A Red Blood Cell Agglutination d-Dimer Test to Exclude Deep Venous Thrombosis in Pregnancy

ABSTRACT Along with compression ultrasonography of the legs, widely viewed as the preferred test for diagnosing deep venous thrombosis (DVT), d-dimer testing frequently is carried out because it is highly sensitive and a negative result may preclude the need for further testing. The latter test has, however, not been evaluated for diagnosing DVT in pregnancy because of its relatively low specificity. This study sought to determine the utility of the easily performed SimpliRED assay for diagnosing DVT in pregnant women. A prospective cohort study, conducted at university-affiliated Canadian hospitals, enrolled 149 consecutive pregnant women suspected of having DVT. They underwent compression ultrasonography on days 0, 3, and 7, and were followed up clinically after 3 months to determine the presence or absence of DVT. Whole blood samples were obtained at initial presentation for the SimpliRED assay. In this rapid test, blood is mixed with a conjugate of monoclonal antibody to d-dimer linked to a monoclonal antibody that binds to the red cell surface. High levels of d-dimer cause agglutination. DVT was diagnosed in 13 of the 149 participants (8.7%). All cases but one were diagnosed from initial compression ultrasonography. The SimpliRED assay was 100% sensitive and 60% specific, and had a negative predictive value of 100%. When stratifying the results of d-dimer testing by the presence or absence of patient risk factors, women having positive d-dimer test results in the first trimester of pregnancy had a high prevalence of DVT (62.5%) regardless of whether or not risk factors were present. Among pregnant women in whom DVT was not diagnosed, the assay was positive in no women in the first trimester, 24% of second-trimester pregnancies, and 51% of women in the third trimester. These findings indicate that the SimpliRED d-dimer assay is clinically useful in pregnant women suspected of having DVT. A negative result effectively excludes this diagnosis. The test appears to be useful for at least the first 2 trimesters of pregnancy.