The proportion of culture-negative Shigella cases that tested positive by PCR was multiplied by the number of culturenegative Shigella cases of diarrhoea and dysentery detected during the surveillance period.This product expresses the estimated number of cases missed by culture methods. The fraction of cases missed by passive detection was estimated on the basis of a health-care utilization survey (6). The total number of cases (missing + detected cases) was computed as follows: (detected cases)(1/fraction missed by culture)(1/fraction missed by incomplete health-care utilization). The calculation is shown in the Appendix (available on web version only
at: http://www.who.int/bulletin). Incidence calculations were computed using Excel XP spreadsheets (Microsoft, Redmond, WA, USA).