Monitoring BK virus in kidney transplant patients

Since July last year, the Department of Renal Medicine and Transplantation and the Centre for Transplant and Renal Research at Westmead Hospital in Sydney have been using a new molecular method to quantitatively detect BK virus in kidney transplant patients in a bid to improve patient outcomes.

The new technique quantitatively measures BK virus DNA in a single reaction and has successfully reduced turnaround times of results, as well as improving workflows and costs.

BK virus

The BK virus was first isolated in 1971 and was named after the renal transplant patient in which it was found, who had the initials BK. BK virus (BKV) is a Polyomavirus, which is a small non-enveloped virus with a circular, double-stranded DNA genome.

Although BKV is prevalent around the world, infection is usually asymptomatic or associated with mild respiratory tract symptoms in healthy individuals. Primary infection typically occurs in early childhood, after which the virus persists in a latent form in the kidneys and urinary tract of its host.

In most people reactivation of the virus is benign, but it is one of the most common viral complications to affect renal transplant patients and it can cause serious complications in these patients.

Reactivation of the virus can lead to BKV-associated nephropathy (BKVAN) in up to 10% of renal transplant recipients, and it is associated with graft failure in 15-80% of affected patients. In addition to immunosuppression, intragraft inflammation and host-specific immunity have been suggested. The progression of BKVAN may occur without obvious signs or symptoms, other than raised serum creatinine, so it is often misdiagnosed.

Thus, early detection of viral reactivation and accurate monitoring of viral loads is important.

Monitoring BK viral loads

Quantitative measurements of BK viral load in urine and blood by molecular techniques are useful for monitoring the course of BKV infection and for predicting the development of BKVAN. Such measurements are also valuable in monitoring response to therapy.

Viral reactivation can be detected in the urine several weeks before the virus is detected in the blood, and viremia can be detected months before histological evidence of BKVAN is present.

Although suggested BKV load thresholds for quantitative molecular measurements vary, and laboratories are encouraged to establish their own cut-off values for the purpose of clinical management, BK viral loads of greater than 10,000 copies/mL in blood and greater than 10 million copies/mL in urine are considered predictive for BKVAN.

Current guidelines recommend screening for BKV in the serum or plasma of kidney transplant patients monthly for the first 3-6 months after transplantation, and then every 3 months up to one year post-transplantation. These guidelines also recommend that patients are screened for BKV if there is an unexplained rise in serum creatinine or following treatment for acute rejection.

Detecting BK virus

Due to the specialist nature of BKV testing and the resources and expertise required to perform BKV measurements by urine cytology or nucleic acid testing, many centres are required to send samples to a reference laboratory for analysis. Some laboratories have adopted in-house polymerase chain reaction (PCR) BKV assays.

Up until recently, the Westmead laboratory used an in-house conventional qualitative PCR method for detecting BKV followed by monthly quantification of viral loads in BKV-positive patients using a commercially available real-time PCR assay. In-house PCR methods can be labour intensive, have variable specificity and can be time-consuming due to the need for confirmatory tests on positive results - which can potentially impact patient management.

The new Iam BKV assay the Westmead lab is now using, produced by DiaSorin, detects and quantifies BKV with high specificity and is scalable, allowing for large or small workloads.

The lab provides a BKV testing service to the hospital’s renal transplant outpatient clinics as well as other specialist clinics associated with the centre. They also test samples from other pathology groups in their reference capacity.

The assay also produces quantitative results on the same day as sample receipt, expediting the process of determining whether a patient is likely to develop BKVAN, which may lead to premature graft loss.

“Renal transplant patients are tested routinely using the new assay at 1, 2, 3, 6, 9 and 12 months post-transplantation. Patients that test positive for BKV are tested more frequently, every 2-4 weeks,” Dr Neisha Jeoffreys, senior hospital scientist at the Westmead lab, said.

“Patients with high BKV levels will have their immunosuppression regime modified in order to reduce BKV levels while preventing graft rejection. Ongoing monitoring of BK viral load then assists the renal physicians to ensure the right amount of immunosuppression is delivered to reduce the risk of BKVAN and maintain a healthy graft. Quantitative results allow the physicians to determine the appropriate point at which to modify the treatment.”

This is a summary of an article written by Dawn Wallace, a US-based healthcare writer.