External quality assessment of CMV-DNA detection on dried blood spots

Testing for viral DNA in neonatal blood dried on paper (DBS) has proved a valid means of diagnosing congenital CMV infection with both clinical and epidemiological relevance. Most laboratories use an in-house developed assay for detection of CMV-DNA in DBS. However, no Internal Standard is available and previous external quality assessment studies have shown that the quality of nucleic acid amplification methods varies considerably between laboratories. Therefore, a quality assessment study for the detection of CMV-DNA on DBS was organised by QCMD (Quality Control for Molecular Diagnostics) in collaboration with ECCI (European Congenital CMV Initiative). Thirty-three laboratories from 14 countries participated in the study and received a panel consisting of 9 samples with 2 blood spots on DFS paper each. Six samples were derived from whole blood, negative for CMV DNA and antibody, and spiked with cell-grown CMV Towne in various concentrations ( 4x103 - 7 x105 copies/ml), one was a CMV positive clinical specimen (3x106 copies/ml), and two samples were non-spiked whole blood. The 29 responding participants submitted 33 datasets obtained by means of conventional PCR (5 sets) or RT-PCR (28 sets) technology. Positive results were reported by at least 91% of sets down to the 9x104 copies/ml concentration, 59% identified the 6x103 copies/ml samples and 12% only reported the lowest concentration samples as positive. A false positive result was reported 5 times (7.6%). These results indicate a clear need for improvement of methods as sensitivity and cross-contamination still appear to be a problem in some laboratories.