• To determine the incidence of arterial inflow problems and venous outflow problems as causes of impaired renal function and/or treatment-resistant hypertension after kidney transplantation, when all kidney-transplant recipients in Denmark are evaluated according to uniform, well-defined clinical criteria.
• To investigate the efficacy and safety of catheter-based balloon treatment (percutaneous transluminal angioplasty, PTA) for these vascular complications, of which transplant renal artery stenosis is by far the most common.
• To assess whether novel imaging and functional diagnostic methods can predict treatment response.

Kidney transplantation is performed 250-300 times annually in Denmark and substantially improves survival, quality of life, and reduces the burden of comorbidities in patients with end-stage kidney disease. Despite these benefits, vascular complications, particularly transplant renal artery stenosis (TRAS), remain a major cause of morbidity. Reported incidence of TRAS varies widely (1-23%), reflecting retrospective study designs and inconsistent diagnostic criteria. TRAS are classified into three main types: anastomotic (TRAS-A), post-anastomotic (TRAS-P), and long-segment bend/kink (TRAS-B), with most cases diagnosed within the first two years post-transplant. Severe stenoses can critically impair graft perfusion, leading to reduced renal function and treatment-resistant hypertension.

Percutaneous transluminal angioplasty (PTA) for TRAS is a well-established procedure performed according to the same principles as coronary balloon angioplasty; however, the role of stent placement remains uncertain. PTA without stenting is associated with higher restenosis rates compared to PTA with stenting, yet evidence regarding graft function, survival, and blood-pressure control remains conflicting.

Adverse events related to PTA occur in approximately 10% of patients and are generally mild. Serious adverse events are observed in fewer than 5% of patients and include procedure-related internal bleeding and vascular access-site complications. Severe internal bleeding may require blood transfusion and endovascular vessel occlusion and can, in rare cases, result in loss of the transplanted kidney. Access-site vascular complications may present as bleeding, thrombosis, or pseudoaneurysm.

Against this background, the nationwide prospective multicentre DAN-PTRAIII study aims to establish the true incidence of arterial inflow and venous outflow problems in Danish kidney-transplant recipients, evaluate the efficacy and safety of balloon angioplasty, and explore novel imaging and functional diagnostic methods for predicting treatment response.

Inclusion Criteria:

1. At least one of the following clinical criteria (1 or 2) must be fulfilled:

1. Graft dysfunction, defined by at least one of the following:

   • Acute reduction in estimated glomerular filtration rate (eGFR) >15% on two consecutive measurements at least 2 weeks apart, with other causes excluded (rejection, obstruction, infection).
   • eGFR <50% of the expected value 30 days after kidney transplantation of unknown cause.
   • Decline in eGFR >30% after initiation of an angiotensin-converting enzyme inhibitor or angiotensin II receptor blocker.

2. Persistent resistant hypertension for more than 6 weeks after kidney transplantation, defined as:

   • 24-hour ambulatory systolic blood pressure >130 mmHg despite treatment with at least three classes of antihypertensive medication at maximally tolerated doses (including diuretics, if tolerated).

Together with at least one of the following radiological criteria:

1. CT or MR angiography demonstrating a lumen reduction ≥50%.

2. Doppler ultrasound showing:

   1. Peak systolic velocity in the renal artery ≥200 cm/s and a renal renal ratio (velocity at stenosis / velocity in distal artery) >4.
   2. Acceleration time >70 ms in intrarenal arteries.

2. In cases of strong clinical suspicion of a vascular complication where CT or MR angiography cannot reliably exclude graft artery or vein stenosis, patients may be referred for confirmatory invasive investigations.

Before PTA, catheter-based angiography and translesional pressure measurements are performed to confirm whether the patient meets the radiological eligibility criterion for PTA:

1. Stenosis ≥70%.

2. Stenosis 50-69% if at least one of the following criteria is met:

   • Mean translesional pressure gradient ≥10 mmHg.
   • Systolic pressure gradient ≥20 mmHg.
   • Renal Pd/Pa ≤0.8.
   • If pressure measurements cannot be obtained, treatment is based on the operator's clinical judgement.

Exclusion Criteria:

1. Inability to provide informed consent.
2. Concurrent biopsy demonstrating rejection requiring treatment.
3. Pregnancy.
4. Previous PTA of the same vessel.
5. Patients unable to tolerate any form of antithrombotic therapy and therefore not eligible for stent placement.

National prospective multicenter study with a primary endpoint at 3 months and annual follow-up for up to 10 years post-intervention, evaluating renal function, blood pressure, and clinical events.

Patients with vascular complications, predominantly transplant renal artery stenosis (TRAS) following kidney transplantation, who meet at least one of the clinical inclusion criteria, may be enrolled in the study.