USE OF COMBINED SHEWHART-CUSUM CONTROL CHARTS IN INTERNAL QUALITY-CONTROL OF ENZYME-LINKED IMMUNOSORBENT ASSAYS FOR THE TYPING OF FOOT-AND-MOUTH-DISEASE VIRUS-ANTIGEN

An enzyme-linked immunosorbent assay (ELISA) for the typing of foot an d mouth disease virus (FMDV) antigen was employed for the routine labo ratory diagnosis of FMD at a regional veterinary laboratory in norther n Thailand. An objective procedure was developed to monitor the test p erformance of the ELISA, using absolute test control limits in a Shewh art-CUSUM (cumulative sum) control chart method. The procedure detecte d significant data trends and 'beyond control limit' situations for ea ch antigen typing system (types O, A and Asia 1), using an assay varia ble (gamma(i)). Retrospective analysis using Shewhart-CUSUM control ch arts of data from 42 ELISAs demonstrated that control limits were exce eded in two assays for FMDV type A. The Shewhart-CUSUM control chart i s a simple and reliable internal quality control method for the detect ion of significant random and systematic variation in assays.