Predicting deep venous thrombosis in pregnancy: external validation of the LEFT clinical prediction rule

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 of 2 centimeters or over (E for edema) and first trimester presentation (Ft) but is lacking an external validation. The LEFt rule was computed among pregnant women with suspected deep vein thrombosis who were included in a multicenter 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. All variables needed to compute the rule could be retrieved in 157 of the 167 pregnant women with suspected deep vein thrombosis. The prevalence of confirmed deep vein thrombosis was 13 of 157 (8.3%). The LEFt rule was negative in 46 (29%) women. A deep vein thrombosis was diagnosed in 13 of 111 (11.7%, 95% Confidence Interval (CI): 8.3-20.9%) of women with at least one of the LEFt criteria, as compared with none of 46 (0.0%, 95%CI: 0.0-7.9%) of women with none of the LEFt criteria. 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 D-dimer measurement and compression ultrasonography.