Focused Ultrasound of the Popliteal Fossa for Detection of Symptomatic Baker's Cysts in Patients With Suspected Deep Vein Thrombosis: A Pilot Diagnostic Accuracy Study

Patients with suspected deep vein thrombosis (DVT) are commonly evaluated using clinical probability assessment, D-dimer testing, and compression ultrasonography. However, a substantial proportion of patients referred for DVT evaluation are ultimately diagnosed with alternative conditions that may explain their symptoms. One such condition is a symptomatic Baker's cyst, which may present with unilateral leg pain, swelling, or discomfort that clinically mimics DVT.

In current clinical practice, radiologist-performed complete compression ultrasound of the lower extremity (CCUS) is primarily used to confirm or exclude DVT. Although Baker's cysts are frequently identified during these examinations, their potential role as an early alternative diagnosis within the diagnostic pathway has not been systematically investigated. A focused ultrasound examination of the popliteal fossa (FUP) may allow emergency department (ED) or primary care clinicians to identify symptomatic Baker's cysts before referral for further DVT workup.

This prospective pilot study primarily aims to evaluate the feasibility of conducting a future definitive study investigating the role of FUP in patients with suspected DVT. Key feasibility outcomes include recruitment rates, scan completion rates, workflow integration, procedure timing, and practical implementation within routine emergency care. In addition, the study will generate preliminary diagnostic accuracy estimates regarding the ability of ED physicians and trained research assistants to identify symptomatic Baker's cysts using FUP, with CCUS serving as the reference standard. These data will be used to estimate outcome variance and inform sample size calculations and study design for a future definitive validation study.

This single-center pilot study will be conducted at Aarhus University Hospital, Denmark, among adult patients referred to the ED with suspected DVT. All participants will receive standard diagnostic evaluation and management according to current guidelines. The study-specific FUP examination will be performed before D-dimer and/or CCUS results are available, and FUP findings will not influence clinical decision-making during the study.

• Index Test (Focused Ultrasound of the Popliteal Fossa) (Diagnostic test): Eligible patients will undergo a focused ultrasound examination of the popliteal fossa performed by a trained research assistant or emergency physician. The examination will be conducted before the results of D-dimer testing and CCUS are available. Operators will assess the presence and characteristics of Baker's cysts, evaluate whether an identified cyst is likely to explain the patient's presenting symptoms, estimate the probability of concurrent DVT, and complete a structured interpretation survey. FUP findings will be recorded for research purposes only and will not influence patient management.
• Reference test (Routine DVT Diagnostic Pathway) (Diagnostic test): Following FUP, all participants will proceed through the routine diagnostic pathway for suspected DVT. Depending on clinical probability and D-dimer results, patients may undergo no further testing or radiologist-performed complete compression ultrasound of the lower extremity (CCUS) according to standard practice. Final diagnoses established through the routine diagnostic pathway will serve as the reference standard.

Primary outcomes

• Participant recruitment rate [Immediately after the procedure]: Number of eligible patients enrolled per study month and proportion of eligible patients successfully enrolled.

Secondary outcomes

• Completion rate of Focused Ultrasound of the Popliteal Fossa [Immediately after the procedure]: Proportion of initiated FUP examinations successfully completed and deemed interpretable.
• Duration of the FUP examination [Immediately after the procedure]: Time required to perform and document the FUP examination.
• Diagnostic accuracy of FUP for detection of symptomatic Baker's cysts [Immediately after the procedure]: Sensitivity, specificity, positive predictive value, and negative predictive value of FUP compared with radiologist-performed complete compression ultrasound of the lower extremity (CCUS).
• Frequency of symptomatic Baker's cysts among patients evaluated for suspected DVT. [Immediately after the procedure]: Proportion of participants with a symptomatic Baker's cyst identified on FUP.
• Concurrent DVT in participants with a symptomatic Baker's cyst identified on FUP [Immediately after the procedure]: Proportion of participants with a symptomatic Baker's cyst on FUP who are subsequently diagnosed with DVT through the routine diagnostic pathway.
• Estimated impact of FUP on clinical decision-making [Immediately after the procedure]: Proportion of examinations in which the operator indicates that the FUP finding would have altered the estimated post-test probability of DVT or influenced further diagnostic workup.
• Ultrasound image quality [Within 60 days of the index examination]: Proportion of FUP examinations rated as of adequate diagnostic quality upon retrospective review by a proficient radiologist.

Sex: All, ages: 18 Years to any

Inclusion Criteria:

* Age ≥ 18 years
* Clinical signs and symptoms of DVT, prompting the treating physician to initiate d-Dimer or CCUS diagnostic strategies

Exclusion Criteria:

* Prior inclusion
* FUP performed following d-Dimer or CCUS result already revealed