What labs should be ordered for a patient presenting with leg swelling?

Laboratory Tests for Patients with Leg Swelling

For patients presenting with leg swelling, the initial diagnostic approach should include D-dimer testing and compression ultrasonography (CUS) to rule out deep vein thrombosis (DVT), which is a potentially life-threatening condition requiring prompt diagnosis and treatment. 1

Initial Risk Assessment

• Perform clinical risk stratification using a validated clinical decision rule to categorize patients as "DVT likely" or "DVT unlikely" before ordering laboratory tests 1
• For patients with "DVT likely" classification, proceed directly to compression ultrasonography rather than starting with D-dimer testing 1
• For patients with "DVT unlikely" classification, begin with D-dimer testing as this can safely rule out DVT when negative 1

Laboratory Testing Algorithm

For "DVT Unlikely" Patients:

• Order a highly sensitive D-dimer test as the initial diagnostic test 1
• If D-dimer is negative, no further testing is needed as DVT can be safely excluded (negative predictive value >99%) 1, 2
• If D-dimer is positive, proceed to proximal compression ultrasonography 1

For "DVT Likely" Patients:

• Order proximal compression ultrasonography or whole-leg ultrasonography as the initial test 1
• If proximal CUS is negative, additional testing with a highly sensitive D-dimer, whole-leg US, or repeat proximal CUS in 1 week is recommended 1
• If proximal CUS is positive, no confirmatory venography is needed before initiating treatment 1

Essential Pre-Treatment Laboratory Tests

• Complete blood count with platelet count to establish baseline values before anticoagulation 3
• Coagulation profile including prothrombin time, international normalized ratio, and activated partial thromboplastin time to guide anticoagulant therapy 3
• Comprehensive metabolic panel to assess kidney and liver function, which will influence anticoagulant selection and dosing 3, 4

Special Imaging Considerations

• In patients with extensive unexplained leg swelling, if proximal CUS or whole-leg US is negative and D-dimer testing is positive or not performed, iliac vein imaging should be done to exclude isolated iliac DVT 1, 3
• When ultrasound is impractical (e.g., leg casting, excessive subcutaneous tissue) or nondiagnostic, CT venography or MR venography can be used as alternatives 1, 3
• For patients with suspected pulmonary embolism in addition to DVT, appropriate PE imaging should be considered 3, 2

Common Pitfalls to Avoid

• Do not rely solely on D-dimer testing in patients with high pretest probability, as a negative result does not reliably exclude DVT in this population 1, 4
• Avoid delaying treatment for additional testing when DVT diagnosis is already firmly established by appropriate imaging 3, 4
• Do not overlook the need for iliac vein imaging in patients with extensive leg swelling when proximal ultrasound is negative 1, 3
• Failing to check baseline laboratory values before initiating anticoagulation can lead to complications in patients with undiagnosed bleeding disorders or organ dysfunction 3