Diagnosing deep venous thrombosis in primary care

In patients suspected of deep venous thrombosis (DVT) in primary care, it is a challenge to discriminate the patients with DVT from those without DVT. The risk of missing the diagnosis and the risk of unnecessary referral and treatment with a potential harmful therapy has to be balanced by the primary care physician. This thesis aims to address issues related to the process of diagnosing deep venous thrombosis in daily primary care. These include validation of an existing secondary care diagnostic decision rule for the exclusion of DVT in the primary care setting, development of a new diagnostic rule for DVT specifically for primary care, the role of D-dimer testing in recognizing DVT and the prevalence of unrecognized malignancy in patients with DVT in primary care. In Chapter 2, we describe the validation of the "Wells rule" with and without D-dimer testing, in a primary care setting. In this setting, 12.0% of patients in the low-risk group had DVT; the original study by Wells and colleagues reported a rate of 3% among such patients. When combined with D-dimer testing, the Wells rule yielded a prevalence of DVT of 2.9% in the lowest-risk group, whereas the prevalence was 0.9% in the original study. The Wells rule, alone or in combination with D-dimer testing, does not guarantee accurate estimation of risk in primary care patients in whom DVT is suspected. In Chapter 3 we conclude that the predictive value of the combination of independent variables from patient history and physical examination in primary care is too low (ROC area 0.68) to identify patients in whom the diagnosis DVT could be ruled out. We describe (Chapter 4) a new diagnostic rule for primary care, based on patient history, physical examination plus D-dimer testing. Applying this rule could reduce the number of referrals by at least 23% while only 0.7% of the patients at very low risk had a (missed) DVT and were thus not (yet) referred. Using seven simple diagnostic indicators from patient history, physical examination and the result of D-dimer testing, it is possible to safely rule out DVT in a large number of patients in primary care, reducing unnecessary patient burden and health care costs. As cut-off values of D-dimer tests are not standardized, we determined and described in Chapter 5 the optimal threshold for two frequently used D-dimer assays in diagnosing DVT. The primary care rule for excluding DVT showed reasonable to good performance when applied to secondary care patients suspected of DVT (Chapter 6). In our primary care study population, the prevalence of malignancy is two times the prevalence in the population at large (Chapter 7). In idiopathic DVT patients, this was 7.4% and 2.6% in secondary DVT patients. The percentage unrecognized malignancy is comparable to the prevalence known from secondary studies. This thesis concluded with a discussion on the topics addressed in earlier chapters. A strategy for diagnosis of DVT in primary care is given, together with a practical flow diagram