Grant: Operating Grant

A D-dimer test is a blood test that measures proteins that are formed when the body breaks down lung or leg vein clots. If a D-dimer test is normal, it is much less likely that a patient has one of these clots. However, because other illnesses also increase D-dimer levels, an abnormal D-dimer level is unhelpful and other tests need to be performed. If there is concern about a lung clot, this test is usually a lung "CAT" (or "CT") scan. We recently completed a study in patients with suspected leg clots that showed that, rather than labelling D-dimer results as "positive" and "negative" in the same way in all patients, it is better to use a higher D-dimer cut-off value (i.e., level that separates a negative from a positive result) if, after examining the patient, the doctor thinks that there is a "Low probability" of clots. We now want to apply this new way of using D-dimer blood testing to reduce how often lung CAT scans are needed in patients who may have lung clots. 2,000 patients in 14 hospitals will be assessed as Low, Moderate or High probability for lung clots by their doctor, and then have D-dimer measured. Lung clots will be ruled-out if D-dimer is <1,000 with Low probability, or <750 with Moderate probability. Those who do not have lung clots ruled-out in this way will have a lung CAT scan. Patients who have had lung clots excluded will then be followed for 3 months, expecting that very few will have clots (<1%), thereby confirming the safety of this diagnostic approach. We will also calculate cost savings. This diagnostic approach will reduce lung CAT scanning by one-third. As 1.1 million North Americans are investigated for lung clots each year, and as CAT scanning is very costly and can have serious side effects, this reduction in CAT scanning is important to patients, health care providers and to society.