohol and eat more fish and folate-rich greens (such as spinach and broccoli). You should also stop smoking and lower your stress levels. None of this is ground-breaking; we just have another measurable chemical to show how much we're messing up our bodies.

So Gus and I decide to take our health in our own hands with a home blood-testing kit that checks for homocysteine and cholesterol. This is the latest in a line of kits that claim to be able to screen for diseases that include osteoporosis, bowel disease and Alzheimer's. The results come with a covering letter, which states, "Please note the test does not provide a definitive diagnosis of increased risk, and it is advisable to discuss your results with your GP should these appear above the normal range." To me, however, this seems to be little more than a get-out clause.

Our Heart In The Box kits from YorkTest laboratories come with clear instructions. We need to put a spot of blood on a test card for the homocysteine, and half fill a small phial with blood for the cholesterol test. The first part is easy. It is when we have to fill the phials we come unstuck. We press, we squeeze, we hold a finger over the tube but one little mean drop trickles round the side and dries up. We consider arm veins, which might not be advised but damn it, it's in the name of science. As I hover over Gus's arm, our daughter walks in, screams and runs out. Then she sticks her head round the door and asks, "Are you junkies?" I ring the help line.

"Er, we're not bleeding enough. Can I use something else?"

"We really couldn't advise that, madam," they say.

They send us spare kits and this time, we manage, with awkwardness, and a spare set of hands.

When we get our results, my cholesterol level is 6.66. Ideally, it should be below five, but my homocysteine levels are so low - 5.3, within the low-risk category of below 6.9 - I can afford to be semi-smug. Gus, who eats more bad fats than I do, oddly has lower cholesterol but a very high level of homocysteine, 19. "It's not a definitive diagnosis!" I splutter, as he rifles through our life insurance papers and tells me he wants the Stray Cats to play at his funeral.

Will he take the recommended B vitamins and eat a healthier diet, with his test results acting as an incentive? Certainly. Is it worth doing? If you know you eat badly and don't take enough exercise, but need to see a scary number to motivate you to do something about it, then this could be the test for you.

· For details of the Heart In The Box test by YorkTest call 0800 074 6185.