What diagnostic tests to order for a patient presenting with right thigh and knee pain, erythema (redness), and warmth to touch?

From the Guidelines

For a patient with right thigh and knee pain, redness, and warmth, I recommend ordering a highly sensitive D-dimer test or proximal compression ultrasonography (CUS) to assess for deep vein thrombosis (DVT), as suggested by the American College of Chest Physicians evidence-based clinical practice guidelines 1. The patient's symptoms of pain, redness, and warmth in the right thigh and knee could be indicative of DVT, and prompt diagnosis is crucial to prevent complications.

• A highly sensitive D-dimer test is a suitable initial test, especially if it is readily available and the patient has a moderate pretest probability of DVT 1.
• Alternatively, proximal CUS can be used as the initial test, especially if the patient has a high pretest probability of DVT or if D-dimer testing is not readily available 1.
• If the proximal CUS is negative, no further testing is recommended, unless there is a high clinical suspicion of DVT 1.
• If the highly sensitive D-dimer is positive, further testing with proximal CUS or whole-leg ultrasound may be necessary to confirm the diagnosis of DVT 1. It is essential to note that the choice between a highly sensitive D-dimer test and US as the initial test will depend on local availability, access to testing, costs of testing, and the probability of obtaining a negative D-dimer result if DVT is not present 1. Additionally, other potential causes of the patient's symptoms, such as cellulitis or septic arthritis, should also be considered and investigated accordingly, with tests such as complete blood count (CBC), C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), and blood cultures, as well as X-rays and joint aspiration if necessary 1. However, the most recent and highest quality study 1 suggests that CRP and ESR may not be useful in diagnosing infection in patients with suspected prosthetic joint infection, and other tests such as alpha-defensin and synovial fluid culture may be more accurate. Therefore, the initial focus should be on ruling out DVT with a highly sensitive D-dimer test or proximal CUS, and then further testing can be guided by the results and clinical suspicion.

From the Research

Diagnostic Approach

To diagnose the patient's condition, the following steps can be taken:

• Evaluate the patient's symptoms and medical history to determine the pre-test probability of deep vein thrombosis (DVT) 2
• Use a clinical decision rule, such as the modified Wells score, to stratify the patient's risk of DVT 3, 4
• If DVT is considered "unlikely", order a D-dimer test to help exclude the diagnosis 5, 2
• If the D-dimer level is normal, DVT can be excluded; if the D-dimer level is increased, order compression ultrasound to confirm the diagnosis 2

Imaging Tests

The following imaging tests can be used to diagnose DVT:

• Compression ultrasound: this is the primary modality for diagnosing DVT 3, 2
• Computed tomography (CT) and magnetic resonance venography: these can be used as alternative tests in select patients 3
• Nuclear medicine techniques: these are under development and may be used in the future 3

Biomarkers

The following biomarkers can be used to help diagnose DVT:

• D-dimer: this is the only clinically applied biomarker for DVT diagnosis, and it has a high negative predictive value 3, 5, 2
• Soluble P-selectin: this is a promising novel biomarker for DVT diagnosis 3

Specific Considerations

When interpreting the results of the D-dimer test, consider the following:

• Small thrombi may be associated with low D-dimer levels, increasing false negatives 6
• Age-adjusted D-dimer cut-offs may be below the standard cut-off more frequently in subjects with isolated distal DVT (IDDVT) 6