External quality assessment of cytomegalovirus DNA detection on dried blood spots

Background. 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. To assess the quality of the detection of CMV-DNA on DBS in laboratories performing this test a proficiency panel consisting of nine samples with two blood spots on each filter paper was produced and distributed. Six samples were derived from whole blood, negative for CMV DNA and antibody, and spiked with cell-grown CMV Towne in various concentrations (7.3 × 102 - 9.6 × 105 copies/ml), one was a CMV positive clinical specimen (3.9 × 106 copies/ml), and two samples were CMV-negative whole blood. Results. The 27 responding laboratories from 14 countries submitted 33 datasets obtained by means of conventional PCR (n = 5) or real-time PCR (n = 28) technologies. A correct positive result was reported in at least 91% of datasets in samples with a viral load of 8.8 × 104 copies/ml or higher. However only 59% and 12% identified the 9.4 × 103 and 7.3 × 10 2 copies/ml samples, respectively, correctly as positive. False positive results were reported by 9% of laboratories and in 11% of datasets. Conclusion. These results indicate a clear need for improvement of methods as sensitivity and false-positivity still appear to be a major problem in a considerable number of laboratories.