Comparison of two commercial methods for measurement of cytomegalovirus load in blood samples after renal transplantation

A cohort of 77 renal transplant recipients was prospectively studied for co mparison of two commercially available cytomegalovirus (CMV) load assays, i .e., the COBAS AMPLICOR CMV Monitor test (Amplicor), using plasma samples, and the Murex Hybrid Capture System (HCS), using whole blood. The manufactu rer of the HCS assay changed the version of the test from 1.0 (HCS-1) to 2. 0 (HCS-2) after the first 37 patients had been tested. Despite the differen ces in principle and type of specimen used, the Amplicor and HCS assays gav e comparable results. The regression line correlating the HCS-1 assay to th e Amplicor assay was similar to that correlating the HCS-2 assay to the Amp licor assay. The HCS results could be converted to Amplicor-equivalent unit s by using linear-regression equations [log(10) HCS-1 result = 0.49 (log(10 ) Amplicor result) + 2.58, and log(10) HCS-2 result = 0.61 (log(10) Amplico r result) + 2.18]. The HCS-2 assay appeared to have the lowest detection li mit, followed by the Amplicor assay and then the HCS-1 assay. When a slidin g scale of cutoff values in Amplicor-equivalent units (>1,000, >2,500, >6,0 00, > 16,000, >40,000, and > 100,000 copies/ml) was applied to diagnose CMV disease, similar patterns of sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) were observed with the Am plicor and HCS assays. A cutoff value of >40,000 copies/ml has a low sensit ivity (Amplicor, 29.4%; HCS, 41.2%) but is specific (Amplicor, 96.7%; HCS, 93.3%) and can be used far the differential diagnosis of CMV disease (PPV, 71.4% [Amplicor] or 63.6% [HCS]; NPV, 82.9% [Amplicor] or 84.8% [HCS]). A l ower cutoff value of >1,000 copies/ml improves the sensitivity (Amplicor, 7 6.5%; HCS, 82.4%) and has a high NPV (Amplicor, 91.8%; HCS, 94.2%) but, due to the low PPV (Amplicor, 46.2%; HCS, 56%), is useful only for exclusion o f CMV disease.