Diagnosis of DVT
A positive diagnosis of DVT can only be made if a venogram is positive or there is a positive venous ultrasound at two or more sites of the proximal veins. The diagnosis of DVT can be ruled out if there is a negative venogram, a negative D-dimer test or a normal venous ultrasound, assuming the venous ultrasound is accompanied by one of the following findings: 1) low clinical suspicion for DVT, 2) normal D-dimer test, or 3) negative serial ultrasound performed one week later. In an email survey 56% of respondents from acute centers reported the use of venous duplex ultrasonography (VDU) to screen for DVTs post ABI, whereas 13% rehabilitation hospital responders use D-dimer along with venous duplex ultrasonography for routine screening of DVT post injury (Carlile et al. 2006).
Venous ultrasound is often used to diagnose a DVT. There are several types of venous ultrasonography. They include compression ultrasound, duplex ultrasound and color Doppler imaging. Although these types of venous ultrasonography are sometimes used interchangeably, their sensitivities and specificities for detecting acute DVT vary (Zierler 2004). The sensitivity and specificity of compression ultrasonography for detecting DVTs is 43% and 85%, respectively (Girard et al. 2005). The weighted mean sensitivity and specificity of venous ultrasonography for the diagnosis of symptomatic proximal DVT are 97% and 94%, respectively; the sensitivity falls to 73% for distal DVT (Kearon et al. 1998; Zierler 2004). Importantly, distal DVT do not confer the same risk of extension to PE as do proximal DVT. Typically, if a distal clot is going to extend proximally, this occurs within one week of its development. Consequently, serial ultrasound could be used in symptomatic patients in whom the test is initially negative as the test would become positive with the clot extension.
Venography is considered a definitive test for DVT but it is an invasive test whereby contrast dye is injected into the leg veins. Diagnosis of DVT is made if an intraluminal-filling defect is noted.
D-dimer assay is a rapid, non-invasive and inexpensive test. Fibrin is the main component of thrombus formation and fibrin degradation products include D-dimers (Gill & Nahum 2000). A positive D-dimer test is highly sensitive for the presence of a thrombus but lacks specificity since D-dimers are found in other disease states, including cancer, congestive heart failure and inflammatory conditions (Raimondi et al. 1993). As a result, D-dimer assays have a high negative predictive value but a poor positive predictive value. To illustrate, Akman et al. (2004) reported that the sensitivity and negative predictive values of the D-dimer test were high, at 95.2% and 96.2% respectively, in a group of 68 rehabilitation patients (stroke, spinal cord injury, TBI, hip arthroplasty). The specificity and positive predictive values were low at 55.3% and 48.7%, respectively.