When blood clots form in veins (blood vessels that carry blood back to the heart), serious and even fatal complications can occur. Doctors use blood thinners to promote clot breakdown and prevent additional clots. For many years, the most commonly used blood thinner has been a drug known as unfractionated heparin. To be effective, unfractionated heparin must be injected every 6 hours (or given continuously through a needle in a vein). This requires hospitalization and careful daily monitoring of the dose to confirm effective blood thinning. More recently, another preparation of heparin, low-molecular-weight heparin (LMWH), has been introduced. It can be given at home, requires less frequent injections, and does not require daily monitoring of the dose. However, LMWH is about 10 times as expensive as unfractionated heparin. Several studies have suggested that LMWH is more effective than unfractionated heparin. Since the implications for cost and effectiveness are important, doctors need to be confident that these studies fairly compare unfractionated heparin and LMWH. The problem is that to make a fair comparison, researchers must be certain that the correct dose of unfractionated heparin was administered. This depends on accurate laboratory tests. Over the past 25 years, many different laboratory tests have been used, each having a different standard for effective blood thinning dose.