Predicting deep venous thrombosis in pregnancy: externalvalidation of the "left" clinical prediction rule

Background. The assessment of clinical probability represents an important step in the diagnostic strategy of patients with suspected deep vein thrombosis. The recently derived "LEFt" clinical prediction rule for pregnant women combines three variables: symptoms in the left leg ("L"), calf circumference difference ≥ 2 centimeters ("E" for edema) and first trimester presentation ("Ft"), but is lacking an external validation. Design and Methods. The LEFt rule was computed among pregnant women with suspected deep vein thrombosis who were included in a multicentre prospective diagnostic management outcome study. We calculated the proportion of women and the prevalence of deep vein thrombosis in each probability group, along with the diagnostic performances of the LEFt rule. Results. All variables needed to compute the rule could be retrieved in 157 out of the 167 pregnant women with suspected deep vein thrombosis. The prevalence of confirmed deep vein thrombosis was 13/157 (8.3%). The "LEFt" rule was negative in 46 (29%) women. A deep vein thrombosis was diagnosed in 13/111 (11.7%, 95% CI: 8.3 to 20.9%) of women with at least one of the "LEFt" criteria, as compared with 0/46 (0.0%, 95% CI:0.0 to 7.9%) of women with none of the "LEFt" criteria. Conclusions. These results suggest that a negative "LEFt" rule accurately identifies pregnant women in whom the proportion of confirmed deep vein thrombosis appears to be very low. The rule should not be used as stand-alone test for excluding DVT during pregnancy but might rather be implemented in a diagnostic strategy in association with Ddimer measurement and compression ultrasonography.