Tests to screen BK Virus – BK screening
BK virus is detectable both in the blood and the urine. The virus first appears in the urine (viruria) and then is detectable in the blood (viremia) several weeks later. The preferred screening test at most transplant centers is the blood BK quantitative viral DNA polymerase chain reaction (PCR). The sensitivity between the serum and urine BK viral testing is the same at 100%, but the specificity of serum testing is 90% versus 80% for urine testing. BK viremia is also a better predictor of BK nephropathy. There is an alternative strategy that does use BK viruria for screening because it appears earlier that BK viremia. Once BK viruria is present, the clinician considers lowering immunosuppression and now switching to BK viremia for screening.
Recommended screening times are monthly for the first 6 months and then every 3 months up to 24 months post transplant.
A BK viral load greater than 10,000 copies is strongly associated with BK nephropathy. BK viruria greater than 1,000,000 copies is associated with BK nephropathy. The next step is a kidney transplant biopsy, which is the gold standard to confirm the diagnosis and see the degree of nephropathy.