Impedance measurements were conducted on 50 limbs of 25 normal subjects, and on 43 limbs of 24 patients with clinical suspicion of deep venous thrombosis. On all patients, the impedance studies were followed by a phlebogram. Thirty-four out of 50 normal limbs showed a reduction of impedance during deep inspiration of more than 0.2%. The remaining 16 limbs demonstrated an impedance deflection of less than 0.2%. In patients, there were six false positive results, and one false negative result. Using the method described, it was concluded that the overall accuracy of impedance testing for diagnosing presence or absence of recent clots was 53.5%. Nevertheless, impedance measurement is a useful screening procedure for detection of deep thrombophlebitic disease of the lower extremity if its limitations are taken into account, primarily because of the very low rate of "false negative" results.
Dmochowski JR, Adams DF, Couch NP. Impedance Measurement in the Diagnosis of Deep Venous Thrombosis. Arch Surg. 1972;104(2):170-173. doi:10.1001/archsurg.1972.04180020050009