Comparison of two quantitative real-time CMV-PCR tests calibrated against the 1st WHO international standard for viral load monitoring of renal transplant patients.

Cytomegalovirus (CMV) replication in organ transplant recipients is commonly diagnosed by quantitative PCR methods. However, there has been a poor inter-laboratory correlation of viral load values due to the lack of an international reference standard. In a recent study, the COBAS® AmpliPrep/COBAS® TaqMan® (CAP/CTM) CMV test calibrated to the 1st WHO CMV standard, showed good reproducibility in CMV load values across multiple laboratories. Fifty-seven follow-up plasma specimens from 10 kidney transplant recipients with CMV replication were examined using the new quantitative CAP/CTM CMV test and the "in-house" quantitative CMV real-time PCR method, also calibrated against the 1st WHO CMV standard for their clinical applicability for monitoring CMV load in renal transplant patients. By CAP/CTM CMV test 49/57 specimens were CMV-DNA positive compared to 44/57 by the "in-house" PCR test. The "in-house" PCR and CAP/CTM CMV test correlated well in monitoring individual kidney transplant patients. Conversion of the CMV-DNA copies to IUs made the results of the "in-house" PCR and CAP/CTM CMV test less uniform in analysis of the patient samples. In specimens of one patient, significant underquantification of CMV load with "in-house" PCR emerged during follow-up due to a point mutation in the "in-house" PCR primer sequence. The CAP/CTM CMV test was found suitable for diagnosing and monitoring CMV replication in renal transplant patients. Multicenter studies are needed to provide more information of the commutability of the 1st WHO CMV standard and to define the clinical thresholds.