United Kingdom scheme for external quality assessment in virology. Part I. General method of operation.

Developments in the United Kingdom national external quality assessment scheme for virology are described. There are about 198 participants (170 in the UK) who are enrolled for examination of any or all of five categories of specimen (distribution types). These are detection of rubella antibody (128 UK participants), detection of hepatitis B surface antigen (130 UK participants), general virus serology (86 UK participants), virus identification (85 UK participants), and electron microscopy (56 UK participants). Specimens of a sixth category (rubella IgM antibody), not yet formally established, have also been distributed to 67 UK participants. Specimens in each distribution type are sent out once or twice a year, and, except for rubella IgM antibody, participants have been given a score of 2, 1, 0 or -1 marks for their reports on each specimen. Their cumulative scores and performance ratings are calculated retrospectively over a 12 month period for each distribution type separately and for combined distributions. The performance rating is defined by the number of standard errors by which the individual's cumulative score differs from the mean for all participants and carries a + or - sign depending on whether the cumulative score lies above or below the mean. Performance ratings have been found generally to be close to the mean in rubella serology and detection of hepatitis B surface antigen but are more variable in virus identification and electron microscopy. Ratings of less than -1.96 are considered to be significantly worse than average and to constitute poor performance.